                      FreeBSD Developers' Handbook

  The FreeBSD Documentation Project

   Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
   2008, 2009, 2010 The FreeBSD Documentation Project

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   Apple, AirPort, FireWire, Mac, Macintosh, Mac OS, Quicktime, and
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   Microsoft Corporation in the United States and/or other countries.

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   Many of the designations used by manufacturers and sellers to
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     --------------------------------------------------------------

   Table of Contents

   I. Basics

                1 Introduction

                             1.1 Developing on FreeBSD

                             1.2 The BSD Vision

                             1.3 Architectural Guidelines

                             1.4 The Layout of /usr/src

                2 Programming Tools

                             2.1 Synopsis

                             2.2 Introduction

                             2.3 Introduction to Programming

                             2.4 Compiling with cc

                             2.5 Make

                             2.6 Debugging

                             2.7 Using Emacs as a Development
                             Environment

                             2.8 Further Reading

                3 Secure Programming

                             3.1 Synopsis

                             3.2 Secure Design Methodology

                             3.3 Buffer Overflows

                             3.4 SetUID issues

                             3.5 Limiting your program's environment

                             3.6 Trust

                             3.7 Race Conditions

                4 Localization and Internationalization - L10N and
                I18N

                             4.1 Programming I18N Compliant
                             Applications

                             4.2 Localized Messages with POSIX.1
                             Native Language Support (NLS)

                5 Source Tree Guidelines and Policies

                             5.1 Style Guidelines

                             5.2 MAINTAINER on Makefiles

                             5.3 Contributed Software

                             5.4 Encumbered Files

                             5.5 Shared Libraries

                6 Regression and Performance Testing

                             6.1. Micro Benchmark Checklist

                             6.2. The FreeBSD Source Tinderbox

   II. Interprocess Communication

                7 Sockets

                             7.1 Synopsis

                             7.2 Networking and Diversity

                             7.3 Protocols

                             7.4 The Sockets Model

                             7.5 Essential Socket Functions

                             7.6 Helper Functions

                             7.7 Concurrent Servers

                8 IPv6 Internals

                             8.1 IPv6/IPsec Implementation

   III. Kernel

                9 Building and Installing a FreeBSD Kernel

                             9.1 Building a Kernel the "Traditional"
                             Way

                             9.2 Building a Kernel the "New" Way

                10 Kernel Debugging

                             10.1 Obtaining a Kernel Crash Dump

                             10.2 Debugging a Kernel Crash Dump with
                             kgdb

                             10.3 Debugging a Crash Dump with DDD

                             10.4 On-Line Kernel Debugging Using DDB

                             10.5 On-Line Kernel Debugging Using
                             Remote GDB

                             10.6 Debugging a Console Driver

                             10.7 Debugging Deadlocks

                             10.8 Kernel debugging with Dcons

                             10.9 Glossary of Kernel Options for
                             Debugging

   IV. Architectures

                11 x86 Assembly Language Programming

                             11.1 Synopsis

                             11.2 The Tools

                             11.3 System Calls

                             11.4 Return Values

                             11.5 Creating Portable Code

                             11.6 Our First Program

                             11.7 Writing UNIX(R) Filters

                             11.8 Buffered Input and Output

                             11.9 Command Line Arguments

                             11.10 UNIX Environment

                             11.11 Working with Files

                             11.12 One-Pointed Mind

                             11.13 Using the FPU

                             11.14 Caveats

                             11.15 Acknowledgements

   V. Appendices

                Bibliography

   List of Examples

   2-1. A sample .emacs file

                               I. Basics

   Table of Contents

   1 Introduction

   2 Programming Tools

   3 Secure Programming

   4 Localization and Internationalization - L10N and I18N

   5 Source Tree Guidelines and Policies

   6 Regression and Performance Testing

     --------------------------------------------------------------

                         Chapter 1 Introduction

   Contributed by Murray Stokely and Jeroen Ruigrok van der Werven.

1.1 Developing on FreeBSD

   So here we are. System all installed and you are ready to start
   programming. But where to start? What does FreeBSD provide? What
   can it do for me, as a programmer?

   These are some questions which this chapter tries to answer. Of
   course, programming has different levels of proficiency like any
   other trade. For some it is a hobby, for others it is their
   profession. The information in this chapter might be aimed toward
   the beginning programmer; indeed, it could serve useful for the
   programmer unfamiliar with the FreeBSD platform.

     --------------------------------------------------------------

1.2 The BSD Vision

   To produce the best UNIX(R) like operating system package
   possible, with due respect to the original software tools ideology
   as well as usability, performance and stability.

     --------------------------------------------------------------

1.3 Architectural Guidelines

   Our ideology can be described by the following guidelines

     * Do not add new functionality unless an implementor cannot
       complete a real application without it.

     * It is as important to decide what a system is not as to decide
       what it is. Do not serve all the world's needs; rather, make
       the system extensible so that additional needs can be met in
       an upwardly compatible fashion.

     * The only thing worse than generalizing from one example is
       generalizing from no examples at all.

     * If a problem is not completely understood, it is probably best
       to provide no solution at all.

     * If you can get 90 percent of the desired effect for 10 percent
       of the work, use the simpler solution.

     * Isolate complexity as much as possible.

     * Provide mechanism, rather than policy. In particular, place
       user interface policy in the client's hands.

   From Scheifler & Gettys: "X Window System"

     --------------------------------------------------------------

1.4 The Layout of /usr/src

   The complete source code to FreeBSD is available from our public
   repository. The source code is normally installed in /usr/src
   which contains the following subdirectories:



   Directory                        Description                       
   bin/       Source for files in /bin                                
   cddl/      Utilities covered by the Common Development and         
              Distribution License                                    
   contrib/   Source for files from contributed software.             
   crypto/    Cryptographical sources                                 
   etc/       Source for files in /etc                                
   games/     Source for files in /usr/games                          
   gnu/       Utilities covered by the GNU Public License             
   include/   Source for files in /usr/include                        
   kerberos5/ Source for Kerberos version 5                           
   lib/       Source for files in /usr/lib                            
   libexec/   Source for files in /usr/libexec                        
   release/   Files required to produce a FreeBSD release             
   rescue/    Build system for the /rescue utilities                  
   sbin/      Source for files in /sbin                               
   secure/    FreeSec sources                                         
   share/     Source for files in /usr/share                          
   sys/       Kernel source files                                     
   tools/     Tools used for maintenance and testing of FreeBSD       
   usr.bin/   Source for files in /usr/bin                            
   usr.sbin/  Source for files in /usr/sbin                           

     --------------------------------------------------------------

                      Chapter 2 Programming Tools

   Contributed by James Raynard and Murray Stokely.

2.1 Synopsis

   This chapter is an introduction to using some of the programming
   tools supplied with FreeBSD, although much of it will be
   applicable to many other versions of UNIX. It does not attempt to
   describe coding in any detail. Most of the chapter assumes little
   or no previous programming knowledge, although it is hoped that
   most programmers will find something of value in it.

     --------------------------------------------------------------

2.2 Introduction

   FreeBSD offers an excellent development environment. Compilers for
   C and C++ and an assembler come with the basic system, not to
   mention classic UNIX tools such as sed and awk. If that is not
   enough, there are many more compilers and interpreters in the
   Ports collection. The following section, Introduction to
   Programming, lists some of the available options. FreeBSD is very
   compatible with standards such as POSIX(R) and ANSI C, as well
   with its own BSD heritage, so it is possible to write applications
   that will compile and run with little or no modification on a wide
   range of platforms.

   However, all this power can be rather overwhelming at first if you
   have never written programs on a UNIX platform before. This
   document aims to help you get up and running, without getting too
   deeply into more advanced topics. The intention is that this
   document should give you enough of the basics to be able to make
   some sense of the documentation.

   Most of the document requires little or no knowledge of
   programming, although it does assume a basic competence with using
   UNIX and a willingness to learn!

     --------------------------------------------------------------

2.3 Introduction to Programming

   A program is a set of instructions that tell the computer to do
   various things; sometimes the instruction it has to perform
   depends on what happened when it performed a previous instruction.
   This section gives an overview of the two main ways in which you
   can give these instructions, or "commands" as they are usually
   called. One way uses an interpreter, the other a compiler. As
   human languages are too difficult for a computer to understand in
   an unambiguous way, commands are usually written in one or other
   languages specially designed for the purpose.

     --------------------------------------------------------------

  2.3.1 Interpreters

   With an interpreter, the language comes as an environment, where
   you type in commands at a prompt and the environment executes them
   for you. For more complicated programs, you can type the commands
   into a file and get the interpreter to load the file and execute
   the commands in it. If anything goes wrong, many interpreters will
   drop you into a debugger to help you track down the problem.

   The advantage of this is that you can see the results of your
   commands immediately, and mistakes can be corrected readily. The
   biggest disadvantage comes when you want to share your programs
   with someone. They must have the same interpreter, or you must
   have some way of giving it to them, and they need to understand
   how to use it. Also users may not appreciate being thrown into a
   debugger if they press the wrong key! From a performance point of
   view, interpreters can use up a lot of memory, and generally do
   not generate code as efficiently as compilers.

   In my opinion, interpreted languages are the best way to start if
   you have not done any programming before. This kind of environment
   is typically found with languages like Lisp, Smalltalk, Perl and
   Basic. It could also be argued that the UNIX shell (sh, csh) is
   itself an interpreter, and many people do in fact write shell
   "scripts" to help with various "housekeeping" tasks on their
   machine. Indeed, part of the original UNIX philosophy was to
   provide lots of small utility programs that could be linked
   together in shell scripts to perform useful tasks.

     --------------------------------------------------------------

  2.3.2 Interpreters available with FreeBSD

   Here is a list of interpreters that are available from the FreeBSD
   Ports Collection, with a brief discussion of some of the more
   popular interpreted languages.

   Instructions on how to get and install applications from the Ports
   Collection can be found in the   Ports section of the handbook.

   BASIC

           Short for Beginner's All-purpose Symbolic Instruction
           Code. Developed in the 1950s for teaching University
           students to program and provided with every
           self-respecting personal computer in the 1980s, BASIC has
           been the first programming language for many programmers.
           It is also the foundation for Visual Basic.

           The Bywater Basic Interpreter can be found in the Ports
           Collection as lang/bwbasic and the Phil Cockroft's Basic
           Interpreter (formerly Rabbit Basic) is available as
           lang/pbasic.

   Lisp

           A language that was developed in the late 1950s as an
           alternative to the "number-crunching" languages that were
           popular at the time. Instead of being based on numbers,
           Lisp is based on lists; in fact the name is short for
           "List Processing". Very popular in AI (Artificial
           Intelligence) circles.

           Lisp is an extremely powerful and sophisticated language,
           but can be rather large and unwieldy.

           Various implementations of Lisp that can run on UNIX
           systems are available in the Ports Collection for FreeBSD.
           GNU Common Lisp can be found as lang/gcl. CLISP by Bruno
           Haible and Michael Stoll is available as lang/clisp. For
           CMUCL, which includes a highly-optimizing compiler too, or
           simpler Lisp implementations like SLisp, which implements
           most of the Common Lisp constructs in a few hundred lines
           of C code, lang/cmucl and lang/slisp are available
           respectively.

   Perl

           Very popular with system administrators for writing
           scripts; also often used on World Wide Web servers for
           writing CGI scripts.

           Perl is available in the Ports Collection as lang/perl5.16
           for all FreeBSD releases.

   Scheme

           A dialect of Lisp that is rather more compact and cleaner
           than Common Lisp. Popular in Universities as it is simple
           enough to teach to undergraduates as a first language,
           while it has a high enough level of abstraction to be used
           in research work.

           Scheme is available from the Ports Collection as lang/elk
           for the Elk Scheme Interpreter. The MIT Scheme Interpreter
           can be found in lang/mit-scheme and the SCM Scheme
           Interpreter in lang/scm.

   Icon

           Icon is a high-level language with extensive facilities
           for processing strings and structures. The version of Icon
           for FreeBSD can be found in the Ports Collection as
           lang/icon.

   Logo

           Logo is a language that is easy to learn, and has been
           used as an introductory programming language in various
           courses. It is an excellent tool to work with when
           teaching programming in small ages, as it makes the
           creation of elaborate geometric shapes an easy task even
           for very small children.

           The latest version of Logo for FreeBSD is available from
           the Ports Collection in lang/logo.

   Python

           Python is an Object-Oriented, interpreted language. Its
           advocates argue that it is one of the best languages to
           start programming with, since it is relatively easy to
           start with, but is not limited in comparison to other
           popular interpreted languages that are used for the
           development of large, complex applications (Perl and Tcl
           are two other languages that are popular for such tasks).

           The latest version of Python is available from the Ports
           Collection in lang/python.

   Ruby

           Ruby is an interpreter, pure object-oriented programming
           language. It has become widely popular because of its easy
           to understand syntax, flexibility when writing code, and
           the ability to easily develop and maintain large, complex
           programs.

           Ruby is available from the Ports Collection as
           lang/ruby18.

   Tcl and Tk

           Tcl is an embeddable, interpreted language, that has
           become widely used and became popular mostly because of
           its portability to many platforms. It can be used both for
           quickly writing small, prototype applications, or (when
           combined with Tk, a GUI toolkit) fully-fledged, featureful
           programs.

           Various versions of Tcl are available as ports for
           FreeBSD. The latest version, Tcl 8.5, can be found in
           lang/tcl85.

     --------------------------------------------------------------

  2.3.3 Compilers

   Compilers are rather different. First of all, you write your code
   in a file (or files) using an editor. You then run the compiler
   and see if it accepts your program. If it did not compile, grit
   your teeth and go back to the editor; if it did compile and gave
   you a program, you can run it either at a shell command prompt or
   in a debugger to see if it works properly. [1]

   Obviously, this is not quite as direct as using an interpreter.
   However it allows you to do a lot of things which are very
   difficult or even impossible with an interpreter, such as writing
   code which interacts closely with the operating system-or even
   writing your own operating system! It is also useful if you need
   to write very efficient code, as the compiler can take its time
   and optimize the code, which would not be acceptable in an
   interpreter. Moreover, distributing a program written for a
   compiler is usually more straightforward than one written for an
   interpreter-you can just give them a copy of the executable,
   assuming they have the same operating system as you.

   As the edit-compile-run-debug cycle is rather tedious when using
   separate programs, many commercial compiler makers have produced
   Integrated Development Environments (IDEs for short). FreeBSD does
   not include an IDE in the base system, but devel/kdevelop is
   available in the Ports Collection and many use Emacs for this
   purpose. Using Emacs as an IDE is discussed in Section 2.7.

     --------------------------------------------------------------

2.4 Compiling with cc

   This section deals with the gcc and clang compilers for C and C++,
   since they come with the FreeBSD base system. Starting with
   FreeBSD 10.X clang is installed as cc. The details of producing a
   program with an interpreter vary considerably between
   interpreters, and are usually well covered in the documentation
   and on-line help for the interpreter.

   Once you have written your masterpiece, the next step is to
   convert it into something that will (hopefully!) run on FreeBSD.
   This usually involves several steps, each of which is done by a
   separate program.

    1. Pre-process your source code to remove comments and do other
       tricks like expanding macros in C.

    2. Check the syntax of your code to see if you have obeyed the
       rules of the language. If you have not, it will complain!

    3. Convert the source code into assembly language-this is very
       close to machine code, but still understandable by humans.
       Allegedly.

    4. Convert the assembly language into machine code-yep, we are
       talking bits and bytes, ones and zeros here.

    5. Check that you have used things like functions and global
       variables in a consistent way. For example, if you have called
       a non-existent function, it will complain.

    6. If you are trying to produce an executable from several source
       code files, work out how to fit them all together.

    7. Work out how to produce something that the system's run-time
       loader will be able to load into memory and run.

    8. Finally, write the executable on the filesystem.

   The word compiling is often used to refer to just steps 1 to 4-the
   others are referred to as linking. Sometimes step 1 is referred to
   as pre-processing and steps 3-4 as assembling.

   Fortunately, almost all this detail is hidden from you, as cc is a
   front end that manages calling all these programs with the right
   arguments for you; simply typing

 % cc foobar.c

   will cause foobar.c to be compiled by all the steps above. If you
   have more than one file to compile, just do something like

 % cc foo.c bar.c

   Note that the syntax checking is just that-checking the syntax. It
   will not check for any logical mistakes you may have made, like
   putting the program into an infinite loop, or using a bubble sort
   when you meant to use a binary sort. [2]

   There are lots and lots of options for cc, which are all in the
   manual page. Here are a few of the most important ones, with
   examples of how to use them.

   -o filename

           The output name of the file. If you do not use this
           option, cc will produce an executable called a.out. [3]

 % cc foobar.c               executable is a.out
 % cc -o foobar foobar.c     executable is foobar
            

   -c

           Just compile the file, do not link it. Useful for toy
           programs where you just want to check the syntax, or if
           you are using a Makefile.

 % cc -c foobar.c
            

           This will produce an object file (not an executable)
           called foobar.o. This can be linked together with other
           object files into an executable.

   -g

           Create a debug version of the executable. This makes the
           compiler put information into the executable about which
           line of which source file corresponds to which function
           call. A debugger can use this information to show the
           source code as you step through the program, which is very
           useful; the disadvantage is that all this extra
           information makes the program much bigger. Normally, you
           compile with -g while you are developing a program and
           then compile a "release version" without -g when you are
           satisfied it works properly.

 % cc -g foobar.c
            

           This will produce a debug version of the program. [4]

   -O

           Create an optimized version of the executable. The
           compiler performs various clever tricks to try to produce
           an executable that runs faster than normal. You can add a
           number after the -O to specify a higher level of
           optimization, but this often exposes bugs in the
           compiler's optimizer.

 % cc -O -o foobar foobar.c
            

           This will produce an optimized version of foobar.

   The following three flags will force cc to check that your code
   complies to the relevant international standard, often referred to
   as the ANSI standard, though strictly speaking it is an ISO
   standard.

   -Wall

           Enable all the warnings which the authors of cc believe
           are worthwhile. Despite the name, it will not enable all
           the warnings cc is capable of.

   -ansi

           Turn off most, but not all, of the non-ANSI C features
           provided by cc. Despite the name, it does not guarantee
           strictly that your code will comply to the standard.

   -pedantic

           Turn off all cc's non-ANSI C features.

   Without these flags, cc will allow you to use some of its
   non-standard extensions to the standard. Some of these are very
   useful, but will not work with other compilers-in fact, one of the
   main aims of the standard is to allow people to write code that
   will work with any compiler on any system. This is known as
   portable code.

   Generally, you should try to make your code as portable as
   possible, as otherwise you may have to completely rewrite the
   program later to get it to work somewhere else-and who knows what
   you may be using in a few years time?

 % cc -Wall -ansi -pedantic -o foobar foobar.c

   This will produce an executable foobar after checking foobar.c for
   standard compliance.

   -llibrary

           Specify a function library to be used at link time.

           The most common example of this is when compiling a
           program that uses some of the mathematical functions in C.
           Unlike most other platforms, these are in a separate
           library from the standard C one and you have to tell the
           compiler to add it.

           The rule is that if the library is called libsomething.a,
           you give cc the argument -lsomething. For example, the
           math library is libm.a, so you give cc the argument -lm. A
           common "gotcha" with the math library is that it has to be
           the last library on the command line.

 % cc -o foobar foobar.c -lm
            

           This will link the math library functions into foobar.

           If you are compiling C++ code, you need to add -lg++, or
           -lstdc++ if you are using FreeBSD 2.2 or later, to the
           command line argument to link the C++ library functions.
           Alternatively, you can run c++ instead of cc, which does
           this for you. c++ can also be invoked as g++ on FreeBSD.

 % cc -o foobar foobar.cc -lg++     For FreeBSD 2.1.6 and earlier
 % cc -o foobar foobar.cc -lstdc++  For FreeBSD 2.2 and later
 % c++ -o foobar foobar.cc
            

           Each of these will both produce an executable foobar from
           the C++ source file foobar.cc. Note that, on UNIX systems,
           C++ source files traditionally end in .C, .cxx or .cc,
           rather than the MS-DOS(R) style .cpp (which was already
           used for something else). gcc used to rely on this to work
           out what kind of compiler to use on the source file;
           however, this restriction no longer applies, so you may
           now call your C++ files .cpp with impunity!

     --------------------------------------------------------------

  2.4.1 Common cc Queries and Problems

   2.4.1.1. I am trying to write a program which uses the sin()
   function and I get an error like this. What does it mean?

   2.4.1.2. All right, I wrote this simple program to practice using
   -lm. All it does is raise 2.1 to the power of 6.

   2.4.1.3. So how do I fix this?

   2.4.1.4. I compiled a file called foobar.c and I cannot find an
   executable called foobar. Where has it gone?

   2.4.1.5. OK, I have an executable called foobar, I can see it when
   I run ls, but when I type in foobar at the command prompt it tells
   me there is no such file. Why can it not find it?

   2.4.1.6. I called my executable test, but nothing happens when I
   run it. What is going on?

   2.4.1.7. I compiled my program and it seemed to run all right at
   first, then there was an error and it said something about "core
   dumped". What does that mean?

   2.4.1.8. Fascinating stuff, but what I am supposed to do now?

   2.4.1.9. When my program dumped core, it said something about a
   "segmentation fault". What is that?

   2.4.1.10. Sometimes when I get a core dump it says "bus error". It
   says in my UNIX book that this means a hardware problem, but the
   computer still seems to be working. Is this true?

   2.4.1.11. This dumping core business sounds as though it could be
   quite useful, if I can make it happen when I want to. Can I do
   this, or do I have to wait until there is an error?

   2.4.1.1. I am trying to write a program which uses the sin()
   function and I get an error like this. What does it mean?

 /var/tmp/cc0143941.o: Undefined symbol `_sin' referenced from text segment
              

   When using mathematical functions like sin(), you have to tell cc
   to link in the math library, like so:

 % cc -o foobar foobar.c -lm
              

   2.4.1.2. All right, I wrote this simple program to practice using
   -lm. All it does is raise 2.1 to the power of 6.

 #include <stdio.h>

 int main() {
         float f;

         f = pow(2.1, 6);
         printf("2.1 ^ 6 = %f\n", f);
         return 0;
 }
              

   and I compiled it as:

 % cc temp.c -lm
              

   like you said I should, but I get this when I run it:

 % ./a.out
 2.1 ^ 6 = 1023.000000
              

   This is not the right answer! What is going on?

   When the compiler sees you call a function, it checks if it has
   already seen a prototype for it. If it has not, it assumes the
   function returns an int, which is definitely not what you want
   here.

   2.4.1.3. So how do I fix this?

   The prototypes for the mathematical functions are in math.h. If
   you include this file, the compiler will be able to find the
   prototype and it will stop doing strange things to your
   calculation!

 #include <math.h>
 #include <stdio.h>

 int main() {
 ...
              

   After recompiling it as you did before, run it:

 % ./a.out
 2.1 ^ 6 = 85.766121
              

   If you are using any of the mathematical functions, always include
   math.h and remember to link in the math library.

   2.4.1.4. I compiled a file called foobar.c and I cannot find an
   executable called foobar. Where has it gone?

   Remember, cc will call the executable a.out unless you tell it
   differently. Use the -o filename option:

 % cc -o foobar foobar.c
              

   2.4.1.5. OK, I have an executable called foobar, I can see it when
   I run ls, but when I type in foobar at the command prompt it tells
   me there is no such file. Why can it not find it?

   Unlike MS-DOS, UNIX does not look in the current directory when it
   is trying to find out which executable you want it to run, unless
   you tell it to. Either type ./foobar, which means "run the file
   called foobar in the current directory", or change your PATH
   environment variable so that it looks something like

 bin:/usr/bin:/usr/local/bin:.
              

   The dot at the end means "look in the current directory if it is
   not in any of the others".

   2.4.1.6. I called my executable test, but nothing happens when I
   run it. What is going on?

   Most UNIX systems have a program called test in /usr/bin and the
   shell is picking that one up before it gets to checking the
   current directory. Either type:

 % ./test
              

   or choose a better name for your program!

   2.4.1.7. I compiled my program and it seemed to run all right at
   first, then there was an error and it said something about "core
   dumped". What does that mean?

   The name core dump dates back to the very early days of UNIX, when
   the machines used core memory for storing data. Basically, if the
   program failed under certain conditions, the system would write
   the contents of core memory to disk in a file called core, which
   the programmer could then pore over to find out what went wrong.

   2.4.1.8. Fascinating stuff, but what I am supposed to do now?

   Use gdb to analyze the core (see Section 2.6).

   2.4.1.9. When my program dumped core, it said something about a
   "segmentation fault". What is that?

   This basically means that your program tried to perform some sort
   of illegal operation on memory; UNIX is designed to protect the
   operating system and other programs from rogue programs.

   Common causes for this are:

     * Trying to write to a NULL pointer, eg

 char *foo = NULL;
 strcpy(foo, "bang!");
                

     * Using a pointer that has not been initialized, eg

 char *foo;
 strcpy(foo, "bang!");
                

       The pointer will have some random value that, with luck, will
       point into an area of memory that is not available to your
       program and the kernel will kill your program before it can do
       any damage. If you are unlucky, it will point somewhere inside
       your own program and corrupt one of your data structures,
       causing the program to fail mysteriously.

     * Trying to access past the end of an array, eg

 int bar[20];
 bar[27] = 6;
                

     * Trying to store something in read-only memory, eg

 char *foo = "My string";
 strcpy(foo, "bang!");
                

       UNIX compilers often put string literals like "My string" into
       read-only areas of memory.

     * Doing naughty things with malloc() and free(), eg

 char bar[80];
 free(bar);
                

       or

 char *foo = malloc(27);
 free(foo);
 free(foo);
                

   Making one of these mistakes will not always lead to an error, but
   they are always bad practice. Some systems and compilers are more
   tolerant than others, which is why programs that ran well on one
   system can crash when you try them on an another.

   2.4.1.10. Sometimes when I get a core dump it says "bus error". It
   says in my UNIX book that this means a hardware problem, but the
   computer still seems to be working. Is this true?

   No, fortunately not (unless of course you really do have a
   hardware problem...). This is usually another way of saying that
   you accessed memory in a way you should not have.

   2.4.1.11. This dumping core business sounds as though it could be
   quite useful, if I can make it happen when I want to. Can I do
   this, or do I have to wait until there is an error?

   Yes, just go to another console or xterm, do

 % ps
            

   to find out the process ID of your program, and do

 % kill -ABRT pid
            

   where pid is the process ID you looked up.

   This is useful if your program has got stuck in an infinite loop,
   for instance. If your program happens to trap SIGABRT, there are
   several other signals which have a similar effect.

   Alternatively, you can create a core dump from inside your
   program, by calling the abort() function. See the manual page of
   abort(3) to learn more.

   If you want to create a core dump from outside your program, but
   do not want the process to terminate, you can use the gcore
   program. See the manual page of gcore(1) for more information.

     --------------------------------------------------------------

2.5 Make

  2.5.1 What is make?

   When you are working on a simple program with only one or two
   source files, typing in

 % cc file1.c file2.c

   is not too bad, but it quickly becomes very tedious when there are
   several files-and it can take a while to compile, too.

   One way to get around this is to use object files and only
   recompile the source file if the source code has changed. So we
   could have something like:

 % cc file1.o file2.o ... file37.c ...

   if we had changed file37.c, but not any of the others, since the
   last time we compiled. This may speed up the compilation quite a
   bit, but does not solve the typing problem.

   Or we could write a shell script to solve the typing problem, but
   it would have to re-compile everything, making it very inefficient
   on a large project.

   What happens if we have hundreds of source files lying about? What
   if we are working in a team with other people who forget to tell
   us when they have changed one of their source files that we use?

   Perhaps we could put the two solutions together and write
   something like a shell script that would contain some kind of
   magic rule saying when a source file needs compiling. Now all we
   need now is a program that can understand these rules, as it is a
   bit too complicated for the shell.

   This program is called make. It reads in a file, called a
   makefile, that tells it how different files depend on each other,
   and works out which files need to be re-compiled and which ones do
   not. For example, a rule could say something like "if fromboz.o is
   older than fromboz.c, that means someone must have changed
   fromboz.c, so it needs to be re-compiled." The makefile also has
   rules telling make how to re-compile the source file, making it a
   much more powerful tool.

   Makefiles are typically kept in the same directory as the source
   they apply to, and can be called makefile, Makefile or MAKEFILE.
   Most programmers use the name Makefile, as this puts it near the
   top of a directory listing, where it can easily be seen. [5]

     --------------------------------------------------------------

  2.5.2 Example of using make

   Here is a very simple make file:

 foo: foo.c
         cc -o foo foo.c

   It consists of two lines, a dependency line and a creation line.

   The dependency line here consists of the name of the program
   (known as the target), followed by a colon, then whitespace, then
   the name of the source file. When make reads this line, it looks
   to see if foo exists; if it exists, it compares the time foo was
   last modified to the time foo.c was last modified. If foo does not
   exist, or is older than foo.c, it then looks at the creation line
   to find out what to do. In other words, this is the rule for
   working out when foo.c needs to be re-compiled.

   The creation line starts with a tab (press the tab key) and then
   the command you would type to create foo if you were doing it at a
   command prompt. If foo is out of date, or does not exist, make
   then executes this command to create it. In other words, this is
   the rule which tells make how to re-compile foo.c.

   So, when you type make, it will make sure that foo is up to date
   with respect to your latest changes to foo.c. This principle can
   be extended to Makefiles with hundreds of targets-in fact, on
   FreeBSD, it is possible to compile the entire operating system
   just by typing make world in the appropriate directory!

   Another useful property of makefiles is that the targets do not
   have to be programs. For instance, we could have a make file that
   looks like this:

 foo: foo.c
         cc -o foo foo.c

 install:
         cp foo /home/me

   We can tell make which target we want to make by typing:

 % make target

   make will then only look at that target and ignore any others. For
   example, if we type make foo with the makefile above, make will
   ignore the install target.

   If we just type make on its own, make will always look at the
   first target and then stop without looking at any others. So if we
   typed make here, it will just go to the foo target, re-compile foo
   if necessary, and then stop without going on to the install
   target.

   Notice that the install target does not actually depend on
   anything! This means that the command on the following line is
   always executed when we try to make that target by typing make
   install. In this case, it will copy foo into the user's home
   directory. This is often used by application makefiles, so that
   the application can be installed in the correct directory when it
   has been correctly compiled.

   This is a slightly confusing subject to try to explain. If you do
   not quite understand how make works, the best thing to do is to
   write a simple program like "hello world" and a make file like the
   one above and experiment. Then progress to using more than one
   source file, or having the source file include a header file. The
   touch command is very useful here-it changes the date on a file
   without you having to edit it.

     --------------------------------------------------------------

  2.5.3 Make and include-files

   C code often starts with a list of files to include, for example
   stdio.h. Some of these files are system-include files, some of
   them are from the project you are now working on:

 #include <stdio.h>
 #include "foo.h"

 int main(....

   To make sure that this file is recompiled the moment foo.h is
   changed, you have to add it in your Makefile:

 foo: foo.c foo.h

   The moment your project is getting bigger and you have more and
   more own include-files to maintain, it will be a pain to keep
   track of all include files and the files which are depending on
   it. If you change an include-file but forget to recompile all the
   files which are depending on it, the results will be devastating.
   gcc has an option to analyze your files and to produce a list of
   include-files and their dependencies: -MM.

   If you add this to your Makefile:

 depend:
         gcc -E -MM *.c > .depend

   and run make depend, the file .depend will appear with a list of
   object-files, C-files and the include-files:

 foo.o: foo.c foo.h

   If you change foo.h, next time you run make all files depending on
   foo.h will be recompiled.

   Do not forget to run make depend each time you add an include-file
   to one of your files.

     --------------------------------------------------------------

  2.5.4 FreeBSD Makefiles

   Makefiles can be rather complicated to write. Fortunately,
   BSD-based systems like FreeBSD come with some very powerful ones
   as part of the system. One very good example of this is the
   FreeBSD ports system. Here is the essential part of a typical
   ports Makefile:

 MASTER_SITES=   ftp://freefall.cdrom.com/pub/FreeBSD/LOCAL_PORTS/
 DISTFILES=      scheme-microcode+dist-7.3-freebsd.tgz

 .include <bsd.port.mk>

   Now, if we go to the directory for this port and type make, the
   following happens:

    1. A check is made to see if the source code for this port is
       already on the system.

    2. If it is not, an FTP connection to the URL in MASTER_SITES is
       set up to download the source.

    3. The checksum for the source is calculated and compared it with
       one for a known, good, copy of the source. This is to make
       sure that the source was not corrupted while in transit.

    4. Any changes required to make the source work on FreeBSD are
       applied-this is known as patching.

    5. Any special configuration needed for the source is done. (Many
       UNIX program distributions try to work out which version of
       UNIX they are being compiled on and which optional UNIX
       features are present-this is where they are given the
       information in the FreeBSD ports scenario).

    6. The source code for the program is compiled. In effect, we
       change to the directory where the source was unpacked and do
       make-the program's own make file has the necessary information
       to build the program.

    7. We now have a compiled version of the program. If we wish, we
       can test it now; when we feel confident about the program, we
       can type make install. This will cause the program and any
       supporting files it needs to be copied into the correct
       location; an entry is also made into a package database, so
       that the port can easily be uninstalled later if we change our
       mind about it.

   Now I think you will agree that is rather impressive for a four
   line script!

   The secret lies in the last line, which tells make to look in the
   system makefile called bsd.port.mk. It is easy to overlook this
   line, but this is where all the clever stuff comes from-someone
   has written a makefile that tells make to do all the things above
   (plus a couple of other things I did not mention, including
   handling any errors that may occur) and anyone can get access to
   that just by putting a single line in their own make file!

   If you want to have a look at these system makefiles, they are in
   /usr/share/mk, but it is probably best to wait until you have had
   a bit of practice with makefiles, as they are very complicated
   (and if you do look at them, make sure you have a flask of strong
   coffee handy!)

     --------------------------------------------------------------

  2.5.5 More advanced uses of make

   Make is a very powerful tool, and can do much more than the simple
   example above shows. Unfortunately, there are several different
   versions of make, and they all differ considerably. The best way
   to learn what they can do is probably to read the
   documentation-hopefully this introduction will have given you a
   base from which you can do this.

   The version of make that comes with FreeBSD is the Berkeley make;
   there is a tutorial for it in /usr/share/doc/psd/12.make. To view
   it, do

 % zmore paper.ascii.gz

   in that directory.

   Many applications in the ports use GNU make, which has a very good
   set of "info" pages. If you have installed any of these ports, GNU
   make will automatically have been installed as gmake. It is also
   available as a port and package in its own right.

   To view the info pages for GNU make, you will have to edit the dir
   file in the /usr/local/info directory to add an entry for it. This
   involves adding a line like

  * Make: (make).                 The GNU Make utility.

   to the file. Once you have done this, you can type info and then
   select make from the menu (or in Emacs, do C-h i).

     --------------------------------------------------------------

2.6 Debugging

  2.6.1 The Debugger

   The debugger that comes with FreeBSD is called gdb (GNU debugger).
   You start it up by typing

 % gdb progname

   although many people prefer to run it inside Emacs. You can do
   this by:

 M-x gdb RET progname RET

   Using a debugger allows you to run the program under more
   controlled circumstances. Typically, you can step through the
   program a line at a time, inspect the value of variables, change
   them, tell the debugger to run up to a certain point and then
   stop, and so on. You can even attach to a program that is already
   running, or load a core file to investigate why the program
   crashed. It is even possible to debug the kernel, though that is a
   little trickier than the user applications we will be discussing
   in this section.

   gdb has quite good on-line help, as well as a set of info pages,
   so this section will concentrate on a few of the basic commands.

   Finally, if you find its text-based command-prompt style
   off-putting, there is a graphical front-end for it (devel/xxgdb)
   in the Ports Collection.

   This section is intended to be an introduction to using gdb and
   does not cover specialized topics such as debugging the kernel.

     --------------------------------------------------------------

  2.6.2 Running a program in the debugger

   You will need to have compiled the program with the -g option to
   get the most out of using gdb. It will work without, but you will
   only see the name of the function you are in, instead of the
   source code. If you see a line like:

 ... (no debugging symbols found) ...

   when gdb starts up, you will know that the program was not
   compiled with the -g option.

   At the gdb prompt, type break main. This will tell the debugger
   that you are not interested in watching the preliminary set-up
   code in the program being run, and that it should stop execution
   at the beginning of your code. Now type run to start the
   program-it will start at the beginning of the set-up code and then
   get stopped by the debugger when it calls main(). (If you have
   ever wondered where main() gets called from, now you know!).

   You can now step through the program, a line at a time, by
   pressing n. If you get to a function call, you can step into it by
   pressing s. Once you are in a function call, you can return from
   stepping into a function call by pressing f. You can also use up
   and down to take a quick look at the caller.

   Here is a simple example of how to spot a mistake in a program
   with gdb. This is our program (with a deliberate mistake):

 #include <stdio.h>

 int bazz(int anint);

 main() {
         int i;

         printf("This is my program\n");
         bazz(i);
         return 0;
 }

 int bazz(int anint) {
         printf("You gave me %d\n", anint);
         return anint;
 }

   This program sets i to be 5 and passes it to a function bazz()
   which prints out the number we gave it.

   When we compile and run the program we get

 % cc -g -o temp temp.c
 % ./temp
 This is my program
 anint = 4231

   That was not what we expected! Time to see what is going on!

 % gdb temp
 GDB is free software and you are welcome to distribute copies of it
  under certain conditions; type "show copying" to see the conditions.
 There is absolutely no warranty for GDB; type "show warranty" for details.
 GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
 (gdb) break main                                Skip the set-up code
 Breakpoint 1 at 0x160f: file temp.c, line 9.    gdb puts breakpoint at main()
 (gdb) run                                       Run as far as main()
 Starting program: /home/james/tmp/temp          Program starts running

 Breakpoint 1, main () at temp.c:9               gdb stops at main()
 (gdb) n                                         Go to next line
 This is my program                              Program prints out
 (gdb) s                                         step into bazz()
 bazz (anint=4231) at temp.c:17                  gdb displays stack frame
 (gdb)

   Hang on a minute! How did anint get to be 4231? Did we not we set
   it to be 5 in main()? Let's move up to main() and have a look.

 (gdb) up                                        Move up call stack
 #1  0x1625 in main () at temp.c:11              gdb displays stack frame
 (gdb) p i                                       Show us the value of i
 $1 = 4231                                       gdb displays 4231

   Oh dear! Looking at the code, we forgot to initialize i. We meant
   to put

 ...
 main() {
         int i;

         i = 5;
         printf("This is my program\n");
 ...

   but we left the i=5; line out. As we did not initialize i, it had
   whatever number happened to be in that area of memory when the
   program ran, which in this case happened to be 4231.

     Note: gdb displays the stack frame every time we go into or out
     of a function, even if we are using up and down to move around
     the call stack. This shows the name of the function and the
     values of its arguments, which helps us keep track of where we
     are and what is going on. (The stack is a storage area where the
     program stores information about the arguments passed to
     functions and where to go when it returns from a function call).

     --------------------------------------------------------------

  2.6.3 Examining a core file

   A core file is basically a file which contains the complete state
   of the process when it crashed. In "the good old days",
   programmers had to print out hex listings of core files and sweat
   over machine code manuals, but now life is a bit easier.
   Incidentally, under FreeBSD and other 4.4BSD systems, a core file
   is called progname.core instead of just core, to make it clearer
   which program a core file belongs to.

   To examine a core file, start up gdb in the usual way. Instead of
   typing break or run, type

 (gdb) core progname.core

   If you are not in the same directory as the core file, you will
   have to do dir /path/to/core/file first.

   You should see something like this:

 % gdb a.out
 GDB is free software and you are welcome to distribute copies of it
  under certain conditions; type "show copying" to see the conditions.
 There is absolutely no warranty for GDB; type "show warranty" for details.
 GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
 (gdb) core a.out.core
 Core was generated by `a.out'.
 Program terminated with signal 11, Segmentation fault.
 Cannot access memory at address 0x7020796d.
 #0  0x164a in bazz (anint=0x5) at temp.c:17
 (gdb)

   In this case, the program was called a.out, so the core file is
   called a.out.core. We can see that the program crashed due to
   trying to access an area in memory that was not available to it in
   a function called bazz.

   Sometimes it is useful to be able to see how a function was
   called, as the problem could have occurred a long way up the call
   stack in a complex program. The bt command causes gdb to print out
   a back-trace of the call stack:

 (gdb) bt
 #0  0x164a in bazz (anint=0x5) at temp.c:17
 #1  0xefbfd888 in end ()
 #2  0x162c in main () at temp.c:11
 (gdb)

   The end() function is called when a program crashes; in this case,
   the bazz() function was called from main().

     --------------------------------------------------------------

  2.6.4 Attaching to a running program

   One of the neatest features about gdb is that it can attach to a
   program that is already running. Of course, that assumes you have
   sufficient permissions to do so. A common problem is when you are
   stepping through a program that forks, and you want to trace the
   child, but the debugger will only let you trace the parent.

   What you do is start up another gdb, use ps to find the process ID
   for the child, and do

 (gdb) attach pid

   in gdb, and then debug as usual.

   "That is all very well," you are probably thinking, "but by the
   time I have done that, the child process will be over the hill and
   far away". Fear not, gentle reader, here is how to do it (courtesy
   of the gdb info pages):

 ...
 if ((pid = fork()) < 0)         /* _Always_ check this */
         error();
 else if (pid == 0) {            /* child */
         int PauseMode = 1;

         while (PauseMode)
                 sleep(10);      /* Wait until someone attaches to us */
         ...
 } else {                        /* parent */
         ...

   Now all you have to do is attach to the child, set PauseMode to 0,
   and wait for the sleep() call to return!

     --------------------------------------------------------------

2.7 Using Emacs as a Development Environment

  2.7.1 Emacs

   Emacs is a highly customizable editor-indeed, it has been
   customized to the point where it is more like an operating system
   than an editor! Many developers and sysadmins do in fact spend
   practically all their time working inside Emacs, leaving it only
   to log out.

   It is impossible even to summarize everything Emacs can do here,
   but here are some of the features of interest to developers:

     * Very powerful editor, allowing search-and-replace on both
       strings and regular expressions (patterns), jumping to
       start/end of block expression, etc, etc.

     * Pull-down menus and online help.

     * Language-dependent syntax highlighting and indentation.

     * Completely customizable.

     * You can compile and debug programs within Emacs.

     * On a compilation error, you can jump to the offending line of
       source code.

     * Friendly-ish front-end to the info program used for reading
       GNU hypertext documentation, including the documentation on
       Emacs itself.

     * Friendly front-end to gdb, allowing you to look at the source
       code as you step through your program.

   And doubtless many more that have been overlooked.

   Emacs can be installed on FreeBSD using the editors/emacs port.

   Once it is installed, start it up and do C-h t to read an Emacs
   tutorial-that means hold down the control key, press h, let go of
   the control key, and then press t. (Alternatively, you can use the
   mouse to select Emacs Tutorial from the Help menu.)

   Although Emacs does have menus, it is well worth learning the key
   bindings, as it is much quicker when you are editing something to
   press a couple of keys than to try to find the mouse and then
   click on the right place. And, when you are talking to seasoned
   Emacs users, you will find they often casually throw around
   expressions like "M-x replace-s RET foo RET bar RET" so it is
   useful to know what they mean. And in any case, Emacs has far too
   many useful functions for them to all fit on the menu bars.

   Fortunately, it is quite easy to pick up the key-bindings, as they
   are displayed next to the menu item. My advice is to use the menu
   item for, say, opening a file until you understand how it works
   and feel confident with it, then try doing C-x C-f. When you are
   happy with that, move on to another menu command.

   If you can not remember what a particular combination of keys
   does, select Describe Key from the Help menu and type it in-Emacs
   will tell you what it does. You can also use the Command Apropos
   menu item to find out all the commands which contain a particular
   word in them, with the key binding next to it.

   By the way, the expression above means hold down the Meta key,
   press x, release the Meta key, type replace-s (short for
   replace-string-another feature of Emacs is that you can abbreviate
   commands), press the return key, type foo (the string you want
   replaced), press the return key, type bar (the string you want to
   replace foo with) and press return again. Emacs will then do the
   search-and-replace operation you have just requested.

   If you are wondering what on earth the Meta key is, it is a
   special key that many UNIX workstations have. Unfortunately, PC's
   do not have one, so it is usually the alt key (or if you are
   unlucky, the escape key).

   Oh, and to get out of Emacs, do C-x C-c (that means hold down the
   control key, press x, press c and release the control key). If you
   have any unsaved files open, Emacs will ask you if you want to
   save them. (Ignore the bit in the documentation where it says C-z
   is the usual way to leave Emacs-that leaves Emacs hanging around
   in the background, and is only really useful if you are on a
   system which does not have virtual terminals).

     --------------------------------------------------------------

  2.7.2 Configuring Emacs

   Emacs does many wonderful things; some of them are built in, some
   of them need to be configured.

   Instead of using a proprietary macro language for configuration,
   Emacs uses a version of Lisp specially adapted for editors, known
   as Emacs Lisp. Working with Emacs Lisp can be quite helpful if you
   want to go on and learn something like Common Lisp. Emacs Lisp has
   many features of Common Lisp, although it is considerably smaller
   (and thus easier to master).

   The best way to learn Emacs Lisp is to download the Emacs Tutorial

   However, there is no need to actually know any Lisp to get started
   with configuring Emacs, as I have included a sample .emacs file,
   which should be enough to get you started. Just copy it into your
   home directory and restart Emacs if it is already running; it will
   read the commands from the file and (hopefully) give you a useful
   basic setup.

     --------------------------------------------------------------

  2.7.3 A sample .emacs file

   Unfortunately, there is far too much here to explain it in detail;
   however there are one or two points worth mentioning.

     * Everything beginning with a ; is a comment and is ignored by
       Emacs.

     * In the first line, the -*- Emacs-Lisp -*- is so that we can
       edit the .emacs file itself within Emacs and get all the fancy
       features for editing Emacs Lisp. Emacs usually tries to guess
       this based on the filename, and may not get it right for
       .emacs.

     * The tab key is bound to an indentation function in some modes,
       so when you press the tab key, it will indent the current line
       of code. If you want to put a tab character in whatever you
       are writing, hold the control key down while you are pressing
       the tab key.

     * This file supports syntax highlighting for C, C++, Perl, Lisp
       and Scheme, by guessing the language from the filename.

     * Emacs already has a pre-defined function called next-error. In
       a compilation output window, this allows you to move from one
       compilation error to the next by doing M-n; we define a
       complementary function, previous-error, that allows you to go
       to a previous error by doing M-p. The nicest feature of all is
       that C-c C-c will open up the source file in which the error
       occurred and jump to the appropriate line.

     * We enable Emacs's ability to act as a server, so that if you
       are doing something outside Emacs and you want to edit a file,
       you can just type in

 % emacsclient filename
          

       and then you can edit the file in your Emacs! [6]

   Example 2-1. A sample .emacs file

 ;; -*-Emacs-Lisp-*-

 ;; This file is designed to be re-evaled; use the variable first-time
 ;; to avoid any problems with this.
 (defvar first-time t
   "Flag signifying this is the first time that .emacs has been evaled")

 ;; Meta
 (global-set-key "\M- " 'set-mark-command)
 (global-set-key "\M-\C-h" 'backward-kill-word)
 (global-set-key "\M-\C-r" 'query-replace)
 (global-set-key "\M-r" 'replace-string)
 (global-set-key "\M-g" 'goto-line)
 (global-set-key "\M-h" 'help-command)

 ;; Function keys
 (global-set-key [f1] 'manual-entry)
 (global-set-key [f2] 'info)
 (global-set-key [f3] 'repeat-complex-command)
 (global-set-key [f4] 'advertised-undo)
 (global-set-key [f5] 'eval-current-buffer)
 (global-set-key [f6] 'buffer-menu)
 (global-set-key [f7] 'other-window)
 (global-set-key [f8] 'find-file)
 (global-set-key [f9] 'save-buffer)
 (global-set-key [f10] 'next-error)
 (global-set-key [f11] 'compile)
 (global-set-key [f12] 'grep)
 (global-set-key [C-f1] 'compile)
 (global-set-key [C-f2] 'grep)
 (global-set-key [C-f3] 'next-error)
 (global-set-key [C-f4] 'previous-error)
 (global-set-key [C-f5] 'display-faces)
 (global-set-key [C-f8] 'dired)
 (global-set-key [C-f10] 'kill-compilation)

 ;; Keypad bindings
 (global-set-key [up] "\C-p")
 (global-set-key [down] "\C-n")
 (global-set-key [left] "\C-b")
 (global-set-key [right] "\C-f")
 (global-set-key [home] "\C-a")
 (global-set-key [end] "\C-e")
 (global-set-key [prior] "\M-v")
 (global-set-key [next] "\C-v")
 (global-set-key [C-up] "\M-\C-b")
 (global-set-key [C-down] "\M-\C-f")
 (global-set-key [C-left] "\M-b")
 (global-set-key [C-right] "\M-f")
 (global-set-key [C-home] "\M-<")
 (global-set-key [C-end] "\M->")
 (global-set-key [C-prior] "\M-<")
 (global-set-key [C-next] "\M->")

 ;; Mouse
 (global-set-key [mouse-3] 'imenu)

 ;; Misc
 (global-set-key [C-tab] "\C-q\t")       ; Control tab quotes a tab.
 (setq backup-by-copying-when-mismatch t)

 ;; Treat 'y' or <CR> as yes, 'n' as no.
 (fset 'yes-or-no-p 'y-or-n-p)
 (define-key query-replace-map [return] 'act)
 (define-key query-replace-map [?\C-m] 'act)

 ;; Load packages
 (require 'desktop)
 (require 'tar-mode)

 ;; Pretty diff mode
 (autoload 'ediff-buffers "ediff" "Intelligent Emacs interface to diff" t)
 (autoload 'ediff-files "ediff" "Intelligent Emacs interface to diff" t)
 (autoload 'ediff-files-remote "ediff"
   "Intelligent Emacs interface to diff")

 (if first-time
     (setq auto-mode-alist
           (append '(("\\.cpp$" . c++-mode)
                     ("\\.hpp$" . c++-mode)
                     ("\\.lsp$" . lisp-mode)
                     ("\\.scm$" . scheme-mode)
                     ("\\.pl$" . perl-mode)
                     ) auto-mode-alist)))

 ;; Auto font lock mode
 (defvar font-lock-auto-mode-list
   (list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'lisp-mode 'perl-mode 'scheme-mode)
   "List of modes to always start in font-lock-mode")

 (defvar font-lock-mode-keyword-alist
   '((c++-c-mode . c-font-lock-keywords)
     (perl-mode . perl-font-lock-keywords))
   "Associations between modes and keywords")

 (defun font-lock-auto-mode-select ()
   "Automatically select font-lock-mode if the current major mode is in font-lock-auto-mode-list"
   (if (memq major-mode font-lock-auto-mode-list)
       (progn
         (font-lock-mode t))
     )
   )

 (global-set-key [M-f1] 'font-lock-fontify-buffer)

 ;; New dabbrev stuff
 ;(require 'new-dabbrev)
 (setq dabbrev-always-check-other-buffers t)
 (setq dabbrev-abbrev-char-regexp "\\sw\\|\\s_")
 (add-hook 'emacs-lisp-mode-hook
           '(lambda ()
              (set (make-local-variable 'dabbrev-case-fold-search) nil)
              (set (make-local-variable 'dabbrev-case-replace) nil)))
 (add-hook 'c-mode-hook
           '(lambda ()
              (set (make-local-variable 'dabbrev-case-fold-search) nil)
              (set (make-local-variable 'dabbrev-case-replace) nil)))
 (add-hook 'text-mode-hook
           '(lambda ()
              (set (make-local-variable 'dabbrev-case-fold-search) t)
              (set (make-local-variable 'dabbrev-case-replace) t)))

 ;; C++ and C mode...
 (defun my-c++-mode-hook ()
   (setq tab-width 4)
   (define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
   (define-key c++-mode-map "\C-ce" 'c-comment-edit)
   (setq c++-auto-hungry-initial-state 'none)
   (setq c++-delete-function 'backward-delete-char)
   (setq c++-tab-always-indent t)
   (setq c-indent-level 4)
   (setq c-continued-statement-offset 4)
   (setq c++-empty-arglist-indent 4))

 (defun my-c-mode-hook ()
   (setq tab-width 4)
   (define-key c-mode-map "\C-m" 'reindent-then-newline-and-indent)
   (define-key c-mode-map "\C-ce" 'c-comment-edit)
   (setq c-auto-hungry-initial-state 'none)
   (setq c-delete-function 'backward-delete-char)
   (setq c-tab-always-indent t)
 ;; BSD-ish indentation style
   (setq c-indent-level 4)
   (setq c-continued-statement-offset 4)
   (setq c-brace-offset -4)
   (setq c-argdecl-indent 0)
   (setq c-label-offset -4))

 ;; Perl mode
 (defun my-perl-mode-hook ()
   (setq tab-width 4)
   (define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
   (setq perl-indent-level 4)
   (setq perl-continued-statement-offset 4))

 ;; Scheme mode...
 (defun my-scheme-mode-hook ()
   (define-key scheme-mode-map "\C-m" 'reindent-then-newline-and-indent))

 ;; Emacs-Lisp mode...
 (defun my-lisp-mode-hook ()
   (define-key lisp-mode-map "\C-m" 'reindent-then-newline-and-indent)
   (define-key lisp-mode-map "\C-i" 'lisp-indent-line)
   (define-key lisp-mode-map "\C-j" 'eval-print-last-sexp))

 ;; Add all of the hooks...
 (add-hook 'c++-mode-hook 'my-c++-mode-hook)
 (add-hook 'c-mode-hook 'my-c-mode-hook)
 (add-hook 'scheme-mode-hook 'my-scheme-mode-hook)
 (add-hook 'emacs-lisp-mode-hook 'my-lisp-mode-hook)
 (add-hook 'lisp-mode-hook 'my-lisp-mode-hook)
 (add-hook 'perl-mode-hook 'my-perl-mode-hook)

 ;; Complement to next-error
 (defun previous-error (n)
   "Visit previous compilation error message and corresponding source code."
   (interactive "p")
   (next-error (- n)))

 ;; Misc...
 (transient-mark-mode 1)
 (setq mark-even-if-inactive t)
 (setq visible-bell nil)
 (setq next-line-add-newlines nil)
 (setq compile-command "make")
 (setq suggest-key-bindings nil)
 (put 'eval-expression 'disabled nil)
 (put 'narrow-to-region 'disabled nil)
 (put 'set-goal-column 'disabled nil)
 (if (>= emacs-major-version 21)
         (setq show-trailing-whitespace t))

 ;; Elisp archive searching
 (autoload 'format-lisp-code-directory "lispdir" nil t)
 (autoload 'lisp-dir-apropos "lispdir" nil t)
 (autoload 'lisp-dir-retrieve "lispdir" nil t)
 (autoload 'lisp-dir-verify "lispdir" nil t)

 ;; Font lock mode
 (defun my-make-face (face color &optional bold)
   "Create a face from a color and optionally make it bold"
   (make-face face)
   (copy-face 'default face)
   (set-face-foreground face color)
   (if bold (make-face-bold face))
   )

 (if (eq window-system 'x)
     (progn
       (my-make-face 'blue "blue")
       (my-make-face 'red "red")
       (my-make-face 'green "dark green")
       (setq font-lock-comment-face 'blue)
       (setq font-lock-string-face 'bold)
       (setq font-lock-type-face 'bold)
       (setq font-lock-keyword-face 'bold)
       (setq font-lock-function-name-face 'red)
       (setq font-lock-doc-string-face 'green)
       (add-hook 'find-file-hooks 'font-lock-auto-mode-select)

       (setq baud-rate 1000000)
       (global-set-key "\C-cmm" 'menu-bar-mode)
       (global-set-key "\C-cms" 'scroll-bar-mode)
       (global-set-key [backspace] 'backward-delete-char)
                                         ;      (global-set-key [delete] 'delete-char)
       (standard-display-european t)
       (load-library "iso-transl")))

 ;; X11 or PC using direct screen writes
 (if window-system
     (progn
       ;;      (global-set-key [M-f1] 'hilit-repaint-command)
       ;;      (global-set-key [M-f2] [?\C-u M-f1])
       (setq hilit-mode-enable-list
             '(not text-mode c-mode c++-mode emacs-lisp-mode lisp-mode
                   scheme-mode)
             hilit-auto-highlight nil
             hilit-auto-rehighlight 'visible
             hilit-inhibit-hooks nil
             hilit-inhibit-rebinding t)
       (require 'hilit19)
       (require 'paren))
   (setq baud-rate 2400)                 ; For slow serial connections
   )

 ;; TTY type terminal
 (if (and (not window-system)
          (not (equal system-type 'ms-dos)))
     (progn
       (if first-time
           (progn
             (keyboard-translate ?\C-h ?\C-?)
             (keyboard-translate ?\C-? ?\C-h)))))

 ;; Under UNIX
 (if (not (equal system-type 'ms-dos))
     (progn
       (if first-time
           (server-start))))

 ;; Add any face changes here
 (add-hook 'term-setup-hook 'my-term-setup-hook)
 (defun my-term-setup-hook ()
   (if (eq window-system 'pc)
       (progn
 ;;      (set-face-background 'default "red")
         )))

 ;; Restore the "desktop" - do this as late as possible
 (if first-time
     (progn
       (desktop-load-default)
       (desktop-read)))

 ;; Indicate that this file has been read at least once
 (setq first-time nil)

 ;; No need to debug anything now

 (setq debug-on-error nil)

 ;; All done
 (message "All done, %s%s" (user-login-name) ".")
        

     --------------------------------------------------------------

  2.7.4 Extending the Range of Languages Emacs Understands

   Now, this is all very well if you only want to program in the
   languages already catered for in the .emacs file (C, C++, Perl,
   Lisp and Scheme), but what happens if a new language called
   "whizbang" comes out, full of exciting features?

   The first thing to do is find out if whizbang comes with any files
   that tell Emacs about the language. These usually end in .el,
   short for "Emacs Lisp". For example, if whizbang is a FreeBSD
   port, we can locate these files by doing

 % find /usr/ports/lang/whizbang -name "*.el" -print

   and install them by copying them into the Emacs site Lisp
   directory. On FreeBSD, this is /usr/local/share/emacs/site-lisp.

   So for example, if the output from the find command was

 /usr/ports/lang/whizbang/work/misc/whizbang.el

   we would do

 # cp /usr/ports/lang/whizbang/work/misc/whizbang.el /usr/local/share/emacs/site-lisp

   Next, we need to decide what extension whizbang source files have.
   Let's say for the sake of argument that they all end in .wiz. We
   need to add an entry to our .emacs file to make sure Emacs will be
   able to use the information in whizbang.el.

   Find the auto-mode-alist entry in .emacs and add a line for
   whizbang, such as:

 ...
 ("\\.lsp$" . lisp-mode)
 ("\\.wiz$" . whizbang-mode)
 ("\\.scm$" . scheme-mode)
 ...

   This means that Emacs will automatically go into whizbang-mode
   when you edit a file ending in .wiz.

   Just below this, you will find the font-lock-auto-mode-list entry.
   Add whizbang-mode to it like so:

 ;; Auto font lock mode
 (defvar font-lock-auto-mode-list
   (list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'whizbang-mode 'lisp-mode 'perl-mode 'scheme-mode)
   "List of modes to always start in font-lock-mode")

   This means that Emacs will always enable font-lock-mode (ie syntax
   highlighting) when editing a .wiz file.

   And that is all that is needed. If there is anything else you want
   done automatically when you open up a .wiz file, you can add a
   whizbang-mode hook (see my-scheme-mode-hook for a simple example
   that adds auto-indent).

     --------------------------------------------------------------

2.8 Further Reading

   For information about setting up a development environment for
   contributing fixes to FreeBSD itself, please see development(7).

     * Brian Harvey and Matthew Wright Simply Scheme MIT 1994. ISBN
       0-262-08226-8

     * Randall Schwartz Learning Perl O'Reilly 1993 ISBN
       1-56592-042-2

     * Patrick Henry Winston and Berthold Klaus Paul Horn Lisp (3rd
       Edition) Addison-Wesley 1989 ISBN 0-201-08319-1

     * Brian W. Kernighan and Rob Pike The Unix Programming
       Environment Prentice-Hall 1984 ISBN 0-13-937681-X

     * Brian W. Kernighan and Dennis M. Ritchie The C Programming
       Language (2nd Edition) Prentice-Hall 1988 ISBN 0-13-110362-8

     * Bjarne Stroustrup The C++ Programming Language Addison-Wesley
       1991 ISBN 0-201-53992-6

     * W. Richard Stevens Advanced Programming in the Unix
       Environment Addison-Wesley 1992 ISBN 0-201-56317-7

     * W. Richard Stevens Unix Network Programming Prentice-Hall 1990
       ISBN 0-13-949876-1

     --------------------------------------------------------------

                      Chapter 3 Secure Programming

   Contributed by Murray Stokely.

3.1 Synopsis

   This chapter describes some of the security issues that have
   plagued UNIX programmers for decades and some of the new tools
   available to help programmers avoid writing exploitable code.

     --------------------------------------------------------------

3.2 Secure Design Methodology

   Writing secure applications takes a very scrutinous and
   pessimistic outlook on life. Applications should be run with the
   principle of "least privilege" so that no process is ever running
   with more than the bare minimum access that it needs to accomplish
   its function. Previously tested code should be reused whenever
   possible to avoid common mistakes that others may have already
   fixed.

   One of the pitfalls of the UNIX environment is how easy it is to
   make assumptions about the sanity of the environment. Applications
   should never trust user input (in all its forms), system
   resources, inter-process communication, or the timing of events.
   UNIX processes do not execute synchronously so logical operations
   are rarely atomic.

     --------------------------------------------------------------

3.3 Buffer Overflows

   Buffer Overflows have been around since the very beginnings of the
   Von-Neuman 1 architecture. They first gained widespread notoriety
   in 1988 with the Morris Internet worm. Unfortunately, the same
   basic attack remains effective today. By far the most common type
   of buffer overflow attack is based on corrupting the stack.

   Most modern computer systems use a stack to pass arguments to
   procedures and to store local variables. A stack is a last in
   first out (LIFO) buffer in the high memory area of a process
   image. When a program invokes a function a new "stack frame" is
   created. This stack frame consists of the arguments passed to the
   function as well as a dynamic amount of local variable space. The
   "stack pointer" is a register that holds the current location of
   the top of the stack. Since this value is constantly changing as
   new values are pushed onto the top of the stack, many
   implementations also provide a "frame pointer" that is located
   near the beginning of a stack frame so that local variables can
   more easily be addressed relative to this value. 1 The return
   address for function calls is also stored on the stack, and this
   is the cause of stack-overflow exploits since overflowing a local
   variable in a function can overwrite the return address of that
   function, potentially allowing a malicious user to execute any
   code he or she wants.

   Although stack-based attacks are by far the most common, it would
   also be possible to overrun the stack with a heap-based
   (malloc/free) attack.

   The C programming language does not perform automatic bounds
   checking on arrays or pointers as many other languages do. In
   addition, the standard C library is filled with a handful of very
   dangerous functions.

   strcpy(char *dest, const char *src)      May overflow the dest     
                                            buffer                    
   strcat(char *dest, const char *src)      May overflow the dest     
                                            buffer                    
   getwd(char *buf)                         May overflow the buf      
                                            buffer                    
   gets(char *s)                            May overflow the s buffer 
   [vf]scanf(const char *format, ...)       May overflow its          
                                            arguments.                
   realpath(char *path, char                May overflow the path     
   resolved_path[])                         buffer                    
   [v]sprintf(char *str, const char         May overflow the str      
   *format, ...)                            buffer.                   

     --------------------------------------------------------------

  3.3.1 Example Buffer Overflow

   The following example code contains a buffer overflow designed to
   overwrite the return address and skip the instruction immediately
   following the function call. (Inspired by 4)

 #include <stdio.h>

 void manipulate(char *buffer) {
   char newbuffer[80];
   strcpy(newbuffer,buffer);
 }

 int main() {
   char ch,buffer[4096];
   int i=0;

   while ((buffer[i++] = getchar()) != '\n') {};

   i=1;
   manipulate(buffer);
   i=2;
   printf("The value of i is : %d\n",i);
   return 0;
 }

   Let us examine what the memory image of this process would look
   like if we were to input 160 spaces into our little program before
   hitting return.

   [XXX figure here!]

   Obviously more malicious input can be devised to execute actual
   compiled instructions (such as exec(/bin/sh)).

     --------------------------------------------------------------

  3.3.2 Avoiding Buffer Overflows

   The most straightforward solution to the problem of
   stack-overflows is to always use length restricted memory and
   string copy functions. strncpy and strncat are part of the
   standard C library. These functions accept a length value as a
   parameter which should be no larger than the size of the
   destination buffer. These functions will then copy up to `length'
   bytes from the source to the destination. However there are a
   number of problems with these functions. Neither function
   guarantees NUL termination if the size of the input buffer is as
   large as the destination. The length parameter is also used
   inconsistently between strncpy and strncat so it is easy for
   programmers to get confused as to their proper usage. There is
   also a significant performance loss compared to strcpy when
   copying a short string into a large buffer since strncpy NUL fills
   up the size specified.

   In OpenBSD, another memory copy implementation has been created to
   get around these problem. The strlcpy and strlcat functions
   guarantee that they will always null terminate the destination
   string when given a non-zero length argument. For more information
   about these functions see 6. The OpenBSD strlcpy and strlcat
   instructions have been in FreeBSD since 3.3.

     --------------------------------------------------------------

    3.3.2.1 Compiler based run-time bounds checking

   Unfortunately there is still a very large assortment of code in
   public use which blindly copies memory around without using any of
   the bounded copy routines we just discussed. Fortunately, there is
   a way to help prevent such attacks - run-time bounds checking,
   which is implemented by several C/C++ compilers.

   ProPolice is one such compiler feature, and is integrated into
   gcc(1) versions 4.1 and later. It replaces and extends the earlier
   StackGuard gcc(1) extension.

   ProPolice helps to protect against stack-based buffer overflows
   and other attacks by laying pseudo-random numbers in key areas of
   the stack before calling any function. When a function returns,
   these "canaries" are checked and if they are found to have been
   changed the executable is immediately aborted. Thus any attempt to
   modify the return address or other variable stored on the stack in
   an attempt to get malicious code to run is unlikely to succeed, as
   the attacker would have to also manage to leave the pseudo-random
   canaries untouched.

   Recompiling your application with ProPolice is an effective means
   of stopping most buffer-overflow attacks, but it can still be
   compromised.

     --------------------------------------------------------------

    3.3.2.2 Library based run-time bounds checking

   Compiler-based mechanisms are completely useless for binary-only
   software for which you cannot recompile. For these situations
   there are a number of libraries which re-implement the unsafe
   functions of the C-library (strcpy, fscanf, getwd, etc..) and
   ensure that these functions can never write past the stack
   pointer.

     * libsafe

     * libverify

     * libparanoia

   Unfortunately these library-based defenses have a number of
   shortcomings. These libraries only protect against a very small
   set of security related issues and they neglect to fix the actual
   problem. These defenses may fail if the application was compiled
   with -fomit-frame-pointer. Also, the LD_PRELOAD and
   LD_LIBRARY_PATH environment variables can be overwritten/unset by
   the user.

     --------------------------------------------------------------

3.4 SetUID issues

   There are at least 6 different IDs associated with any given
   process. Because of this you have to be very careful with the
   access that your process has at any given time. In particular, all
   seteuid applications should give up their privileges as soon as it
   is no longer required.

   The real user ID can only be changed by a superuser process. The
   login program sets this when a user initially logs in and it is
   seldom changed.

   The effective user ID is set by the exec() functions if a program
   has its seteuid bit set. An application can call seteuid() at any
   time to set the effective user ID to either the real user ID or
   the saved set-user-ID. When the effective user ID is set by exec()
   functions, the previous value is saved in the saved set-user-ID.

     --------------------------------------------------------------

3.5 Limiting your program's environment

   The traditional method of restricting a process is with the
   chroot() system call. This system call changes the root directory
   from which all other paths are referenced for a process and any
   child processes. For this call to succeed the process must have
   execute (search) permission on the directory being referenced. The
   new environment does not actually take effect until you chdir()
   into your new environment. It should also be noted that a process
   can easily break out of a chroot environment if it has root
   privilege. This could be accomplished by creating device nodes to
   read kernel memory, attaching a debugger to a process outside of
   the chroot(8) environment, or in many other creative ways.

   The behavior of the chroot() system call can be controlled
   somewhat with the kern.chroot_allow_open_directories sysctl
   variable. When this value is set to 0, chroot() will fail with
   EPERM if there are any directories open. If set to the default
   value of 1, then chroot() will fail with EPERM if there are any
   directories open and the process is already subject to a chroot()
   call. For any other value, the check for open directories will be
   bypassed completely.

     --------------------------------------------------------------

  3.5.1 FreeBSD's jail functionality

   The concept of a Jail extends upon the chroot() by limiting the
   powers of the superuser to create a true `virtual server'. Once a
   prison is set up all network communication must take place through
   the specified IP address, and the power of "root privilege" in
   this jail is severely constrained.

   While in a prison, any tests of superuser power within the kernel
   using the suser() call will fail. However, some calls to suser()
   have been changed to a new interface suser_xxx(). This function is
   responsible for recognizing or denying access to superuser power
   for imprisoned processes.

   A superuser process within a jailed environment has the power to:

     * Manipulate credential with setuid, seteuid, setgid, setegid,
       setgroups, setreuid, setregid, setlogin

     * Set resource limits with setrlimit

     * Modify some sysctl nodes (kern.hostname)

     * chroot()

     * Set flags on a vnode: chflags, fchflags

     * Set attributes of a vnode such as file permission, owner,
       group, size, access time, and modification time.

     * Bind to privileged ports in the Internet domain (ports < 1024)

   Jail is a very useful tool for running applications in a secure
   environment but it does have some shortcomings. Currently, the IPC
   mechanisms have not been converted to the suser_xxx so
   applications such as MySQL cannot be run within a jail. Superuser
   access may have a very limited meaning within a jail, but there is
   no way to specify exactly what "very limited" means.

     --------------------------------------------------------------

  3.5.2 POSIX(R).1e Process Capabilities

   POSIX has released a working draft that adds event auditing,
   access control lists, fine grained privileges, information
   labeling, and mandatory access control.

   This is a work in progress and is the focus of the TrustedBSD
   project. Some of the initial work has been committed to
   FreeBSD-CURRENT (cap_set_proc(3)).

     --------------------------------------------------------------

3.6 Trust

   An application should never assume that anything about the users
   environment is sane. This includes (but is certainly not limited
   to): user input, signals, environment variables, resources, IPC,
   mmaps, the filesystem working directory, file descriptors, the #
   of open files, etc.

   You should never assume that you can catch all forms of invalid
   input that a user might supply. Instead, your application should
   use positive filtering to only allow a specific subset of inputs
   that you deem safe. Improper data validation has been the cause of
   many exploits, especially with CGI scripts on the world wide web.
   For filenames you need to be extra careful about paths ("../",
   "/"), symbolic links, and shell escape characters.

   Perl has a really cool feature called "Taint" mode which can be
   used to prevent scripts from using data derived outside the
   program in an unsafe way. This mode will check command line
   arguments, environment variables, locale information, the results
   of certain syscalls (readdir(), readlink(), getpwxxx()), and all
   file input.

     --------------------------------------------------------------

3.7 Race Conditions

   A race condition is anomalous behavior caused by the unexpected
   dependence on the relative timing of events. In other words, a
   programmer incorrectly assumed that a particular event would
   always happen before another.

   Some of the common causes of race conditions are signals, access
   checks, and file opens. Signals are asynchronous events by nature
   so special care must be taken in dealing with them. Checking
   access with access(2) then open(2) is clearly non-atomic. Users
   can move files in between the two calls. Instead, privileged
   applications should seteuid() and then call open() directly. Along
   the same lines, an application should always set a proper umask
   before open() to obviate the need for spurious chmod() calls.

     --------------------------------------------------------------

    Chapter 4 Localization and Internationalization - L10N and I18N

4.1 Programming I18N Compliant Applications

   To make your application more useful for speakers of other
   languages, we hope that you will program I18N compliant. The GNU
   gcc compiler and GUI libraries like QT and GTK support I18N
   through special handling of strings. Making a program I18N
   compliant is very easy. It allows contributors to port your
   application to other languages quickly. Refer to the library
   specific I18N documentation for more details.

   In contrast with common perception, I18N compliant code is easy to
   write. Usually, it only involves wrapping your strings with
   library specific functions. In addition, please be sure to allow
   for wide or multibyte character support.

     --------------------------------------------------------------

  4.1.1 A Call to Unify the I18N Effort

   It has come to our attention that the individual I18N/L10N efforts
   for each country has been repeating each others' efforts. Many of
   us have been reinventing the wheel repeatedly and inefficiently.
   We hope that the various major groups in I18N could congregate
   into a group effort similar to the Core Team's responsibility.

   Currently, we hope that, when you write or port I18N programs, you
   would send it out to each country's related FreeBSD mailing list
   for testing. In the future, we hope to create applications that
   work in all the languages out-of-the-box without dirty hacks.

   The FreeBSD internationalization mailing list has been
   established. If you are an I18N/L10N developer, please send your
   comments, ideas, questions, and anything you deem related to it.

     --------------------------------------------------------------

  4.1.2 Perl and Python

   Perl and Python have I18N and wide character handling libraries.
   Please use them for I18N compliance.

     --------------------------------------------------------------

4.2 Localized Messages with POSIX.1 Native Language Support (NLS)

   Contributed by Gabor Ko:vesdan.

   Beyond the basic I18N functions, like supporting various input
   encodings or supporting national conventions, such as the
   different decimal separators, at a higher level of I18N, it is
   possible to localize the messages written to the output by the
   various programs. A common way of doing this is using the POSIX.1
   NLS functions, which are provided as a part of the FreeBSD base
   system.

     --------------------------------------------------------------

  4.2.1 Organizing Localized Messages into Catalog Files

   POSIX.1 NLS is based on catalog files, which contain the localized
   messages in the desired encoding. The messages are organized into
   sets and each message is identified by an integer number in the
   containing set. The catalog files are conventionally named after
   the locale they contain localized messages for, followed by the
   .msg extension. For instance, the Hungarian messages for ISO8859-2
   encoding should be stored in a file called hu_HU.ISO8859-2.

   These catalog files are common text files that contain the
   numbered messages. It is possible to write comments by starting
   the line with a $ sign. Set boundaries are also separated by
   special comments, where the keyword set must directly follow the $
   sign. The set keyword is then followed by the set number. For
   example:

 $set 1

   The actual message entries start with the message number and
   followed by the localized message. The well-known modifiers from
   printf(3) are accepted:

 15 "File not found: %s\n"

   The language catalog files have to be compiled into a binary form
   before they can be opened from the program. This conversion is
   done with the gencat(1) utility. Its first argument is the
   filename of the compiled catalog and its further arguments are the
   input catalogs. The localized messages can also be organized into
   more catalog files and then all of them can be processed with
   gencat(1).

     --------------------------------------------------------------

  4.2.2 Using the Catalog Files from the Source Code

   Using the catalog files is simple. To use the related functions,
   nl_types.h must be included. Before using a catalog, it has to be
   opened with catopen(3). The function takes two arguments. The
   first parameter is the name of the installed and compiled catalog.
   Usually, the name of the program is used, such as grep. This name
   will be used when looking for the compiled catalog file. The
   catopen(3) call looks for this file in
   /usr/share/nls/locale/catname and in
   /usr/local/share/nls/locale/catname, where locale is the locale
   set and catname is the catalog name being discussed. The second
   parameter is a constant, which can have two values:

     * NL_CAT_LOCALE, which means that the used catalog file will be
       based on LC_MESSAGES.

     * 0, which means that LANG has to be used to open the proper
       catalog.

   The catopen(3) call returns a catalog identifier of type nl_catd.
   Please refer to the manual page for a list of possible returned
   error codes.

   After opening a catalog catgets(3) can be used to retrieve a
   message. The first parameter is the catalog identifier returned by
   catopen(3), the second one is the number of the set, the third one
   is the number of the messages, and the fourth one is a fallback
   message, which will be returned if the requested message cannot be
   retrieved from the catalog file.

   After using the catalog file, it must be closed by calling
   catclose(3), which has one argument, the catalog id.

     --------------------------------------------------------------

  4.2.3 A Practical Example

   The following example will demonstrate an easy solution on how to
   use NLS catalogs in a flexible way.

   The below lines need to be put into a common header file of the
   program, which is included into all source files where localized
   messages are necessary:

 #ifdef WITHOUT_NLS
 #define getstr(n)        nlsstr[n]
 #else
 #include <nl_types.h>

 extern nl_catd           catalog;
 #define getstr(n)        catgets(catalog, 1, n, nlsstr[n])
 #endif

 extern char             *nlsstr[];

   Next, put these lines into the global declaration part of the main
   source file:

 #ifndef WITHOUT_NLS
 #include <nl_types.h>
 nl_catd  catalog;
 #endif

 /*
  * Default messages to use when NLS is disabled or no catalog
  * is found.
  */
 char    *nlsstr[] = {
         "",
 /* 1*/  "some random message",
 /* 2*/  "some other message"
 };

   Next come the real code snippets, which open, read, and close the
   catalog:

 #ifndef WITHOUT_NLS
         catalog = catopen("myapp", NL_CAT_LOCALE);
 #endif

 ...

 printf(getstr(1));

 ...

 #ifndef WITHOUT_NLS
         catclose(catalog);
 #endif

     --------------------------------------------------------------

    4.2.3.1 Reducing Strings to Localize

   There is a good way of reducing the strings that need to be
   localized by using libc error messages. This is also useful to
   just avoid duplication and provide consistent error messages for
   the common errors that can be encountered by a great many of
   programs.

   First, here is an example that does not use libc error messages:

 #include <err.h>
 ...
 if (!S_ISDIR(st.st_mode))
         errx(1, "argument is not a directory");
          

   This can be transformed to print an error message by reading errno
   and printing an error message accordingly:

 #include <err.h>
 #include <errno.h>
 ...
 if (!S_ISDIR(st.st_mode)) {
         errno = ENOTDIR;
         err(1, NULL);
 }
          

   In this example, the custom string is eliminated, thus translators
   will have less work when localizing the program and users will see
   the usual "Not a directory" error message when they encounter this
   error. This message will probably seem more familiar to them.
   Please note that it was necessary to include errno.h in order to
   directly access errno.

   It is worth to note that there are cases when errno is set
   automatically by a preceding call, so it is not necessary to set
   it explicitly:

 #include <err.h>
 ...
 if ((p = malloc(size)) == NULL)
         err(1, NULL);
          

     --------------------------------------------------------------

  4.2.4 Making use of bsd.nls.mk

   Using the catalog files requires few repeatable steps, such as
   compiling the catalogs and installing them to the proper location.
   In order to simplify this process even more, bsd.nls.mk introduces
   some macros. It is not necessary to include bsd.nls.mk explicitly,
   it is pulled in from the common Makefiles, such as bsd.prog.mk or
   bsd.lib.mk.

   Usually it is enough to define NLSNAME, which should have the
   catalog name mentioned as the first argument of catopen(3) and
   list the catalog files in NLS without their .msg extension. Here
   is an example, which makes it possible to to disable NLS when used
   with the code examples before. The WITHOUT_NLS make(1) variable
   has to be defined in order to build the program without NLS
   support.

 .if !defined(WITHOUT_NLS)
 NLS=    es_ES.ISO8859-1
 NLS+=   hu_HU.ISO8859-2
 NLS+=   pt_BR.ISO8859-1
 .else
 CFLAGS+=        -DWITHOUT_NLS
 .endif

   Conventionally, the catalog files are placed under the nls
   subdirectory and this is the default behaviour of bsd.nls.mk. It
   is possible, though to override the location of the catalogs with
   the NLSSRCDIR make(1) variable. The default name of the
   precompiled catalog files also follow the naming convention
   mentioned before. It can be overridden by setting the NLSNAME
   variable. There are other options to fine tune the processing of
   the catalog files but usually it is not needed, thus they are not
   described here. For further information on bsd.nls.mk, please
   refer to the file itself, it is short and easy to understand.

     --------------------------------------------------------------

             Chapter 5 Source Tree Guidelines and Policies

   Contributed by Poul-Henning Kamp and Giorgos Keramidas.

   This chapter documents various guidelines and policies in force
   for the FreeBSD source tree.

     --------------------------------------------------------------

5.1 Style Guidelines

   Consistent coding style is extremely important, particularly with
   large projects like FreeBSD. Code should follow the FreeBSD coding
   styles described in style(9) and style.Makefile(5).

     --------------------------------------------------------------

5.2 MAINTAINER on Makefiles

   If a particular portion of the FreeBSD src/ distribution is being
   maintained by a person or group of persons, this is communicated
   through an entry in the src/MAINTAINERS file. Maintainers of ports
   within the Ports Collection express their maintainership to the
   world by adding a MAINTAINER line to the Makefile of the port in
   question:

 MAINTAINER= email-addresses

     Tip: For other parts of the repository, or for sections not
     listed as having a maintainer, or when you are unsure who the
     active maintainer is, try looking at the recent commit history
     of the relevant parts of the source tree. It is quite often the
     case that a maintainer is not explicitly named, but the people
     who are actively working in a part of the source tree for, say,
     the last couple of years are interested in reviewing changes.
     Even if this is not specifically mentioned in the documentation
     or the source itself, asking for a review as a form of courtesy
     is a very reasonable thing to do.

   The role of the maintainer is as follows:

     * The maintainer owns and is responsible for that code. This
       means that he or she is responsible for fixing bugs and
       answering problem reports pertaining to that piece of the
       code, and in the case of contributed software, for tracking
       new versions, as appropriate.

     * Changes to directories which have a maintainer defined shall
       be sent to the maintainer for review before being committed.
       Only if the maintainer does not respond for an unacceptable
       period of time, to several emails, will it be acceptable to
       commit changes without review by the maintainer. However, it
       is suggested that you try to have the changes reviewed by
       someone else if at all possible.

     * It is of course not acceptable to add a person or group as
       maintainer unless they agree to assume this duty. On the other
       hand it does not have to be a committer and it can easily be a
       group of people.

     --------------------------------------------------------------

5.3 Contributed Software

   Contributed by Poul-Henning Kamp, David O'Brien, and Gavin
   Atkinson.

   Some parts of the FreeBSD distribution consist of software that is
   actively being maintained outside the FreeBSD project. For
   historical reasons, we call this contributed software. Some
   examples are sendmail, gcc and patch.

   Over the last couple of years, various methods have been used in
   dealing with this type of software and all have some number of
   advantages and drawbacks. No clear winner has emerged.

   Since this is the case, after some debate one of these methods has
   been selected as the "official" method and will be required for
   future imports of software of this kind. Furthermore, it is
   strongly suggested that existing contributed software converge on
   this model over time, as it has significant advantages over the
   old method, including the ability to easily obtain diffs relative
   to the "official" versions of the source by everyone (even without
   direct repository access). This will make it significantly easier
   to return changes to the primary developers of the contributed
   software.

   Ultimately, however, it comes down to the people actually doing
   the work. If using this model is particularly unsuited to the
   package being dealt with, exceptions to these rules may be granted
   only with the approval of the core team and with the general
   consensus of the other developers. The ability to maintain the
   package in the future will be a key issue in the decisions.

     Note: Because it makes it harder to import future versions
     minor, trivial and/or cosmetic changes are strongly discouraged
     on files that are still tracking the vendor branch.

     --------------------------------------------------------------

  5.3.1 Vendor Imports with SVN

   Contributed by Dag-Erling Smo/rgrav.

   This section describes the vendor import procedure with Subversion
   in details.

    1. Preparing the Tree

       If this is your first import after the switch to SVN, you will
       have to flatten and clean up the vendor tree, and bootstrap
       merge history in the main tree. If not, you can safely omit
       this step.

       During the conversion from CVS to SVN, vendor branches were
       imported with the same layout as the main tree. For example,
       the foo vendor sources ended up in
       vendor/foo/dist/contrib/foo, but it is pointless and rather
       inconvenient. What we really want is to have the vendor source
       directly in vendor/foo/dist, like this:

 % cd vendor/foo/dist/contrib/foo
 % svn move $(svn list) ../..
 % cd ../..
 % svn remove contrib
 % svn propdel -R svn:mergeinfo
 % svn commit

       Note that, the propdel bit is necessary because starting with
       1.5, Subversion will automatically add svn:mergeinfo to any
       directory you copy or move. In this case, you will not need
       this information, since you are not going to merge anything
       from the tree you deleted.

         Note: You may want to flatten the tags as well. The
         procedure is exactly the same. If you do this, put off the
         commit until the end.

       Check the dist tree and perform any cleanup that is deemed to
       be necessary. You may want to disable keyword expansion, as it
       makes no sense on unmodified vendor code. In some cases, it
       can be even be harmful.

 % svn propdel svn:keywords -R .
 % svn commit

       Bootstrapping of svn:mergeinfo on the target directory (in the
       main tree) to the revision that corresponds to the last change
       was made to the vendor tree prior to importing new sources is
       also needed:

 % cd head/contrib/foo
 % svn merge --record-only svn_base/vendor/foo/dist@12345678 .
 % svn commit

       where svn_base is the base directory of your SVN repository,
       e.g. svn+ssh://svn.FreeBSD.org/base.

    2. Importing New Sources

       Prepare a full, clean tree of the vendor sources. With SVN, we
       can keep a full distribution in the vendor tree without
       bloating the main tree. Import everything but merge only what
       is needed.

       Note that you will need to add any files that were added since
       the last vendor import, and remove any that were removed. To
       facilitate this, you should prepare sorted lists of the
       contents of the vendor tree and of the sources you are about
       to import:

 % cd vendor/foo/dist
 % svn list -R | grep -v '/$' | sort > ../old
 % cd ../foo-9.9
 % find . -type f | cut -c 3- | sort > ../new

       With these two files, the following command will list list
       removed files (files only in old):

 % comm -23 ../old ../new

       While the command below will list added files (files only in
       new):

 % comm -13 ../old ../new

       Let's put this together:

 % cd vendor/foo/foo-9.9
 % tar cf - . | tar xf - -C ../dist
 % cd ../dist
 % comm -23 ../old ../new | xargs svn remove
 % comm -13 ../old ../new | xargs svn add

         Warning: If there are new directories in the new
         distribution, the last command will fail. You will have to
         add the directories, and run it again. Conversely, if any
         directories were removed, you will have to remove them
         manually.

       Check properties on any new files:

          * All text files should have svn:eol-style set to native.

          * All binary files should have svn:mime-type set to
            application/octet-stream, unless there is a more
            appropriate media type.

          * Executable files should have svn:executable set to *.

          * There should be no other properties on any file in the
            tree.

         Note: You are ready to commit, but you should first check
         the output of svn stat and svn diff to make sure everything
         is in order.

       Once you have committed the new vendor release, you should tag
       it for future reference. The best and quickest way is to do it
       directly in the repository:

 % svn copy svn_base/vendor/foo/dist svn_base/vendor/foo/9.9

       To get the new tag, you can update your working copy of
       vendor/foo.

         Note: If you choose to do the copy in the checkout instead,
         do not forget to remove the generated svn:mergeinfo as
         described above.

    3. Merging to -HEAD

       After you have prepared your import, it is time to merge.
       Option --accept=postpone tells SVN not to handle merge
       conflicts yet, because they will be taken care of manually:

 % cd head/contrib/foo
 % svn update
 % svn merge --accept=postpone svn_base/vendor/foo/dist

       Resolve any conflicts, and make sure that any files that were
       added or removed in the vendor tree have been properly added
       or removed in the main tree. It is always a good idea to check
       differences against the vendor branch:

 % svn diff --no-diff-deleted --old=svn_base/vendor/foo/dist --new=.

       The --no-diff-deleted option tells SVN not to check files that
       are in the vendor tree but not in the main tree.

         Note: With SVN, there is no concept of on or off the vendor
         branch. If a file that previously had local modifications no
         longer does, just remove any left-over cruft, such as
         FreeBSD version tags, so it no longer shows up in diffs
         against the vendor tree.

       If any changes are required for the world to build with the
       new sources, make them now - and test until you are satisfied
       that everything build and runs correctly.

    4. Commit

       Now, you are ready to commit. Make sure you get everything in
       one go. Ideally, you would have done all steps in a clean
       tree, in which case you can just commit from the top of that
       tree. That is the best way to avoid surprises. If you do it
       properly, the tree will move atomically from a consistent
       state with the old code to a consistent state with the new
       code.

     --------------------------------------------------------------

5.4 Encumbered Files

   It might occasionally be necessary to include an encumbered file
   in the FreeBSD source tree. For example, if a device requires a
   small piece of binary code to be loaded to it before the device
   will operate, and we do not have the source to that code, then the
   binary file is said to be encumbered. The following policies apply
   to including encumbered files in the FreeBSD source tree.

    1. Any file which is interpreted or executed by the system CPU(s)
       and not in source format is encumbered.

    2. Any file with a license more restrictive than BSD or GNU is
       encumbered.

    3. A file which contains downloadable binary data for use by the
       hardware is not encumbered, unless (1) or (2) apply to it. It
       must be stored in an architecture neutral ASCII format (file2c
       or uuencoding is recommended).

    4. Any encumbered file requires specific approval from the Core
       Team before it is added to the repository.

    5. Encumbered files go in src/contrib or src/sys/contrib.

    6. The entire module should be kept together. There is no point
       in splitting it, unless there is code-sharing with
       non-encumbered code.

    7. Object files are named arch/filename.o.uu>.

    8. Kernel files:

         a. Should always be referenced in conf/files.* (for build
            simplicity).

         b. Should always be in LINT, but the Core Team decides per
            case if it should be commented out or not. The Core Team
            can, of course, change their minds later on.

         c. The Release Engineer decides whether or not it goes into
            the release.

    9. User-land files:

         a. The Core team decides if the code should be part of make
            world.

         b. The Release Engineering decides if it goes into the
            release.

     --------------------------------------------------------------

5.5 Shared Libraries

   Contributed by Satoshi Asami, Peter Wemm, and David O'Brien.

   If you are adding shared library support to a port or other piece
   of software that does not have one, the version numbers should
   follow these rules. Generally, the resulting numbers will have
   nothing to do with the release version of the software.

   The three principles of shared library building are:

     * Start from 1.0

     * If there is a change that is backwards compatible, bump minor
       number (note that ELF systems ignore the minor number)

     * If there is an incompatible change, bump major number

   For instance, added functions and bugfixes result in the minor
   version number being bumped, while deleted functions, changed
   function call syntax, etc. will force the major version number to
   change.

   Stick to version numbers of the form major.minor (x.y). Our a.out
   dynamic linker does not handle version numbers of the form x.y.z
   well. Any version number after the y (i.e. the third digit) is
   totally ignored when comparing shared lib version numbers to
   decide which library to link with. Given two shared libraries that
   differ only in the "micro" revision, ld.so will link with the
   higher one. That is, if you link with libfoo.so.3.3.3, the linker
   only records 3.3 in the headers, and will link with anything
   starting with libfoo.so.3.(anything >= 3).(highest available).

     Note: ld.so will always use the highest "minor" revision. For
     instance, it will use libc.so.2.2 in preference to libc.so.2.0,
     even if the program was initially linked with libc.so.2.0.

   In addition, our ELF dynamic linker does not handle minor version
   numbers at all. However, one should still specify a major and
   minor version number as our Makefiles "do the right thing" based
   on the type of system.

   For non-port libraries, it is also our policy to change the shared
   library version number only once between releases. In addition, it
   is our policy to change the major shared library version number
   only once between major OS releases (i.e. from 6.0 to 7.0). When
   you make a change to a system library that requires the version
   number to be bumped, check the Makefile's commit logs. It is the
   responsibility of the committer to ensure that the first such
   change since the release will result in the shared library version
   number in the Makefile to be updated, and any subsequent changes
   will not.

     --------------------------------------------------------------

              Chapter 6 Regression and Performance Testing

   Regression tests are used to exercise a particular bit of the
   system to check that it works as expected, and to make sure that
   old bugs are not reintroduced.

   The FreeBSD regression testing tools can be found in the FreeBSD
   source tree in the directory src/tools/regression.

     --------------------------------------------------------------

6.1. Micro Benchmark Checklist

   This section contains hints for doing proper micro-benchmarking on
   FreeBSD or of FreeBSD itself.

   It is not possible to use all of the suggestions below every
   single time, but the more used, the better the benchmark's ability
   to test small differences will be.

     * Disable APM and any other kind of clock fiddling (ACPI ?).

     * Run in single user mode. E.g., cron(8), and other daemons only
       add noise. The sshd(8) daemon can also cause problems. If ssh
       access is required during testing either disable the SSHv1 key
       regeneration, or kill the parent sshd daemon during the tests.

     * Do not run ntpd(8).

     * If syslog(3) events are generated, run syslogd(8) with an
       empty /etc/syslogd.conf, otherwise, do not run it.

     * Minimize disk-I/O, avoid it entirely if possible.

     * Do not mount file systems that are not needed.

     * Mount /, /usr, and any other file system as read-only if
       possible. This removes atime updates to disk (etc.) from the
       I/O picture.

     * Reinitialize the read/write test file system with newfs(8) and
       populate it from a tar(1) or dump(8) file before every run.
       Unmount and mount it before starting the test. This results in
       a consistent file system layout. For a worldstone test this
       would apply to /usr/obj (just reinitialize with newfs and
       mount). To get 100% reproducibility, populate the file system
       from a dd(1) file (i.e.: dd if=myimage of=/dev/ad0s1h bs=1m)

     * Use malloc backed or preloaded md(4) partitions.

     * Reboot between individual iterations of the test, this gives a
       more consistent state.

     * Remove all non-essential device drivers from the kernel. For
       instance if USB is not needed for the test, do not put USB in
       the kernel. Drivers which attach often have timeouts ticking
       away.

     * Unconfigure hardware that are not in use. Detach disks with
       atacontrol(8) and camcontrol(8) if the disks are not used for
       the test.

     * Do not configure the network unless it is being tested, or
       wait until after the test has been performed to ship the
       results off to another computer.

       If the system must be connected to a public network, watch out
       for spikes of broadcast traffic. Even though it is hardly
       noticeable, it will take up CPU cycles. Multicast has similar
       caveats.

     * Put each file system on its own disk. This minimizes jitter
       from head-seek optimizations.

     * Minimize output to serial or VGA consoles. Running output into
       files gives less jitter. (Serial consoles easily become a
       bottleneck.) Do not touch keyboard while the test is running,
       even space or back-space shows up in the numbers.

     * Make sure the test is long enough, but not too long. If the
       test is too short, timestamping is a problem. If it is too
       long temperature changes and drift will affect the frequency
       of the quartz crystals in the computer. Rule of thumb: more
       than a minute, less than an hour.

     * Try to keep the temperature as stable as possible around the
       machine. This affects both quartz crystals and disk drive
       algorithms. To get real stable clock, consider stabilized
       clock injection. E.g., get a OCXO + PLL, inject output into
       clock circuits instead of motherboard xtal. Contact
       Poul-Henning Kamp <phk@FreeBSD.org> for more information about
       this.

     * Run the test at least 3 times but it is better to run more
       than 20 times both for "before" and "after" code. Try to
       interleave if possible (i.e.: do not run 20 times before then
       20 times after), this makes it possible to spot environmental
       effects. Do not interleave 1:1, but 3:3, this makes it
       possible to spot interaction effects.

       A good pattern is: bababa{bbbaaa}*. This gives hint after the
       first 1+1 runs (so it is possible to stop the test if it goes
       entirely the wrong way), a standard deviation after the first
       3+3 (gives a good indication if it is going to be worth a long
       run) and trending and interaction numbers later on.

     * Use ministat(1) to see if the numbers are significant.
       Consider buying "Cartoon guide to statistics" ISBN:
       0062731025, highly recommended, if you have forgotten or never
       learned about standard deviation and Student's T.

     * Do not use background fsck(8) unless the test is a benchmark
       of background fsck. Also, disable background_fsck in
       /etc/rc.conf unless the benchmark is not started at least
       60+"fsck runtime" seconds after the boot, as rc(8) wakes up
       and checks if fsck needs to run on any file systems when
       background fsck is enabled. Likewise, make sure there are no
       snapshots lying around unless the benchmark is a test with
       snapshots.

     * If the benchmark show unexpected bad performance, check for
       things like high interrupt volume from an unexpected source.
       Some versions of ACPI have been reported to "misbehave" and
       generate excess interrupts. To help diagnose odd test results,
       take a few snapshots of vmstat -i and look for anything
       unusual.

     * Make sure to be careful about optimization parameters for
       kernel and userspace, likewise debugging. It is easy to let
       something slip through and realize later the test was not
       comparing the same thing.

     * Do not ever benchmark with the WITNESS and INVARIANTS kernel
       options enabled unless the test is interested to benchmarking
       those features. WITNESS can cause 400%+ drops in performance.
       Likewise, userspace malloc(3) parameters default differently
       in -CURRENT from the way they ship in production releases.

     --------------------------------------------------------------

6.2. The FreeBSD Source Tinderbox

   The source Tinderbox consists of:

     * A build script, tinderbox, that automates checking out a
       specific version of the FreeBSD source tree and building it.

     * A supervisor script, tbmaster, that monitors individual
       Tinderbox instances, logs their output, and emails failure
       notices.

     * A CGI script named index.cgi that reads a set of tbmaster logs
       and presents an easy-to-read HTML summary of them.

     * A set of build servers that continually test the tip of the
       most important FreeBSD code branches.

     * A webserver that keeps a complete set of Tinderbox logs and
       displays an up-to-date summary.

   The scripts are maintained and were developed by Dag-Erling C.
   Smo/rgrav <des@FreeBSD.org>, and are now written in Perl, a move
   on from their original incarnation as shell scripts. All scripts
   and configuration files are kept in /projects/tinderbox/.

   For more information about the tinderbox and tbmaster scripts at
   this stage, see their respective man pages: tinderbox(1) and
   tbmaster(1).

     --------------------------------------------------------------

  6.2.1. The index.cgi Script

   The index.cgi script generates the HTML summary of tinderbox and
   tbmaster logs. Although originally intended to be used as a CGI
   script, as indicated by its name, this script can also be run from
   the command line or from a cron(8) job, in which case it will look
   for logs in the directory where the script is located. It will
   automatically detect context, generating HTTP headers when it is
   run as a CGI script. It conforms to XHTML standards and is styled
   using CSS.

   The script starts in the main() block by attempting to verify that
   it is running on the official Tinderbox website. If it is not, a
   page indicating it is not an official website is produced, and a
   URL to the official site is provided.

   Next, it scans the log directory to get an inventory of
   configurations, branches and architectures for which log files
   exist, to avoid hard-coding a list into the script and potentially
   ending up with blank rows or columns. This information is derived
   from the names of the log files matching the following pattern:

 tinderbox-$config-$branch-$arch-$machine.{brief,full}

   The configurations used on the official Tinderbox build servers
   are named for the branches they build. For example, the releng_8
   configuration is used to build RELENG_8 as well as all
   still-supported release branches.

   Once all of this startup procedure has been successfully
   completed, do_config() is called for each configuration.

   The do_config() function generates HTML for a single Tinderbox
   configuration.

   It works by first generating a header row, then iterating over
   each branch build with the specified configuration, producing a
   single row of results for each in the following manner:

     * For each item:

          * For each machine within that architecture:

               * If a brief log file exists, then:

                    * Call success() to detemine the outcome of the
                      build.

                    * Output the modification size.

                    * Output the size of the brief log file with a
                      link to the log file itself.

                    * If a full log file also exists, then:

                         * Output the size of the full log file with
                           a link to the log file itself.

               * Otherwise:

                    * No output.

   The success() function mentioned above scans a brief log file for
   the string "tinderbox run completed" in order to determine whether
   the build was successful.

   Configurations and branches are sorted according to their branch
   rank. This is computed as follows:

     * HEAD and CURRENT have rank 9999.

     * RELENG_x has rank xx99.

     * RELENG_x_y has rank xxyy.

   Tthis means that HEAD always ranks highest, and RELENG branches
   are ranked in numerical order, with each STABLE branch ranking
   higher than the release branches forked off of it. For instance,
   for FreeBSD 8, the order from highest to lowest would be:

     * RELENG_8 (branch rank 899).

     * RELENG_8_3 (branch rank 803).

     * RELENG_8_2 (branch rank 802).

     * RELENG_8_1 (branch rank 801).

     * RELENG_8_0 (branch rank 800).

   The colors that Tinderbox uses for each cell in the table are
   defined by CSS. Successful builds are displayed with green text;
   unsuccessful builds are displayed with red text. The color fades
   as time passes since the corresponding build, with every half an
   hour bringing the color closer to grey.

     --------------------------------------------------------------

  6.2.2. Official Build Servers

   The official Tinderbox build servers are hosted by Sentex Data
   Communications, who also host the FreeBSD Netperf Cluster.

   Three build servers currently exist:

   freebsd-current.sentex.ca builds:

     * HEAD for amd64, arm, i386, i386/pc98, ia64, mips, powerpc,
       powerpc64, and sparc64.

     * RELENG_9 and supported 9.X branches for amd64, arm, i386,
       i386/pc98, ia64, mips, powerpc, powerpc64, and sparc64.

   freebsd-stable.sentex.ca builds:

     * RELENG_8 and supported 8.X branches for amd64, i386,
       i386/pc98, ia64, mips, powerpc and sparc64.

   freebsd-legacy.sentex.ca builds:

     * RELENG_7 and supported 7.X branches for amd64, i386,
       i386/pc98, ia64, powerpc, and sparc64.

     --------------------------------------------------------------

  6.2.3. Official Summary Site

   Summaries and logs from the official build servers are available
   online at http://tinderbox.FreeBSD.org, hosted by Dag-Erling C.
   Smo/rgrav <des@FreeBSD.org> and set up as follows:

     * A cron(8) job checks the build servers at regular intervals
       and downloads any new log files using rsync(1).

     * Apache is set up to use index.cgi as DirectoryIndex.

                     II. Interprocess Communication

   Table of Contents

   7 Sockets

   8 IPv6 Internals

     --------------------------------------------------------------

                           Chapter 7 Sockets

   Contributed by G. Adam Stanislav.

7.1 Synopsis

   BSD sockets take interprocess communications to a new level. It is
   no longer necessary for the communicating processes to run on the
   same machine. They still can, but they do not have to.

   Not only do these processes not have to run on the same machine,
   they do not have to run under the same operating system. Thanks to
   BSD sockets, your FreeBSD software can smoothly cooperate with a
   program running on a Macintosh(R), another one running on a
   Sun(TM) workstation, yet another one running under Windows(R)
   2000, all connected with an Ethernet-based local area network.

   But your software can equally well cooperate with processes
   running in another building, or on another continent, inside a
   submarine, or a space shuttle.

   It can also cooperate with processes that are not part of a
   computer (at least not in the strict sense of the word), but of
   such devices as printers, digital cameras, medical equipment. Just
   about anything capable of digital communications.

     --------------------------------------------------------------

7.2 Networking and Diversity

   We have already hinted on the diversity of networking. Many
   different systems have to talk to each other. And they have to
   speak the same language. They also have to understand the same
   language the same way.

   People often think that body language is universal. But it is not.
   Back in my early teens, my father took me to Bulgaria. We were
   sitting at a table in a park in Sofia, when a vendor approached us
   trying to sell us some roasted almonds.

   I had not learned much Bulgarian by then, so, instead of saying
   no, I shook my head from side to side, the "universal" body
   language for no. The vendor quickly started serving us some
   almonds.

   I then remembered I had been told that in Bulgaria shaking your
   head sideways meant yes. Quickly, I started nodding my head up and
   down. The vendor noticed, took his almonds, and walked away. To an
   uninformed observer, I did not change the body language: I
   continued using the language of shaking and nodding my head. What
   changed was the meaning of the body language. At first, the vendor
   and I interpreted the same language as having completely different
   meaning. I had to adjust my own interpretation of that language so
   the vendor would understand.

   It is the same with computers: The same symbols may have
   different, even outright opposite meaning. Therefore, for two
   computers to understand each other, they must not only agree on
   the same language, but on the same interpretation of the language.

     --------------------------------------------------------------

7.3 Protocols

   While various programming languages tend to have complex syntax
   and use a number of multi-letter reserved words (which makes them
   easy for the human programmer to understand), the languages of
   data communications tend to be very terse. Instead of multi-byte
   words, they often use individual bits. There is a very convincing
   reason for it: While data travels inside your computer at speeds
   approaching the speed of light, it often travels considerably
   slower between two computers.

   Because the languages used in data communications are so terse, we
   usually refer to them as protocols rather than languages.

   As data travels from one computer to another, it always uses more
   than one protocol. These protocols are layered. The data can be
   compared to the inside of an onion: You have to peel off several
   layers of "skin" to get to the data. This is best illustrated with
   a picture:

 +----------------+
 |    Ethernet    |
 |+--------------+|
 ||      IP      ||
 ||+------------+||
 |||     TCP    |||
 |||+----------+|||
 ||||   HTTP   ||||
 ||||+--------+||||
 |||||   PNG  |||||
 |||||+------+|||||
 |||||| Data ||||||
 |||||+------+|||||
 ||||+--------+||||
 |||+----------+|||
 ||+------------+||
 |+--------------+|
 +----------------+

   In this example, we are trying to get an image from a web page we
   are connected to via an Ethernet.

   The image consists of raw data, which is simply a sequence of RGB
   values that our software can process, i.e., convert into an image
   and display on our monitor.

   Alas, our software has no way of knowing how the raw data is
   organized: Is it a sequence of RGB values, or a sequence of
   grayscale intensities, or perhaps of CMYK encoded colors? Is the
   data represented by 8-bit quanta, or are they 16 bits in size, or
   perhaps 4 bits? How many rows and columns does the image consist
   of? Should certain pixels be transparent?

   I think you get the picture...

   To inform our software how to handle the raw data, it is encoded
   as a PNG file. It could be a GIF, or a JPEG, but it is a PNG.

   And PNG is a protocol.

   At this point, I can hear some of you yelling, "No, it is not! It
   is a file format!"

   Well, of course it is a file format. But from the perspective of
   data communications, a file format is a protocol: The file
   structure is a language, a terse one at that, communicating to our
   process how the data is organized. Ergo, it is a protocol.

   Alas, if all we received was the PNG file, our software would be
   facing a serious problem: How is it supposed to know the data is
   representing an image, as opposed to some text, or perhaps a
   sound, or what not? Secondly, how is it supposed to know the image
   is in the PNG format as opposed to GIF, or JPEG, or some other
   image format?

   To obtain that information, we are using another protocol: HTTP.
   This protocol can tell us exactly that the data represents an
   image, and that it uses the PNG protocol. It can also tell us some
   other things, but let us stay focused on protocol layers here.

   So, now we have some data wrapped in the PNG protocol, wrapped in
   the HTTP protocol. How did we get it from the server?

   By using TCP/IP over Ethernet, that is how. Indeed, that is three
   more protocols. Instead of continuing inside out, I am now going
   to talk about Ethernet, simply because it is easier to explain the
   rest that way.

   Ethernet is an interesting system of connecting computers in a
   local area network (LAN). Each computer has a network interface
   card (NIC), which has a unique 48-bit ID called its address. No
   two Ethernet NICs in the world have the same address.

   These NICs are all connected with each other. Whenever one
   computer wants to communicate with another in the same Ethernet
   LAN, it sends a message over the network. Every NIC sees the
   message. But as part of the Ethernet protocol, the data contains
   the address of the destination NIC (among other things). So, only
   one of all the network interface cards will pay attention to it,
   the rest will ignore it.

   But not all computers are connected to the same network. Just
   because we have received the data over our Ethernet does not mean
   it originated in our own local area network. It could have come to
   us from some other network (which may not even be Ethernet based)
   connected with our own network via the Internet.

   All data is transferred over the Internet using IP, which stands
   for Internet Protocol. Its basic role is to let us know where in
   the world the data has arrived from, and where it is supposed to
   go to. It does not guarantee we will receive the data, only that
   we will know where it came from if we do receive it.

   Even if we do receive the data, IP does not guarantee we will
   receive various chunks of data in the same order the other
   computer has sent it to us. So, we can receive the center of our
   image before we receive the upper left corner and after the lower
   right, for example.

   It is TCP (Transmission Control Protocol) that asks the sender to
   resend any lost data and that places it all into the proper order.

   All in all, it took five different protocols for one computer to
   communicate to another what an image looks like. We received the
   data wrapped into the PNG protocol, which was wrapped into the
   HTTP protocol, which was wrapped into the TCP protocol, which was
   wrapped into the IP protocol, which was wrapped into the Ethernet
   protocol.

   Oh, and by the way, there probably were several other protocols
   involved somewhere on the way. For example, if our LAN was
   connected to the Internet through a dial-up call, it used the PPP
   protocol over the modem which used one (or several) of the various
   modem protocols, et cetera, et cetera, et cetera...

   As a developer you should be asking by now, "How am I supposed to
   handle it all?"

   Luckily for you, you are not supposed to handle it all. You are
   supposed to handle some of it, but not all of it. Specifically,
   you need not worry about the physical connection (in our case
   Ethernet and possibly PPP, etc). Nor do you need to handle the
   Internet Protocol, or the Transmission Control Protocol.

   In other words, you do not have to do anything to receive the data
   from the other computer. Well, you do have to ask for it, but that
   is almost as simple as opening a file.

   Once you have received the data, it is up to you to figure out
   what to do with it. In our case, you would need to understand the
   HTTP protocol and the PNG file structure.

   To use an analogy, all the internetworking protocols become a gray
   area: Not so much because we do not understand how it works, but
   because we are no longer concerned about it. The sockets interface
   takes care of this gray area for us:

 +----------------+
 |xxxxEthernetxxxx|
 |+--------------+|
 ||xxxxxxIPxxxxxx||
 ||+------------+||
 |||xxxxxTCPxxxx|||
 |||+----------+|||
 ||||   HTTP   ||||
 ||||+--------+||||
 |||||   PNG  |||||
 |||||+------+|||||
 |||||| Data ||||||
 |||||+------+|||||
 ||||+--------+||||
 |||+----------+|||
 ||+------------+||
 |+--------------+|
 +----------------+

   We only need to understand any protocols that tell us how to
   interpret the data, not how to receive it from another process,
   nor how to send it to another process.

     --------------------------------------------------------------

7.4 The Sockets Model

   BSD sockets are built on the basic UNIX model: Everything is a
   file. In our example, then, sockets would let us receive an HTTP
   file, so to speak. It would then be up to us to extract the PNG
   file from it.

   Because of the complexity of internetworking, we cannot just use
   the open system call, or the open() C function. Instead, we need
   to take several steps to "opening" a socket.

   Once we do, however, we can start treating the socket the same way
   we treat any file descriptor: We can read from it, write to it,
   pipe it, and, eventually, close it.

     --------------------------------------------------------------

7.5 Essential Socket Functions

   While FreeBSD offers different functions to work with sockets, we
   only need four to "open" a socket. And in some cases we only need
   two.

     --------------------------------------------------------------

  7.5.1 The Client-Server Difference

   Typically, one of the ends of a socket-based data communication is
   a server, the other is a client.

     --------------------------------------------------------------

    7.5.1.1 The Common Elements

      7.5.1.1.1 socket

   The one function used by both, clients and servers, is socket(2).
   It is declared this way:

 int socket(int domain, int type, int protocol);

   The return value is of the same type as that of open, an integer.
   FreeBSD allocates its value from the same pool as that of file
   handles. That is what allows sockets to be treated the same way as
   files.

   The domain argument tells the system what protocol family you want
   it to use. Many of them exist, some are vendor specific, others
   are very common. They are declared in sys/socket.h.

   Use PF_INET for UDP, TCP and other Internet protocols (IPv4).

   Five values are defined for the type argument, again, in
   sys/socket.h. All of them start with "SOCK_". The most common one
   is SOCK_STREAM, which tells the system you are asking for a
   reliable stream delivery service (which is TCP when used with
   PF_INET).

   If you asked for SOCK_DGRAM, you would be requesting a
   connectionless datagram delivery service (in our case, UDP).

   If you wanted to be in charge of the low-level protocols (such as
   IP), or even network interfaces (e.g., the Ethernet), you would
   need to specify SOCK_RAW.

   Finally, the protocol argument depends on the previous two
   arguments, and is not always meaningful. In that case, use 0 for
   its value.

     The Unconnected Socket: Nowhere, in the socket function have we
     specified to what other system we should be connected. Our newly
     created socket remains unconnected.

     This is on purpose: To use a telephone analogy, we have just
     attached a modem to the phone line. We have neither told the
     modem to make a call, nor to answer if the phone rings.

     --------------------------------------------------------------

      7.5.1.1.2 sockaddr

   Various functions of the sockets family expect the address of (or
   pointer to, to use C terminology) a small area of the memory. The
   various C declarations in the sys/socket.h refer to it as struct
   sockaddr. This structure is declared in the same file:

 /*
  * Structure used by kernel to store most
  * addresses.
  */
 struct sockaddr {
         unsigned char   sa_len;         /* total length */
         sa_family_t     sa_family;      /* address family */
         char            sa_data[14];    /* actually longer; address value */
 };
 #define SOCK_MAXADDRLEN 255             /* longest possible addresses */

   Please note the vagueness with which the sa_data field is
   declared, just as an array of 14 bytes, with the comment hinting
   there can be more than 14 of them.

   This vagueness is quite deliberate. Sockets is a very powerful
   interface. While most people perhaps think of it as nothing more
   than the Internet interface-and most applications probably use it
   for that nowadays-sockets can be used for just about any kind of
   interprocess communications, of which the Internet (or, more
   precisely, IP) is only one.

   The sys/socket.h refers to the various types of protocols sockets
   will handle as address families, and lists them right before the
   definition of sockaddr:

 /*
  * Address families.
  */
 #define AF_UNSPEC       0               /* unspecified */
 #define AF_LOCAL        1               /* local to host (pipes, portals) */
 #define AF_UNIX         AF_LOCAL        /* backward compatibility */
 #define AF_INET         2               /* internetwork: UDP, TCP, etc. */
 #define AF_IMPLINK      3               /* arpanet imp addresses */
 #define AF_PUP          4               /* pup protocols: e.g. BSP */
 #define AF_CHAOS        5               /* mit CHAOS protocols */
 #define AF_NS           6               /* XEROX NS protocols */
 #define AF_ISO          7               /* ISO protocols */
 #define AF_OSI          AF_ISO
 #define AF_ECMA         8               /* European computer manufacturers */
 #define AF_DATAKIT      9               /* datakit protocols */
 #define AF_CCITT        10              /* CCITT protocols, X.25 etc */
 #define AF_SNA          11              /* IBM SNA */
 #define AF_DECnet       12              /* DECnet */
 #define AF_DLI          13              /* DEC Direct data link interface */
 #define AF_LAT          14              /* LAT */
 #define AF_HYLINK       15              /* NSC Hyperchannel */
 #define AF_APPLETALK    16              /* Apple Talk */
 #define AF_ROUTE        17              /* Internal Routing Protocol */
 #define AF_LINK         18              /* Link layer interface */
 #define pseudo_AF_XTP   19              /* eXpress Transfer Protocol (no AF) */
 #define AF_COIP         20              /* connection-oriented IP, aka ST II */
 #define AF_CNT          21              /* Computer Network Technology */
 #define pseudo_AF_RTIP  22              /* Help Identify RTIP packets */
 #define AF_IPX          23              /* Novell Internet Protocol */
 #define AF_SIP          24              /* Simple Internet Protocol */
 #define pseudo_AF_PIP   25              /* Help Identify PIP packets */
 #define AF_ISDN         26              /* Integrated Services Digital Network*/
 #define AF_E164         AF_ISDN         /* CCITT E.164 recommendation */
 #define pseudo_AF_KEY   27              /* Internal key-management function */
 #define AF_INET6        28              /* IPv6 */
 #define AF_NATM         29              /* native ATM access */
 #define AF_ATM          30              /* ATM */
 #define pseudo_AF_HDRCMPLT 31           /* Used by BPF to not rewrite headers
                                          * in interface output routine
                                          */
 #define AF_NETGRAPH     32              /* Netgraph sockets */
 #define AF_SLOW         33              /* 802.3ad slow protocol */
 #define AF_SCLUSTER     34              /* Sitara cluster protocol */
 #define AF_ARP          35
 #define AF_BLUETOOTH    36              /* Bluetooth sockets */
 #define AF_MAX          37


   The one used for IP is AF_INET. It is a symbol for the constant 2.

   It is the address family listed in the sa_family field of sockaddr
   that decides how exactly the vaguely named bytes of sa_data will
   be used.

   Specifically, whenever the address family is AF_INET, we can use
   struct sockaddr_in found in netinet/in.h, wherever sockaddr is
   expected:

 /*
  * Socket address, internet style.
  */
 struct sockaddr_in {
         uint8_t         sin_len;
         sa_family_t     sin_family;
         in_port_t       sin_port;
         struct  in_addr sin_addr;
         char    sin_zero[8];
 };

   We can visualize its organization this way:

         0        1        2       3
    +--------+--------+-----------------+
  0 |    0   | Family |       Port      |
    +--------+--------+-----------------+
  4 |             IP Address            |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   The three important fields are sin_family, which is byte 1 of the
   structure, sin_port, a 16-bit value found in bytes 2 and 3, and
   sin_addr, a 32-bit integer representation of the IP address,
   stored in bytes 4-7.

   Now, let us try to fill it out. Let us assume we are trying to
   write a client for the daytime protocol, which simply states that
   its server will write a text string representing the current date
   and time to port 13. We want to use TCP/IP, so we need to specify
   AF_INET in the address family field. AF_INET is defined as 2. Let
   us use the IP address of 192.43.244.18, which is the time server
   of US federal government (time.nist.gov).

         0        1        2       3
    +--------+--------+-----------------+
  0 |    0   |   2    |        13       |
    +-----------------+-----------------+
  4 |           192.43.244.18           |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   By the way the sin_addr field is declared as being of the struct
   in_addr type, which is defined in netinet/in.h:

 /*
  * Internet address (a structure for historical reasons)
  */
 struct in_addr {
         in_addr_t s_addr;
 };

   In addition, in_addr_t is a 32-bit integer.

   The 192.43.244.18 is just a convenient notation of expressing a
   32-bit integer by listing all of its 8-bit bytes, starting with
   the most significant one.

   So far, we have viewed sockaddr as an abstraction. Our computer
   does not store short integers as a single 16-bit entity, but as a
   sequence of 2 bytes. Similarly, it stores 32-bit integers as a
   sequence of 4 bytes.

   Suppose we coded something like this:

         sa.sin_family      = AF_INET;
         sa.sin_port        = 13;
         sa.sin_addr.s_addr = (((((192 << 8) | 43) << 8) | 244) << 8) | 18;

   What would the result look like?

   Well, that depends, of course. On a Pentium(R), or other x86,
   based computer, it would look like this:

         0        1        2       3
    +--------+--------+--------+--------+
  0 |    0   |   2    |   13   |   0    |
    +--------+--------+--------+--------+
  4 |   18   |  244   |   43   |  192   |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   On a different system, it might look like this:

         0        1        2       3
    +--------+--------+--------+--------+
  0 |    0   |   2    |    0   |   13   |
    +--------+--------+--------+--------+
  4 |   192  |   43   |   244  |   18   |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   And on a PDP it might look different yet. But the above two are
   the most common ways in use today.

   Ordinarily, wanting to write portable code, programmers pretend
   that these differences do not exist. And they get away with it
   (except when they code in assembly language). Alas, you cannot get
   away with it that easily when coding for sockets.

   Why?

   Because when communicating with another computer, you usually do
   not know whether it stores data most significant byte (MSB) or
   least significant byte (LSB) first.

   You might be wondering, "So, will sockets not handle it for me?"

   It will not.

   While that answer may surprise you at first, remember that the
   general sockets interface only understands the sa_len and
   sa_family fields of the sockaddr structure. You do not have to
   worry about the byte order there (of course, on FreeBSD sa_family
   is only 1 byte anyway, but many other UNIX systems do not have
   sa_len and use 2 bytes for sa_family, and expect the data in
   whatever order is native to the computer).

   But the rest of the data is just sa_data[14] as far as sockets
   goes. Depending on the address family, sockets just forwards that
   data to its destination.

   Indeed, when we enter a port number, it is because we want the
   other computer to know what service we are asking for. And, when
   we are the server, we read the port number so we know what service
   the other computer is expecting from us. Either way, sockets only
   has to forward the port number as data. It does not interpret it
   in any way.

   Similarly, we enter the IP address to tell everyone on the way
   where to send our data to. Sockets, again, only forwards it as
   data.

   That is why, we (the programmers, not the sockets) have to
   distinguish between the byte order used by our computer and a
   conventional byte order to send the data in to the other computer.

   We will call the byte order our computer uses the host byte order,
   or just the host order.

   There is a convention of sending the multi-byte data over IP MSB
   first. This, we will refer to as the network byte order, or simply
   the network order.

   Now, if we compiled the above code for an Intel based computer,
   our host byte order would produce:

         0        1        2       3
    +--------+--------+--------+--------+
  0 |    0   |   2    |   13   |   0    |
    +--------+--------+--------+--------+
  4 |   18   |  244   |   43   |  192   |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   But the network byte order requires that we store the data MSB
   first:

         0        1        2       3
    +--------+--------+--------+--------+
  0 |    0   |   2    |    0   |   13   |
    +--------+--------+--------+--------+
  4 |   192  |   43   |   244  |   18   |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

   Unfortunately, our host order is the exact opposite of the network
   order.

   We have several ways of dealing with it. One would be to reverse
   the values in our code:

         sa.sin_family      = AF_INET;
         sa.sin_port        = 13 << 8;
         sa.sin_addr.s_addr = (((((18 << 8) | 244) << 8) | 43) << 8) | 192;

   This will trick our compiler into storing the data in the network
   byte order. In some cases, this is exactly the way to do it (e.g.,
   when programming in assembly language). In most cases, however, it
   can cause a problem.

   Suppose, you wrote a sockets-based program in C. You know it is
   going to run on a Pentium, so you enter all your constants in
   reverse and force them to the network byte order. It works well.

   Then, some day, your trusted old Pentium becomes a rusty old
   Pentium. You replace it with a system whose host order is the same
   as the network order. You need to recompile all your software. All
   of your software continues to perform well, except the one program
   you wrote.

   You have since forgotten that you had forced all of your constants
   to the opposite of the host order. You spend some quality time
   tearing out your hair, calling the names of all gods you ever
   heard of (and some you made up), hitting your monitor with a nerf
   bat, and performing all the other traditional ceremonies of trying
   to figure out why something that has worked so well is suddenly
   not working at all.

   Eventually, you figure it out, say a couple of swear words, and
   start rewriting your code.

   Luckily, you are not the first one to face the problem. Someone
   else has created the htons(3) and htonl(3) C functions to convert
   a short and long respectively from the host byte order to the
   network byte order, and the ntohs(3) and ntohl(3) C functions to
   go the other way.

   On MSB-first systems these functions do nothing. On LSB-first
   systems they convert values to the proper order.

   So, regardless of what system your software is compiled on, your
   data will end up in the correct order if you use these functions.

     --------------------------------------------------------------

    7.5.1.2 Client Functions

   Typically, the client initiates the connection to the server. The
   client knows which server it is about to call: It knows its IP
   address, and it knows the port the server resides at. It is akin
   to you picking up the phone and dialing the number (the address),
   then, after someone answers, asking for the person in charge of
   wingdings (the port).

     --------------------------------------------------------------

      7.5.1.2.1 connect

   Once a client has created a socket, it needs to connect it to a
   specific port on a remote system. It uses connect(2):

 int connect(int s, const struct sockaddr *name, socklen_t namelen);

   The s argument is the socket, i.e., the value returned by the
   socket function. The name is a pointer to sockaddr, the structure
   we have talked about extensively. Finally, namelen informs the
   system how many bytes are in our sockaddr structure.

   If connect is successful, it returns 0. Otherwise it returns -1
   and stores the error code in errno.

   There are many reasons why connect may fail. For example, with an
   attempt to an Internet connection, the IP address may not exist,
   or it may be down, or just too busy, or it may not have a server
   listening at the specified port. Or it may outright refuse any
   request for specific code.

     --------------------------------------------------------------

      7.5.1.2.2 Our First Client

   We now know enough to write a very simple client, one that will
   get current time from 192.43.244.18 and print it to stdout.

 /*
  * daytime.c
  *
  * Programmed by G. Adam Stanislav
  */
 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>

 int main() {
   register int s;
   register int bytes;
   struct sockaddr_in sa;
   char buffer[BUFSIZ+1];

   if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
     perror("socket");
     return 1;
   }

   bzero(&sa, sizeof sa);

   sa.sin_family = AF_INET;
   sa.sin_port = htons(13);
   sa.sin_addr.s_addr = htonl((((((192 << 8) | 43) << 8) | 244) << 8) | 18);
   if (connect(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
     perror("connect");
     close(s);
     return 2;
   }

   while ((bytes = read(s, buffer, BUFSIZ)) > 0)
     write(1, buffer, bytes);

   close(s);
   return 0;
 }

   Go ahead, enter it in your editor, save it as daytime.c, then
   compile and run it:

 % cc -O3 -o daytime daytime.c
 % ./daytime

 52079 01-06-19 02:29:25 50 0 1 543.9 UTC(NIST) *
 %

   In this case, the date was June 19, 2001, the time was 02:29:25
   UTC. Naturally, your results will vary.

     --------------------------------------------------------------

    7.5.1.3 Server Functions

   The typical server does not initiate the connection. Instead, it
   waits for a client to call it and request services. It does not
   know when the client will call, nor how many clients will call. It
   may be just sitting there, waiting patiently, one moment, The next
   moment, it can find itself swamped with requests from a number of
   clients, all calling in at the same time.

   The sockets interface offers three basic functions to handle this.

     --------------------------------------------------------------

      7.5.1.3.1 bind

   Ports are like extensions to a phone line: After you dial a
   number, you dial the extension to get to a specific person or
   department.

   There are 65535 IP ports, but a server usually processes requests
   that come in on only one of them. It is like telling the phone
   room operator that we are now at work and available to answer the
   phone at a specific extension. We use bind(2) to tell sockets
   which port we want to serve.

 int bind(int s, const struct sockaddr *addr, socklen_t addrlen);

   Beside specifying the port in addr, the server may include its IP
   address. However, it can just use the symbolic constant INADDR_ANY
   to indicate it will serve all requests to the specified port
   regardless of what its IP address is. This symbol, along with
   several similar ones, is declared in netinet/in.h

 #define INADDR_ANY              (u_int32_t)0x00000000

   Suppose we were writing a server for the daytime protocol over
   TCP/IP. Recall that it uses port 13. Our sockaddr_in structure
   would look like this:

         0        1        2       3
    +--------+--------+--------+--------+
  0 |    0   |   2    |    0   |   13   |
    +--------+--------+--------+--------+
  4 |                 0                 |
    +-----------------------------------+
  8 |                 0                 |
    +-----------------------------------+
 12 |                 0                 |
    +-----------------------------------+

     --------------------------------------------------------------

      7.5.1.3.2 listen

   To continue our office phone analogy, after you have told the
   phone central operator what extension you will be at, you now walk
   into your office, and make sure your own phone is plugged in and
   the ringer is turned on. Plus, you make sure your call waiting is
   activated, so you can hear the phone ring even while you are
   talking to someone.

   The server ensures all of that with the listen(2) function.

 int listen(int s, int backlog);

   In here, the backlog variable tells sockets how many incoming
   requests to accept while you are busy processing the last request.
   In other words, it determines the maximum size of the queue of
   pending connections.

     --------------------------------------------------------------

      7.5.1.3.3 accept

   After you hear the phone ringing, you accept the call by answering
   the call. You have now established a connection with your client.
   This connection remains active until either you or your client
   hang up.

   The server accepts the connection by using the accept(2) function.

 int accept(int s, struct sockaddr *addr, socklen_t *addrlen);

   Note that this time addrlen is a pointer. This is necessary
   because in this case it is the socket that fills out addr, the
   sockaddr_in structure.

   The return value is an integer. Indeed, the accept returns a new
   socket. You will use this new socket to communicate with the
   client.

   What happens to the old socket? It continues to listen for more
   requests (remember the backlog variable we passed to listen?)
   until we close it.

   Now, the new socket is meant only for communications. It is fully
   connected. We cannot pass it to listen again, trying to accept
   additional connections.

     --------------------------------------------------------------

      7.5.1.3.4 Our First Server

   Our first server will be somewhat more complex than our first
   client was: Not only do we have more sockets functions to use, but
   we need to write it as a daemon.

   This is best achieved by creating a child process after binding
   the port. The main process then exits and returns control to the
   shell (or whatever program invoked it).

   The child calls listen, then starts an endless loop, which accepts
   a connection, serves it, and eventually closes its socket.

 /*
  * daytimed - a port 13 server
  *
  * Programmed by G. Adam Stanislav
  * June 19, 2001
  */
 #include <stdio.h>
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>

 #define BACKLOG 4

 int main() {
     register int s, c;
     int b;
     struct sockaddr_in sa;
     time_t t;
     struct tm *tm;
     FILE *client;

     if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
         perror("socket");
         return 1;
     }

     bzero(&sa, sizeof sa);

     sa.sin_family = AF_INET;
     sa.sin_port   = htons(13);

     if (INADDR_ANY)
         sa.sin_addr.s_addr = htonl(INADDR_ANY);

     if (bind(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
         perror("bind");
         return 2;
     }

     switch (fork()) {
         case -1:
             perror("fork");
             return 3;
             break;
         default:
             close(s);
             return 0;
             break;
         case 0:
             break;
     }

     listen(s, BACKLOG);

     for (;;) {
         b = sizeof sa;

         if ((c = accept(s, (struct sockaddr *)&sa, &b)) < 0) {
             perror("daytimed accept");
             return 4;
         }

         if ((client = fdopen(c, "w")) == NULL) {
             perror("daytimed fdopen");
             return 5;
         }

         if ((t = time(NULL)) < 0) {
             perror("daytimed time");

             return 6;
         }

         tm = gmtime(&t);
         fprintf(client, "%.4i-%.2i-%.2iT%.2i:%.2i:%.2iZ\n",
             tm->tm_year + 1900,
             tm->tm_mon + 1,
             tm->tm_mday,
             tm->tm_hour,
             tm->tm_min,
             tm->tm_sec);

         fclose(client);
     }
 }

   We start by creating a socket. Then we fill out the sockaddr_in
   structure in sa. Note the conditional use of INADDR_ANY:

     if (INADDR_ANY)
         sa.sin_addr.s_addr = htonl(INADDR_ANY);

   Its value is 0. Since we have just used bzero on the entire
   structure, it would be redundant to set it to 0 again. But if we
   port our code to some other system where INADDR_ANY is perhaps not
   a zero, we need to assign it to sa.sin_addr.s_addr. Most modern C
   compilers are clever enough to notice that INADDR_ANY is a
   constant. As long as it is a zero, they will optimize the entire
   conditional statement out of the code.

   After we have called bind successfully, we are ready to become a
   daemon: We use fork to create a child process. In both, the parent
   and the child, the s variable is our socket. The parent process
   will not need it, so it calls close, then it returns 0 to inform
   its own parent it had terminated successfully.

   Meanwhile, the child process continues working in the background.
   It calls listen and sets its backlog to 4. It does not need a
   large value here because daytime is not a protocol many clients
   request all the time, and because it can process each request
   instantly anyway.

   Finally, the daemon starts an endless loop, which performs the
   following steps:

    1. Call accept. It waits here until a client contacts it. At that
       point, it receives a new socket, c, which it can use to
       communicate with this particular client.

    2. It uses the C function fdopen to turn the socket from a
       low-level file descriptor to a C-style FILE pointer. This will
       allow the use of fprintf later on.

    3. It checks the time, and prints it in the ISO 8601 format to
       the client "file". It then uses fclose to close the file. That
       will automatically close the socket as well.

   We can generalize this, and use it as a model for many other
   servers:

 +-----------------+
 |  Create Socket  |
 +-----------------+
           |
 +-----------------+
 |    Bind Port    |       Daemon Process
 +-----------------+
           |                 +--------+
           +-------------+-->|  Init  |
           |             |   +--------+
 +-----------------+     |         |
 |        Exit     |     |   +--------+
 +-----------------+     |   | Listen |
                         |   +--------+
                         |         |
                         |   +--------+
                         |   | Accept |
                         |   +--------+
                         |         |
                         |   +--------+
                         |   | Serve  |
                         |   +--------+
                         |         |
                         |   +--------+
                         |   | Close  |
                         |<--------+

   This flowchart is good for sequential servers, i.e., servers that
   can serve one client at a time, just as we were able to with our
   daytime server. This is only possible whenever there is no real
   "conversation" going on between the client and the server: As soon
   as the server detects a connection to the client, it sends out
   some data and closes the connection. The entire operation may take
   nanoseconds, and it is finished.

   The advantage of this flowchart is that, except for the brief
   moment after the parent forks and before it exits, there is always
   only one process active: Our server does not take up much memory
   and other system resources.

   Note that we have added initialize daemon in our flowchart. We did
   not need to initialize our own daemon, but this is a good place in
   the flow of the program to set up any signal handlers, open any
   files we may need, etc.

   Just about everything in the flow chart can be used literally on
   many different servers. The serve entry is the exception. We think
   of it as a "black box", i.e., something you design specifically
   for your own server, and just "plug it into the rest."

   Not all protocols are that simple. Many receive a request from the
   client, reply to it, then receive another request from the same
   client. Because of that, they do not know in advance how long they
   will be serving the client. Such servers usually start a new
   process for each client. While the new process is serving its
   client, the daemon can continue listening for more connections.

   Now, go ahead, save the above source code as daytimed.c (it is
   customary to end the names of daemons with the letter d). After
   you have compiled it, try running it:

 % ./daytimed
 bind: Permission denied
 %

   What happened here? As you will recall, the daytime protocol uses
   port 13. But all ports below 1024 are reserved to the superuser
   (otherwise, anyone could start a daemon pretending to serve a
   commonly used port, while causing a security breach).

   Try again, this time as the superuser:

 # ./daytimed
 #

   What... Nothing? Let us try again:

 # ./daytimed

 bind: Address already in use
 #

   Every port can only be bound by one program at a time. Our first
   attempt was indeed successful: It started the child daemon and
   returned quietly. It is still running and will continue to run
   until you either kill it, or any of its system calls fail, or you
   reboot the system.

   Fine, we know it is running in the background. But is it working?
   How do we know it is a proper daytime server? Simple:

 % telnet localhost 13

 Trying ::1...
 telnet: connect to address ::1: Connection refused
 Trying 127.0.0.1...
 Connected to localhost.
 Escape character is '^]'.
 2001-06-19T21:04:42Z
 Connection closed by foreign host.
 %

   telnet tried the new IPv6, and failed. It retried with IPv4 and
   succeeded. The daemon works.

   If you have access to another UNIX system via telnet, you can use
   it to test accessing the server remotely. My computer does not
   have a static IP address, so this is what I did:

 % who

 whizkid          ttyp0   Jun 19 16:59   (216.127.220.143)
 xxx              ttyp1   Jun 19 16:06   (xx.xx.xx.xx)
 % telnet 216.127.220.143 13

 Trying 216.127.220.143...
 Connected to r47.bfm.org.
 Escape character is '^]'.
 2001-06-19T21:31:11Z
 Connection closed by foreign host.
 %

   Again, it worked. Will it work using the domain name?

 % telnet r47.bfm.org 13

 Trying 216.127.220.143...
 Connected to r47.bfm.org.
 Escape character is '^]'.
 2001-06-19T21:31:40Z
 Connection closed by foreign host.
 %

   By the way, telnet prints the Connection closed by foreign host
   message after our daemon has closed the socket. This shows us
   that, indeed, using fclose(client); in our code works as
   advertised.

     --------------------------------------------------------------

7.6 Helper Functions

   FreeBSD C library contains many helper functions for sockets
   programming. For example, in our sample client we hard coded the
   time.nist.gov IP address. But we do not always know the IP
   address. Even if we do, our software is more flexible if it allows
   the user to enter the IP address, or even the domain name.

     --------------------------------------------------------------

  7.6.1 gethostbyname

   While there is no way to pass the domain name directly to any of
   the sockets functions, the FreeBSD C library comes with the
   gethostbyname(3) and gethostbyname2(3) functions, declared in
   netdb.h.

 struct hostent * gethostbyname(const char *name);
 struct hostent * gethostbyname2(const char *name, int af);

   Both return a pointer to the hostent structure, with much
   information about the domain. For our purposes, the h_addr_list[0]
   field of the structure points at h_length bytes of the correct
   address, already stored in the network byte order.

   This allows us to create a much more flexible-and much more
   useful-version of our daytime program:

 /*
  * daytime.c
  *
  * Programmed by G. Adam Stanislav
  * 19 June 2001
  */
 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <netdb.h>

 int main(int argc, char *argv[]) {
   register int s;
   register int bytes;
   struct sockaddr_in sa;
   struct hostent *he;
   char buf[BUFSIZ+1];
   char *host;

   if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
     perror("socket");
     return 1;
   }

   bzero(&sa, sizeof sa);

   sa.sin_family = AF_INET;
   sa.sin_port = htons(13);

   host = (argc > 1) ? (char *)argv[1] : "time.nist.gov";

   if ((he = gethostbyname(host)) == NULL) {
     herror(host);
     return 2;
   }

   bcopy(he->h_addr_list[0],&sa.sin_addr, he->h_length);

   if (connect(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
     perror("connect");
     return 3;
   }

   while ((bytes = read(s, buf, BUFSIZ)) > 0)
     write(1, buf, bytes);

   close(s);
   return 0;
 }

   We now can type a domain name (or an IP address, it works both
   ways) on the command line, and the program will try to connect to
   its daytime server. Otherwise, it will still default to
   time.nist.gov. However, even in this case we will use
   gethostbyname rather than hard coding 192.43.244.18. That way,
   even if its IP address changes in the future, we will still find
   it.

   Since it takes virtually no time to get the time from your local
   server, you could run daytime twice in a row: First to get the
   time from time.nist.gov, the second time from your own system. You
   can then compare the results and see how exact your system clock
   is:

 % daytime ; daytime localhost


 52080 01-06-20 04:02:33 50 0 0 390.2 UTC(NIST) *
 2001-06-20T04:02:35Z
 %

   As you can see, my system was two seconds ahead of the NIST time.

     --------------------------------------------------------------

  7.6.2 getservbyname

   Sometimes you may not be sure what port a certain service uses.
   The getservbyname(3) function, also declared in netdb.h comes in
   very handy in those cases:

 struct servent * getservbyname(const char *name, const char *proto);

   The servent structure contains the s_port, which contains the
   proper port, already in network byte order.

   Had we not known the correct port for the daytime service, we
   could have found it this way:

   struct servent *se;
   ...
   if ((se = getservbyname("daytime", "tcp")) == NULL {
     fprintf(stderr, "Cannot determine which port to use.\n");
     return 7;
   }
   sa.sin_port = se->s_port;

   You usually do know the port. But if you are developing a new
   protocol, you may be testing it on an unofficial port. Some day,
   you will register the protocol and its port (if nowhere else, at
   least in your /etc/services, which is where getservbyname looks).
   Instead of returning an error in the above code, you just use the
   temporary port number. Once you have listed the protocol in
   /etc/services, your software will find its port without you having
   to rewrite the code.

     --------------------------------------------------------------

7.7 Concurrent Servers

   Unlike a sequential server, a concurrent server has to be able to
   serve more than one client at a time. For example, a chat server
   may be serving a specific client for hours-it cannot wait till it
   stops serving a client before it serves the next one.

   This requires a significant change in our flowchart:

 +-----------------+
 |  Create Socket  |
 +-----------------+
           |
 +-----------------+
 |    Bind Port    |       Daemon Process
 +-----------------+
           |                 +--------+
           +-------------+-->|  Init  |
           |             |   +--------+
 +-----------------+     |         |
 |        Exit     |     |   +--------+
 +-----------------+     |   | Listen |
                         |   +--------+
                         |         |
                         |   +--------+
                         |   | Accept |
                         |   +--------+
                         |         |       +------------------+
                         |         +------>| Close Top Socket |
                         |         |       +------------------+
                         |   +--------+             |
                         |   | Close  |    +------------------+
                         |   +--------+    |     Serve        |
                         |         |       +------------------+
                         |<--------+                |
                                           +------------------+
                                           | Close Acc Socket |
                             +--------+    +------------------+
                             | Signal |             |
                             +--------+    +------------------+
                                           |      Exit        |
                                           +------------------+

   We moved the serve from the daemon process to its own server
   process. However, because each child process inherits all open
   files (and a socket is treated just like a file), the new process
   inherits not only the "accepted handle," i.e., the socket returned
   by the accept call, but also the top socket, i.e., the one opened
   by the top process right at the beginning.

   However, the server process does not need this socket and should
   close it immediately. Similarly, the daemon process no longer
   needs the accepted socket, and not only should, but must close
   it-otherwise, it will run out of available file descriptors sooner
   or later.

   After the server process is done serving, it should close the
   accepted socket. Instead of returning to accept, it now exits.

   Under UNIX, a process does not really exit. Instead, it returns to
   its parent. Typically, a parent process waits for its child
   process, and obtains a return value. However, our daemon process
   cannot simply stop and wait. That would defeat the whole purpose
   of creating additional processes. But if it never does wait, its
   children will become zombies-no longer functional but still
   roaming around.

   For that reason, the daemon process needs to set signal handlers
   in its initialize daemon phase. At least a SIGCHLD signal has to
   be processed, so the daemon can remove the zombie return values
   from the system and release the system resources they are taking
   up.

   That is why our flowchart now contains a process signals box,
   which is not connected to any other box. By the way, many servers
   also process SIGHUP, and typically interpret as the signal from
   the superuser that they should reread their configuration files.
   This allows us to change settings without having to kill and
   restart these servers.

     --------------------------------------------------------------

                        Chapter 8 IPv6 Internals

8.1 IPv6/IPsec Implementation

   Contributed by Yoshinobu Inoue.

   This section should explain IPv6 and IPsec related implementation
   internals. These functionalities are derived from KAME project

     --------------------------------------------------------------

  8.1.1 IPv6

    8.1.1.1 Conformance

   The IPv6 related functions conforms, or tries to conform to the
   latest set of IPv6 specifications. For future reference we list
   some of the relevant documents below (NOTE: this is not a complete
   list - this is too hard to maintain...).

   For details please refer to specific chapter in the document,
   RFCs, manual pages, or comments in the source code.

   Conformance tests have been performed on the KAME STABLE kit at
   TAHI project. Results can be viewed at
   http://www.tahi.org/report/KAME/. We also attended Univ. of New
   Hampshire IOL tests (http://www.iol.unh.edu/) in the past, with
   our past snapshots.

     * RFC1639: FTP Operation Over Big Address Records (FOOBAR)

          * RFC2428 is preferred over RFC1639. FTP clients will first
            try RFC2428, then RFC1639 if failed.

     * RFC1886: DNS Extensions to support IPv6

     * RFC1933: Transition Mechanisms for IPv6 Hosts and Routers

          * IPv4 compatible address is not supported.

          * automatic tunneling (described in 4.3 of this RFC) is not
            supported.

          * gif(4) interface implements IPv[46]-over-IPv[46] tunnel
            in a generic way, and it covers "configured tunnel"
            described in the spec. See 23.5.1.5 in this document for
            details.

     * RFC1981: Path MTU Discovery for IPv6

     * RFC2080: RIPng for IPv6

          * usr.sbin/route6d support this.

     * RFC2292: Advanced Sockets API for IPv6

          * For supported library functions/kernel APIs, see
            sys/netinet6/ADVAPI.

     * RFC2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)

          * RFC2362 defines packet formats for PIM-SM.
            draft-ietf-pim-ipv6-01.txt is written based on this.

     * RFC2373: IPv6 Addressing Architecture

          * supports node required addresses, and conforms to the
            scope requirement.

     * RFC2374: An IPv6 Aggregatable Global Unicast Address Format

          * supports 64-bit length of Interface ID.

     * RFC2375: IPv6 Multicast Address Assignments

          * Userland applications use the well-known addresses
            assigned in the RFC.

     * RFC2428: FTP Extensions for IPv6 and NATs

          * RFC2428 is preferred over RFC1639. FTP clients will first
            try RFC2428, then RFC1639 if failed.

     * RFC2460: IPv6 specification

     * RFC2461: Neighbor discovery for IPv6

          * See 23.5.1.2 in this document for details.

     * RFC2462: IPv6 Stateless Address Autoconfiguration

          * See 23.5.1.4 in this document for details.

     * RFC2463: ICMPv6 for IPv6 specification

          * See 23.5.1.9 in this document for details.

     * RFC2464: Transmission of IPv6 Packets over Ethernet Networks

     * RFC2465: MIB for IPv6: Textual Conventions and General Group

          * Necessary statistics are gathered by the kernel. Actual
            IPv6 MIB support is provided as a patchkit for ucd-snmp.

     * RFC2466: MIB for IPv6: ICMPv6 group

          * Necessary statistics are gathered by the kernel. Actual
            IPv6 MIB support is provided as patchkit for ucd-snmp.

     * RFC2467: Transmission of IPv6 Packets over FDDI Networks

     * RFC2497: Transmission of IPv6 packet over ARCnet Networks

     * RFC2553: Basic Socket Interface Extensions for IPv6

          * IPv4 mapped address (3.7) and special behavior of IPv6
            wildcard bind socket (3.8) are supported. See 23.5.1.12
            in this document for details.

     * RFC2675: IPv6 Jumbograms

          * See 23.5.1.7 in this document for details.

     * RFC2710: Multicast Listener Discovery for IPv6

     * RFC2711: IPv6 router alert option

     * draft-ietf-ipngwg-router-renum-08: Router renumbering for IPv6

     * draft-ietf-ipngwg-icmp-namelookups-02: IPv6 Name Lookups
       Through ICMP

     * draft-ietf-ipngwg-icmp-name-lookups-03: IPv6 Name Lookups
       Through ICMP

     * draft-ietf-pim-ipv6-01.txt: PIM for IPv6

          * pim6dd(8) implements dense mode. pim6sd(8) implements
            sparse mode.

     * draft-itojun-ipv6-tcp-to-anycast-00: Disconnecting TCP
       connection toward IPv6 anycast address

     * draft-yamamoto-wideipv6-comm-model-00

          * See 23.5.1.6 in this document for details.

     * draft-ietf-ipngwg-scopedaddr-format-00.txt : An Extension of
       Format for IPv6 Scoped Addresses

     --------------------------------------------------------------

    8.1.1.2 Neighbor Discovery

   Neighbor Discovery is fairly stable. Currently Address Resolution,
   Duplicated Address Detection, and Neighbor Unreachability
   Detection are supported. In the near future we will be adding
   Proxy Neighbor Advertisement support in the kernel and Unsolicited
   Neighbor Advertisement transmission command as admin tool.

   If DAD fails, the address will be marked "duplicated" and message
   will be generated to syslog (and usually to console). The
   "duplicated" mark can be checked with ifconfig(8). It is
   administrators' responsibility to check for and recover from DAD
   failures. The behavior should be improved in the near future.

   Some of the network driver loops multicast packets back to itself,
   even if instructed not to do so (especially in promiscuous mode).
   In such cases DAD may fail, because DAD engine sees inbound NS
   packet (actually from the node itself) and considers it as a sign
   of duplicate. You may want to look at #if condition marked
   "heuristics" in sys/netinet6/nd6_nbr.c:nd6_dad_timer() as
   workaround (note that the code fragment in "heuristics" section is
   not spec conformant).

   Neighbor Discovery specification (RFC2461) does not talk about
   neighbor cache handling in the following cases:

    1. when there was no neighbor cache entry, node received
       unsolicited RS/NS/NA/redirect packet without link-layer
       address

    2. neighbor cache handling on medium without link-layer address
       (we need a neighbor cache entry for IsRouter bit)

   For first case, we implemented workaround based on discussions on
   IETF ipngwg mailing list. For more details, see the comments in
   the source code and email thread started from (IPng 7155), dated
   Feb 6 1999.

   IPv6 on-link determination rule (RFC2461) is quite different from
   assumptions in BSD network code. At this moment, no on-link
   determination rule is supported where default router list is empty
   (RFC2461, section 5.2, last sentence in 2nd paragraph - note that
   the spec misuse the word "host" and "node" in several places in
   the section).

   To avoid possible DoS attacks and infinite loops, only 10 options
   on ND packet is accepted now. Therefore, if you have 20 prefix
   options attached to RA, only the first 10 prefixes will be
   recognized. If this troubles you, please ask it on FREEBSD-CURRENT
   mailing list and/or modify nd6_maxndopt in sys/netinet6/nd6.c. If
   there are high demands we may provide sysctl knob for the
   variable.

     --------------------------------------------------------------

    8.1.1.3 Scope Index

   IPv6 uses scoped addresses. Therefore, it is very important to
   specify scope index (interface index for link-local address, or
   site index for site-local address) with an IPv6 address. Without
   scope index, scoped IPv6 address is ambiguous to the kernel, and
   kernel will not be able to determine the outbound interface for a
   packet.

   Ordinary userland applications should use advanced API (RFC2292)
   to specify scope index, or interface index. For similar purpose,
   sin6_scope_id member in sockaddr_in6 structure is defined in
   RFC2553. However, the semantics for sin6_scope_id is rather vague.
   If you care about portability of your application, we suggest you
   to use advanced API rather than sin6_scope_id.

   In the kernel, an interface index for link-local scoped address is
   embedded into 2nd 16bit-word (3rd and 4th byte) in IPv6 address.
   For example, you may see something like:

         fe80:1::200:f8ff:fe01:6317
        

   in the routing table and interface address structure (struct
   in6_ifaddr). The address above is a link-local unicast address
   which belongs to a network interface whose interface identifier is
   1. The embedded index enables us to identify IPv6 link local
   addresses over multiple interfaces effectively and with only a
   little code change.

   Routing daemons and configuration programs, like route6d(8) and
   ifconfig(8), will need to manipulate the "embedded" scope index.
   These programs use routing sockets and ioctls (like
   SIOCGIFADDR_IN6) and the kernel API will return IPv6 addresses
   with 2nd 16bit-word filled in. The APIs are for manipulating
   kernel internal structure. Programs that use these APIs have to be
   prepared about differences in kernels anyway.

   When you specify scoped address to the command line, NEVER write
   the embedded form (such as ff02:1::1 or fe80:2::fedc). This is not
   supposed to work. Always use standard form, like ff02::1 or
   fe80::fedc, with command line option for specifying interface
   (like ping6 -I ne0 ff02::1). In general, if a command does not
   have command line option to specify outgoing interface, that
   command is not ready to accept scoped address. This may seem to be
   opposite from IPv6's premise to support "dentist office"
   situation. We believe that specifications need some improvements
   for this.

   Some of the userland tools support extended numeric IPv6 syntax,
   as documented in draft-ietf-ipngwg-scopedaddr-format-00.txt. You
   can specify outgoing link, by using name of the outgoing interface
   like "fe80::1%ne0". This way you will be able to specify
   link-local scoped address without much trouble.

   To use this extension in your program, you will need to use
   getaddrinfo(3), and getnameinfo(3) with NI_WITHSCOPEID. The
   implementation currently assumes 1-to-1 relationship between a
   link and an interface, which is stronger than what specs say.

     --------------------------------------------------------------

    8.1.1.4 Plug and Play

   Most of the IPv6 stateless address autoconfiguration is
   implemented in the kernel. Neighbor Discovery functions are
   implemented in the kernel as a whole. Router Advertisement (RA)
   input for hosts is implemented in the kernel. Router Solicitation
   (RS) output for endhosts, RS input for routers, and RA output for
   routers are implemented in the userland.

     --------------------------------------------------------------

      8.1.1.4.1 Assignment of link-local, and special addresses

   IPv6 link-local address is generated from IEEE802 address
   (Ethernet MAC address). Each of interface is assigned an IPv6
   link-local address automatically, when the interface becomes up
   (IFF_UP). Also, direct route for the link-local address is added
   to routing table.

   Here is an output of netstat command:

 Internet6:
 Destination                   Gateway                   Flags      Netif Expire
 fe80:1::%ed0/64               link#1                    UC          ed0
 fe80:2::%ep0/64               link#2                    UC          ep0

   Interfaces that has no IEEE802 address (pseudo interfaces like
   tunnel interfaces, or ppp interfaces) will borrow IEEE802 address
   from other interfaces, such as Ethernet interfaces, whenever
   possible. If there is no IEEE802 hardware attached, a last resort
   pseudo-random value, MD5(hostname), will be used as source of
   link-local address. If it is not suitable for your usage, you will
   need to configure the link-local address manually.

   If an interface is not capable of handling IPv6 (such as lack of
   multicast support), link-local address will not be assigned to
   that interface. See section 2 for details.

   Each interface joins the solicited multicast address and the
   link-local all-nodes multicast addresses (e.g. fe80::1:ff01:6317
   and ff02::1, respectively, on the link the interface is attached).
   In addition to a link-local address, the loopback address (::1)
   will be assigned to the loopback interface. Also, ::1/128 and
   ff01::/32 are automatically added to routing table, and loopback
   interface joins node-local multicast group ff01::1.

     --------------------------------------------------------------

      8.1.1.4.2 Stateless address autoconfiguration on hosts

   In IPv6 specification, nodes are separated into two categories:
   routers and hosts. Routers forward packets addressed to others,
   hosts does not forward the packets. net.inet6.ip6.forwarding
   defines whether this node is router or host (router if it is 1,
   host if it is 0).

   When a host hears Router Advertisement from the router, a host may
   autoconfigure itself by stateless address autoconfiguration. This
   behavior can be controlled by net.inet6.ip6.accept_rtadv (host
   autoconfigures itself if it is set to 1). By autoconfiguration,
   network address prefix for the receiving interface (usually global
   address prefix) is added. Default route is also configured.
   Routers periodically generate Router Advertisement packets. To
   request an adjacent router to generate RA packet, a host can
   transmit Router Solicitation. To generate a RS packet at any time,
   use the rtsol command. rtsold(8) daemon is also available.
   rtsold(8) generates Router Solicitation whenever necessary, and it
   works great for nomadic usage (notebooks/laptops). If one wishes
   to ignore Router Advertisements, use sysctl to set
   net.inet6.ip6.accept_rtadv to 0.

   To generate Router Advertisement from a router, use the rtadvd(8)
   daemon.

   Note that, IPv6 specification assumes the following items, and
   nonconforming cases are left unspecified:

     * Only hosts will listen to router advertisements

     * Hosts have single network interface (except loopback)

   Therefore, this is unwise to enable net.inet6.ip6.accept_rtadv on
   routers, or multi-interface host. A misconfigured node can behave
   strange (nonconforming configuration allowed for those who would
   like to do some experiments).

   To summarize the sysctl knob:

         accept_rtadv    forwarding      role of the node
         ---             ---             ---
         0               0               host (to be manually configured)
         0               1               router
         1               0               autoconfigured host
                                         (spec assumes that host has single
                                         interface only, autoconfigured host
                                         with multiple interface is
                                         out-of-scope)
         1               1               invalid, or experimental
                                         (out-of-scope of spec)

   RFC2462 has validation rule against incoming RA prefix information
   option, in 5.5.3 (e). This is to protect hosts from malicious (or
   misconfigured) routers that advertise very short prefix lifetime.
   There was an update from Jim Bound to ipngwg mailing list (look
   for "(ipng 6712)" in the archive) and it is implemented Jim's
   update.

   See 23.5.1.2 in the document for relationship between DAD and
   autoconfiguration.

     --------------------------------------------------------------

    8.1.1.5 Generic tunnel interface

   GIF (Generic InterFace) is a pseudo interface for configured
   tunnel. Details are described in gif(4). Currently

     * v6 in v6

     * v6 in v4

     * v4 in v6

     * v4 in v4

   are available. Use gifconfig(8) to assign physical (outer) source
   and destination address to gif interfaces. Configuration that uses
   same address family for inner and outer IP header (v4 in v4, or v6
   in v6) is dangerous. It is very easy to configure interfaces and
   routing tables to perform infinite level of tunneling. Please be
   warned.

   gif can be configured to be ECN-friendly. See 23.5.4.5 for
   ECN-friendliness of tunnels, and gif(4) for how to configure.

   If you would like to configure an IPv4-in-IPv6 tunnel with gif
   interface, read gif(4) carefully. You will need to remove IPv6
   link-local address automatically assigned to the gif interface.

     --------------------------------------------------------------

    8.1.1.6 Source Address Selection

   Current source selection rule is scope oriented (there are some
   exceptions - see below). For a given destination, a source IPv6
   address is selected by the following rule:

    1. If the source address is explicitly specified by the user
       (e.g. via the advanced API), the specified address is used.

    2. If there is an address assigned to the outgoing interface
       (which is usually determined by looking up the routing table)
       that has the same scope as the destination address, the
       address is used.

       This is the most typical case.

    3. If there is no address that satisfies the above condition,
       choose a global address assigned to one of the interfaces on
       the sending node.

    4. If there is no address that satisfies the above condition, and
       destination address is site local scope, choose a site local
       address assigned to one of the interfaces on the sending node.

    5. If there is no address that satisfies the above condition,
       choose the address associated with the routing table entry for
       the destination. This is the last resort, which may cause
       scope violation.

   For instance, ::1 is selected for ff01::1,
   fe80:1::200:f8ff:fe01:6317 for fe80:1::2a0:24ff:feab:839b (note
   that embedded interface index - described in 23.5.1.3 - helps us
   choose the right source address. Those embedded indices will not
   be on the wire). If the outgoing interface has multiple address
   for the scope, a source is selected longest match basis (rule 3).
   Suppose 2001:0DB8:808:1:200:f8ff:fe01:6317 and
   2001:0DB8:9:124:200:f8ff:fe01:6317 are given to the outgoing
   interface. 2001:0DB8:808:1:200:f8ff:fe01:6317 is chosen as the
   source for the destination 2001:0DB8:800::1.

   Note that the above rule is not documented in the IPv6 spec. It is
   considered "up to implementation" item. There are some cases where
   we do not use the above rule. One example is connected TCP
   session, and we use the address kept in tcb as the source. Another
   example is source address for Neighbor Advertisement. Under the
   spec (RFC2461 7.2.2) NA's source should be the target address of
   the corresponding NS's target. In this case we follow the spec
   rather than the above longest-match rule.

   For new connections (when rule 1 does not apply), deprecated
   addresses (addresses with preferred lifetime = 0) will not be
   chosen as source address if other choices are available. If no
   other choices are available, deprecated address will be used as a
   last resort. If there are multiple choice of deprecated addresses,
   the above scope rule will be used to choose from those deprecated
   addresses. If you would like to prohibit the use of deprecated
   address for some reason, configure net.inet6.ip6.use_deprecated to
   0. The issue related to deprecated address is described in RFC2462
   5.5.4 (NOTE: there is some debate underway in IETF ipngwg on how
   to use "deprecated" address).

     --------------------------------------------------------------

    8.1.1.7 Jumbo Payload

   The Jumbo Payload hop-by-hop option is implemented and can be used
   to send IPv6 packets with payloads longer than 65,535 octets. But
   currently no physical interface whose MTU is more than 65,535 is
   supported, so such payloads can be seen only on the loopback
   interface (i.e. lo0).

   If you want to try jumbo payloads, you first have to reconfigure
   the kernel so that the MTU of the loopback interface is more than
   65,535 bytes; add the following to the kernel configuration file:

   options "LARGE_LOMTU" #To test jumbo payload

   and recompile the new kernel.

   Then you can test jumbo payloads by the ping6(8) command with -b
   and -s options. The -b option must be specified to enlarge the
   size of the socket buffer and the -s option specifies the length
   of the packet, which should be more than 65,535. For example, type
   as follows:

 % ping6 -b 70000 -s 68000 ::1

   The IPv6 specification requires that the Jumbo Payload option must
   not be used in a packet that carries a fragment header. If this
   condition is broken, an ICMPv6 Parameter Problem message must be
   sent to the sender. specification is followed, but you cannot
   usually see an ICMPv6 error caused by this requirement.

   When an IPv6 packet is received, the frame length is checked and
   compared to the length specified in the payload length field of
   the IPv6 header or in the value of the Jumbo Payload option, if
   any. If the former is shorter than the latter, the packet is
   discarded and statistics are incremented. You can see the
   statistics as output of netstat(8) command with `-s -p ip6'
   option:

 % netstat -s -p ip6
           ip6:
                 (snip)
                 1 with data size < data length

   So, kernel does not send an ICMPv6 error unless the erroneous
   packet is an actual Jumbo Payload, that is, its packet size is
   more than 65,535 bytes. As described above, currently no physical
   interface with such a huge MTU is supported, so it rarely returns
   an ICMPv6 error.

   TCP/UDP over jumbogram is not supported at this moment. This is
   because we have no medium (other than loopback) to test this.
   Contact us if you need this.

   IPsec does not work on jumbograms. This is due to some
   specification twists in supporting AH with jumbograms (AH header
   size influences payload length, and this makes it real hard to
   authenticate inbound packet with jumbo payload option as well as
   AH).

   There are fundamental issues in *BSD support for jumbograms. We
   would like to address those, but we need more time to finalize
   these. To name a few:

     * mbuf pkthdr.len field is typed as "int" in 4.4BSD, so it will
       not hold jumbogram with len > 2G on 32bit architecture CPUs.
       If we would like to support jumbogram properly, the field must
       be expanded to hold 4G + IPv6 header + link-layer header.
       Therefore, it must be expanded to at least int64_t (u_int32_t
       is NOT enough).

     * We mistakingly use "int" to hold packet length in many places.
       We need to convert them into larger integral type. It needs a
       great care, as we may experience overflow during packet length
       computation.

     * We mistakingly check for ip6_plen field of IPv6 header for
       packet payload length in various places. We should be checking
       mbuf pkthdr.len instead. ip6_input() will perform sanity check
       on jumbo payload option on input, and we can safely use mbuf
       pkthdr.len afterwards.

     * TCP code needs a careful update in bunch of places, of course.

     --------------------------------------------------------------

    8.1.1.8 Loop prevention in header processing

   IPv6 specification allows arbitrary number of extension headers to
   be placed onto packets. If we implement IPv6 packet processing
   code in the way BSD IPv4 code is implemented, kernel stack may
   overflow due to long function call chain. sys/netinet6 code is
   carefully designed to avoid kernel stack overflow. Because of
   this, sys/netinet6 code defines its own protocol switch structure,
   as "struct ip6protosw" (see netinet6/ip6protosw.h). There is no
   such update to IPv4 part (sys/netinet) for compatibility, but
   small change is added to its pr_input() prototype. So "struct
   ipprotosw" is also defined. Because of this, if you receive
   IPsec-over-IPv4 packet with massive number of IPsec headers,
   kernel stack may blow up. IPsec-over-IPv6 is okay. (Off-course,
   for those all IPsec headers to be processed, each such IPsec
   header must pass each IPsec check. So an anonymous attacker will
   not be able to do such an attack.)

     --------------------------------------------------------------

    8.1.1.9 ICMPv6

   After RFC2463 was published, IETF ipngwg has decided to disallow
   ICMPv6 error packet against ICMPv6 redirect, to prevent ICMPv6
   storm on a network medium. This is already implemented into the
   kernel.

     --------------------------------------------------------------

    8.1.1.10 Applications

   For userland programming, we support IPv6 socket API as specified
   in RFC2553, RFC2292 and upcoming Internet drafts.

   TCP/UDP over IPv6 is available and quite stable. You can enjoy
   telnet(1), ftp(1), rlogin(1), rsh(1), ssh(1), etc. These
   applications are protocol independent. That is, they automatically
   chooses IPv4 or IPv6 according to DNS.

     --------------------------------------------------------------

    8.1.1.11 Kernel Internals

   While ip_forward() calls ip_output(), ip6_forward() directly calls
   if_output() since routers must not divide IPv6 packets into
   fragments.

   ICMPv6 should contain the original packet as long as possible up
   to 1280. UDP6/IP6 port unreach, for instance, should contain all
   extension headers and the *unchanged* UDP6 and IP6 headers. So,
   all IP6 functions except TCP never convert network byte order into
   host byte order, to save the original packet.

   tcp_input(), udp6_input() and icmp6_input() can not assume that
   IP6 header is preceding the transport headers due to extension
   headers. So, in6_cksum() was implemented to handle packets whose
   IP6 header and transport header is not continuous. TCP/IP6 nor
   UDP6/IP6 header structures do not exist for checksum calculation.

   To process IP6 header, extension headers and transport headers
   easily, network drivers are now required to store packets in one
   internal mbuf or one or more external mbufs. A typical old driver
   prepares two internal mbufs for 96 - 204 bytes data, however, now
   such packet data is stored in one external mbuf.

   netstat -s -p ip6 tells you whether or not your driver conforms
   such requirement. In the following example, "cce0" violates the
   requirement. (For more information, refer to Section 2.)

 Mbuf statistics:
                 317 one mbuf
                 two or more mbuf::
                         lo0 = 8
                         cce0 = 10
                 3282 one ext mbuf
                 0 two or more ext mbuf
        

   Each input function calls IP6_EXTHDR_CHECK in the beginning to
   check if the region between IP6 and its header is continuous.
   IP6_EXTHDR_CHECK calls m_pullup() only if the mbuf has M_LOOP
   flag, that is, the packet comes from the loopback interface.
   m_pullup() is never called for packets coming from physical
   network interfaces.

   Both IP and IP6 reassemble functions never call m_pullup().

     --------------------------------------------------------------

    8.1.1.12 IPv4 mapped address and IPv6 wildcard socket

   RFC2553 describes IPv4 mapped address (3.7) and special behavior
   of IPv6 wildcard bind socket (3.8). The spec allows you to:

     * Accept IPv4 connections by AF_INET6 wildcard bind socket.

     * Transmit IPv4 packet over AF_INET6 socket by using special
       form of the address like ::ffff:10.1.1.1.

   but the spec itself is very complicated and does not specify how
   the socket layer should behave. Here we call the former one
   "listening side" and the latter one "initiating side", for
   reference purposes.

   You can perform wildcard bind on both of the address families, on
   the same port.

   The following table show the behavior of FreeBSD 4.x.

 listening side          initiating side
                 (AF_INET6 wildcard      (connection to ::ffff:10.1.1.1)
                 socket gets IPv4 conn.)
                 ---                     ---
 FreeBSD 4.x     configurable            supported
                 default: enabled
        

   The following sections will give you more details, and how you can
   configure the behavior.

   Comments on listening side:

   It looks that RFC2553 talks too little on wildcard bind issue,
   especially on the port space issue, failure mode and relationship
   between AF_INET/INET6 wildcard bind. There can be several separate
   interpretation for this RFC which conform to it but behaves
   differently. So, to implement portable application you should
   assume nothing about the behavior in the kernel. Using
   getaddrinfo(3) is the safest way. Port number space and wildcard
   bind issues were discussed in detail on ipv6imp mailing list, in
   mid March 1999 and it looks that there is no concrete consensus
   (means, up to implementers). You may want to check the mailing
   list archives.

   If a server application would like to accept IPv4 and IPv6
   connections, there will be two alternatives.

   One is using AF_INET and AF_INET6 socket (you will need two
   sockets). Use getaddrinfo(3) with AI_PASSIVE into ai_flags, and
   socket(2) and bind(2) to all the addresses returned. By opening
   multiple sockets, you can accept connections onto the socket with
   proper address family. IPv4 connections will be accepted by
   AF_INET socket, and IPv6 connections will be accepted by AF_INET6
   socket.

   Another way is using one AF_INET6 wildcard bind socket. Use
   getaddrinfo(3) with AI_PASSIVE into ai_flags and with AF_INET6
   into ai_family, and set the 1st argument hostname to NULL. And
   socket(2) and bind(2) to the address returned. (should be IPv6
   unspecified addr). You can accept either of IPv4 and IPv6 packet
   via this one socket.

   To support only IPv6 traffic on AF_INET6 wildcard binded socket
   portably, always check the peer address when a connection is made
   toward AF_INET6 listening socket. If the address is IPv4 mapped
   address, you may want to reject the connection. You can check the
   condition by using IN6_IS_ADDR_V4MAPPED() macro.

   To resolve this issue more easily, there is system dependent
   setsockopt(2) option, IPV6_BINDV6ONLY, used like below.

         int on;

         setsockopt(s, IPPROTO_IPV6, IPV6_BINDV6ONLY,
                    (char *)&on, sizeof (on)) < 0));
        

   When this call succeed, then this socket only receive IPv6
   packets.

   Comments on initiating side:

   Advise to application implementers: to implement a portable IPv6
   application (which works on multiple IPv6 kernels), we believe
   that the following is the key to the success:

     * NEVER hardcode AF_INET nor AF_INET6.

     * Use getaddrinfo(3) and getnameinfo(3) throughout the system.
       Never use gethostby*(), getaddrby*(), inet_*() or
       getipnodeby*(). (To update existing applications to be IPv6
       aware easily, sometime getipnodeby*() will be useful. But if
       possible, try to rewrite the code to use getaddrinfo(3) and
       getnameinfo(3).)

     * If you would like to connect to destination, use
       getaddrinfo(3) and try all the destination returned, like
       telnet(1) does.

     * Some of the IPv6 stack is shipped with buggy getaddrinfo(3).
       Ship a minimal working version with your application and use
       that as last resort.

   If you would like to use AF_INET6 socket for both IPv4 and IPv6
   outgoing connection, you will need to use getipnodebyname(3). When
   you would like to update your existing application to be IPv6
   aware with minimal effort, this approach might be chosen. But
   please note that it is a temporal solution, because
   getipnodebyname(3) itself is not recommended as it does not handle
   scoped IPv6 addresses at all. For IPv6 name resolution,
   getaddrinfo(3) is the preferred API. So you should rewrite your
   application to use getaddrinfo(3), when you get the time to do it.

   When writing applications that make outgoing connections, story
   goes much simpler if you treat AF_INET and AF_INET6 as totally
   separate address family. {set,get}sockopt issue goes simpler, DNS
   issue will be made simpler. We do not recommend you to rely upon
   IPv4 mapped address.

     --------------------------------------------------------------

      8.1.1.12.1 unified tcp and inpcb code

   FreeBSD 4.x uses shared tcp code between IPv4 and IPv6 (from
   sys/netinet/tcp*) and separate udp4/6 code. It uses unified inpcb
   structure.

   The platform can be configured to support IPv4 mapped address.
   Kernel configuration is summarized as follows:

     * By default, AF_INET6 socket will grab IPv4 connections in
       certain condition, and can initiate connection to IPv4
       destination embedded in IPv4 mapped IPv6 address.

     * You can disable it on entire system with sysctl like below.

       sysctl net.inet6.ip6.mapped_addr=0

     --------------------------------------------------------------

        8.1.1.12.1.1 listening side

   Each socket can be configured to support special AF_INET6 wildcard
   bind (enabled by default). You can disable it on each socket basis
   with setsockopt(2) like below.

         int on;

         setsockopt(s, IPPROTO_IPV6, IPV6_BINDV6ONLY,
                    (char *)&on, sizeof (on)) < 0));
        

   Wildcard AF_INET6 socket grabs IPv4 connection if and only if the
   following conditions are satisfied:

     * there is no AF_INET socket that matches the IPv4 connection

     * the AF_INET6 socket is configured to accept IPv4 traffic, i.e.
       getsockopt(IPV6_BINDV6ONLY) returns 0.

   There is no problem with open/close ordering.

     --------------------------------------------------------------

        8.1.1.12.1.2 initiating side

   FreeBSD 4.x supports outgoing connection to IPv4 mapped address
   (::ffff:10.1.1.1), if the node is configured to support IPv4
   mapped address.

     --------------------------------------------------------------

    8.1.1.13 sockaddr_storage

   When RFC2553 was about to be finalized, there was discussion on
   how struct sockaddr_storage members are named. One proposal is to
   prepend "__" to the members (like "__ss_len") as they should not
   be touched. The other proposal was not to prepend it (like
   "ss_len") as we need to touch those members directly. There was no
   clear consensus on it.

   As a result, RFC2553 defines struct sockaddr_storage as follows:

         struct sockaddr_storage {
                 u_char  __ss_len;       /* address length */
                 u_char  __ss_family;    /* address family */
                 /* and bunch of padding */
         };
        

   On the contrary, XNET draft defines as follows:

         struct sockaddr_storage {
                 u_char  ss_len;         /* address length */
                 u_char  ss_family;      /* address family */
                 /* and bunch of padding */
         };
        

   In December 1999, it was agreed that RFC2553bis should pick the
   latter (XNET) definition.

   Current implementation conforms to XNET definition, based on
   RFC2553bis discussion.

   If you look at multiple IPv6 implementations, you will be able to
   see both definitions. As an userland programmer, the most portable
   way of dealing with it is to:

    1. ensure ss_family and/or ss_len are available on the platform,
       by using GNU autoconf,

    2. have -Dss_family=__ss_family to unify all occurrences
       (including header file) into __ss_family, or

    3. never touch __ss_family. cast to sockaddr * and use sa_family
       like:

         struct sockaddr_storage ss;
         family = ((struct sockaddr *)&ss)->sa_family
            

     --------------------------------------------------------------

  8.1.2 Network Drivers

   Now following two items are required to be supported by standard
   drivers:

    1. mbuf clustering requirement. In this stable release, we
       changed MINCLSIZE into MHLEN+1 for all the operating systems
       in order to make all the drivers behave as we expect.

    2. multicast. If ifmcstat(8) yields no multicast group for a
       interface, that interface has to be patched.

   If any of the drivers do not support the requirements, then the
   drivers can not be used for IPv6 and/or IPsec communication. If
   you find any problem with your card using IPv6/IPsec, then, please
   report it to the FreeBSD problem reports mailing list.

   (NOTE: In the past we required all PCMCIA drivers to have a call
   to in6_ifattach(). We have no such requirement any more)

     --------------------------------------------------------------

  8.1.3 Translator

   We categorize IPv4/IPv6 translator into 4 types:

     * Translator A --- It is used in the early stage of transition
       to make it possible to establish a connection from an IPv6
       host in an IPv6 island to an IPv4 host in the IPv4 ocean.

     * Translator B --- It is used in the early stage of transition
       to make it possible to establish a connection from an IPv4
       host in the IPv4 ocean to an IPv6 host in an IPv6 island.

     * Translator C --- It is used in the late stage of transition to
       make it possible to establish a connection from an IPv4 host
       in an IPv4 island to an IPv6 host in the IPv6 ocean.

     * Translator D --- It is used in the late stage of transition to
       make it possible to establish a connection from an IPv6 host
       in the IPv6 ocean to an IPv4 host in an IPv4 island.

   TCP relay translator for category A is supported. This is called
   "FAITH". We also provide IP header translator for category A. (The
   latter is not yet put into FreeBSD 4.x yet.)

     --------------------------------------------------------------

    8.1.3.1 FAITH TCP relay translator

   FAITH system uses TCP relay daemon called faithd(8) helped by the
   kernel. FAITH will reserve an IPv6 address prefix, and relay TCP
   connection toward that prefix to IPv4 destination.

   For example, if the reserved IPv6 prefix is 2001:0DB8:0200:ffff::,
   and the IPv6 destination for TCP connection is
   2001:0DB8:0200:ffff::163.221.202.12, the connection will be
   relayed toward IPv4 destination 163.221.202.12.

         destination IPv4 node (163.221.202.12)
           ^
           | IPv4 tcp toward 163.221.202.12
         FAITH-relay dual stack node
           ^
           | IPv6 TCP toward 2001:0DB8:0200:ffff::163.221.202.12
         source IPv6 node
        

   faithd(8) must be invoked on FAITH-relay dual stack node.

   For more details, consult src/usr.sbin/faithd/README

     --------------------------------------------------------------

  8.1.4 IPsec

   IPsec is mainly organized by three components.

    1. Policy Management

    2. Key Management

    3. AH and ESP handling

     --------------------------------------------------------------

    8.1.4.1 Policy Management

   The kernel implements experimental policy management code. There
   are two way to manage security policy. One is to configure
   per-socket policy using setsockopt(2). In this cases, policy
   configuration is described in ipsec_set_policy(3). The other is to
   configure kernel packet filter-based policy using PF_KEY
   interface, via setkey(8).

   The policy entry is not re-ordered with its indexes, so the order
   of entry when you add is very significant.

     --------------------------------------------------------------

    8.1.4.2 Key Management

   The key management code implemented in this kit (sys/netkey) is a
   home-brew PFKEY v2 implementation. This conforms to RFC2367.

   The home-brew IKE daemon, "racoon" is included in the kit
   (kame/kame/racoon). Basically you will need to run racoon as
   daemon, then set up a policy to require keys (like ping -P 'out
   ipsec esp/transport//use'). The kernel will contact racoon daemon
   as necessary to exchange keys.

     --------------------------------------------------------------

    8.1.4.3 AH and ESP handling

   IPsec module is implemented as "hooks" to the standard IPv4/IPv6
   processing. When sending a packet, ip{,6}_output() checks if
   ESP/AH processing is required by checking if a matching SPD
   (Security Policy Database) is found. If ESP/AH is needed,
   {esp,ah}{4,6}_output() will be called and mbuf will be updated
   accordingly. When a packet is received, {esp,ah}4_input() will be
   called based on protocol number, i.e. (*inetsw[proto])().
   {esp,ah}4_input() will decrypt/check authenticity of the packet,
   and strips off daisy-chained header and padding for ESP/AH. It is
   safe to strip off the ESP/AH header on packet reception, since we
   will never use the received packet in "as is" form.

   By using ESP/AH, TCP4/6 effective data segment size will be
   affected by extra daisy-chained headers inserted by ESP/AH. Our
   code takes care of the case.

   Basic crypto functions can be found in directory "sys/crypto".
   ESP/AH transform are listed in {esp,ah}_core.c with wrapper
   functions. If you wish to add some algorithm, add wrapper function
   in {esp,ah}_core.c, and add your crypto algorithm code into
   sys/crypto.

   Tunnel mode is partially supported in this release, with the
   following restrictions:

     * IPsec tunnel is not combined with GIF generic tunneling
       interface. It needs a great care because we may create an
       infinite loop between ip_output() and tunnelifp->if_output().
       Opinion varies if it is better to unify them, or not.

     * MTU and Don't Fragment bit (IPv4) considerations need more
       checking, but basically works fine.

     * Authentication model for AH tunnel must be revisited. We will
       need to improve the policy management engine, eventually.

     --------------------------------------------------------------

    8.1.4.4 Conformance to RFCs and IDs

   The IPsec code in the kernel conforms (or, tries to conform) to
   the following standards:

   "old IPsec" specification documented in rfc182[5-9].txt

   "new IPsec" specification documented in rfc240[1-6].txt,
   rfc241[01].txt, rfc2451.txt and
   draft-mcdonald-simple-ipsec-api-01.txt (draft expired, but you can
   take from   ftp://ftp.kame.net/pub/internet-drafts/). (NOTE: IKE
   specifications, rfc241[7-9].txt are implemented in userland, as
   "racoon" IKE daemon)

   Currently supported algorithms are:

     * old IPsec AH

          * null crypto checksum (no document, just for debugging)

          * keyed MD5 with 128bit crypto checksum (rfc1828.txt)

          * keyed SHA1 with 128bit crypto checksum (no document)

          * HMAC MD5 with 128bit crypto checksum (rfc2085.txt)

          * HMAC SHA1 with 128bit crypto checksum (no document)

     * old IPsec ESP

          * null encryption (no document, similar to rfc2410.txt)

          * DES-CBC mode (rfc1829.txt)

     * new IPsec AH

          * null crypto checksum (no document, just for debugging)

          * keyed MD5 with 96bit crypto checksum (no document)

          * keyed SHA1 with 96bit crypto checksum (no document)

          * HMAC MD5 with 96bit crypto checksum (rfc2403.txt)

          * HMAC SHA1 with 96bit crypto checksum (rfc2404.txt)

     * new IPsec ESP

          * null encryption (rfc2410.txt)

          * DES-CBC with derived IV
            (draft-ietf-ipsec-ciph-des-derived-01.txt, draft expired)

          * DES-CBC with explicit IV (rfc2405.txt)

          * 3DES-CBC with explicit IV (rfc2451.txt)

          * BLOWFISH CBC (rfc2451.txt)

          * CAST128 CBC (rfc2451.txt)

          * RC5 CBC (rfc2451.txt)

          * each of the above can be combined with:

               * ESP authentication with HMAC-MD5(96bit)

               * ESP authentication with HMAC-SHA1(96bit)

   The following algorithms are NOT supported:

     * old IPsec AH

          * HMAC MD5 with 128bit crypto checksum + 64bit replay
            prevention (rfc2085.txt)

          * keyed SHA1 with 160bit crypto checksum + 32bit padding
            (rfc1852.txt)

   IPsec (in kernel) and IKE (in userland as "racoon") has been
   tested at several interoperability test events, and it is known to
   interoperate with many other implementations well. Also, current
   IPsec implementation as quite wide coverage for IPsec crypto
   algorithms documented in RFC (we cover algorithms without
   intellectual property issues only).

     --------------------------------------------------------------

    8.1.4.5 ECN consideration on IPsec tunnels

   ECN-friendly IPsec tunnel is supported as described in
   draft-ipsec-ecn-00.txt.

   Normal IPsec tunnel is described in RFC2401. On encapsulation,
   IPv4 TOS field (or, IPv6 traffic class field) will be copied from
   inner IP header to outer IP header. On decapsulation outer IP
   header will be simply dropped. The decapsulation rule is not
   compatible with ECN, since ECN bit on the outer IP TOS/traffic
   class field will be lost.

   To make IPsec tunnel ECN-friendly, we should modify encapsulation
   and decapsulation procedure. This is described in  
   http://www.aciri.org/floyd/papers/draft-ipsec-ecn-00.txt, chapter
   3.

   IPsec tunnel implementation can give you three behaviors, by
   setting net.inet.ipsec.ecn (or net.inet6.ipsec6.ecn) to some
   value:

     * RFC2401: no consideration for ECN (sysctl value -1)

     * ECN forbidden (sysctl value 0)

     * ECN allowed (sysctl value 1)

   Note that the behavior is configurable in per-node manner, not
   per-SA manner (draft-ipsec-ecn-00 wants per-SA configuration, but
   it looks too much for me).

   The behavior is summarized as follows (see source code for more
   detail):

                 encapsulate                     decapsulate
                 ---                             ---
 RFC2401         copy all TOS bits               drop TOS bits on outer
                 from inner to outer.            (use inner TOS bits as is)

 ECN forbidden   copy TOS bits except for ECN    drop TOS bits on outer
                 (masked with 0xfc) from inner   (use inner TOS bits as is)
                 to outer.  set ECN bits to 0.

 ECN allowed     copy TOS bits except for ECN    use inner TOS bits with some
                 CE (masked with 0xfe) from      change.  if outer ECN CE bit
                 inner to outer.                 is 1, enable ECN CE bit on
                 set ECN CE bit to 0.            the inner.

        

   General strategy for configuration is as follows:

     * if both IPsec tunnel endpoint are capable of ECN-friendly
       behavior, you should better configure both end to "ECN
       allowed" (sysctl value 1).

     * if the other end is very strict about TOS bit, use "RFC2401"
       (sysctl value -1).

     * in other cases, use "ECN forbidden" (sysctl value 0).

   The default behavior is "ECN forbidden" (sysctl value 0).

   For more information, please refer to:

   http://www.aciri.org/floyd/papers/draft-ipsec-ecn-00.txt, RFC2481
   (Explicit Congestion Notification),
   src/sys/netinet6/{ah,esp}_input.c

   (Thanks goes to Kenjiro Cho <kjc@csl.sony.co.jp> for detailed
   analysis)

     --------------------------------------------------------------

    8.1.4.6 Interoperability

   Here are (some of) platforms that KAME code have tested IPsec/IKE
   interoperability in the past. Note that both ends may have
   modified their implementation, so use the following list just for
   reference purposes.

   Altiga, Ashley-laurent (vpcom.com), Data Fellows (F-Secure),
   Ericsson ACC, FreeS/WAN, HITACHI, IBM AIX(R), IIJ, Intel,
   Microsoft(R) Windows NT(R), NIST (linux IPsec + plutoplus),
   Netscreen, OpenBSD, RedCreek, Routerware, SSH, Secure Computing,
   Soliton, Toshiba, VPNet, Yamaha RT100i

                              III. Kernel

   Table of Contents

   9 Building and Installing a FreeBSD Kernel

   10 Kernel Debugging

     --------------------------------------------------------------

           Chapter 9 Building and Installing a FreeBSD Kernel

   Being a kernel developer requires understanding of the kernel
   build process. To debug the FreeBSD kernel it is required to be
   able to build one. There are two known ways to do so:

     * The "Traditional" Way

     * The "New" Way

     Note: It is supposed that the reader of this chapter is familiar
     with the information described in the Building and Installing a
     Custom Kernel chapter of the FreeBSD Handbook. If this is not
     the case, please read through the above mentioned chapter to
     understand how the build process works.

     --------------------------------------------------------------

9.1 Building a Kernel the "Traditional" Way

   Up to version 4.X of FreeBSD this was the recommended way to build
   a new kernel. It can still be used on newer versions (instead of
   the "buildkernel" target of the toplevel /usr/src/ makefiles).
   Building the kernel this way may be useful when working on the
   kernel code and it may actually be faster than the "New" procedure
   when only a single option or two were tweaked in the kernel
   configuration file. On the other hand, it might lead to unexpected
   kernel build breakage when used by beginners on newer versions of
   FreeBSD.

    1. Run config(8) to generate the kernel source code:

 # /usr/sbin/config MYKERNEL

    2. Change into the build directory. config(8) will print the name
       of this directory after being run as above.

 # cd ../compile/MYKERNEL

    3. Compile the kernel:

 # make depend
 # make

    4. Install the new kernel:

 # make install

     --------------------------------------------------------------

9.2 Building a Kernel the "New" Way

   This procedure is well supported and recommended under the latest
   FreeBSD releases and is documented in the Building and Installing
   a Custom Kernel chapter of the FreeBSD Handbook.

     --------------------------------------------------------------

                      Chapter 10 Kernel Debugging

   Contributed by Paul Richards, Jo:rg Wunsch, and Robert Watson.

10.1 Obtaining a Kernel Crash Dump

   When running a development kernel (e.g., FreeBSD-CURRENT), such as
   a kernel under extreme conditions (e.g., very high load averages,
   tens of thousands of connections, exceedingly high number of
   concurrent users, hundreds of jail(8)s, etc.), or using a new
   feature or device driver on FreeBSD-STABLE (e.g., PAE), sometimes
   a kernel will panic. In the event that it does, this chapter will
   demonstrate how to extract useful information out of a crash.

   A system reboot is inevitable once a kernel panics. Once a system
   is rebooted, the contents of a system's physical memory (RAM) is
   lost, as well as any bits that are on the swap device before the
   panic. To preserve the bits in physical memory, the kernel makes
   use of the swap device as a temporary place to store the bits that
   are in RAM across a reboot after a crash. In doing this, when
   FreeBSD boots after a crash, a kernel image can now be extracted
   and debugging can take place.

     Note: A swap device that has been configured as a dump device
     still acts as a swap device. Dumps to non-swap devices (such as
     tapes or CDRWs, for example) are not supported at this time. A
     "swap device" is synonymous with a "swap partition."

   Several types of kernel crash dumps are available: full memory
   dumps, which hold the complete contents of physical memory,
   minidumps, which hold only memory pages in use by the kernel
   (FreeBSD  6.2 and higher), and textdumps, which hold captured
   scripted or interactive debugger output (FreeBSD 7.1 and higher).
   Minidumps are the default dump type as of FreeBSD 7.0, and in most
   cases will capture all necessary information present in a full
   memory dump, as most problems can be isolated only using kernel
   state.

     --------------------------------------------------------------

  10.1.1 Configuring the Dump Device

   Before the kernel will dump the contents of its physical memory to
   a dump device, a dump device must be configured. A dump device is
   specified by using the dumpon(8) command to tell the kernel where
   to save kernel crash dumps. The dumpon(8) program must be called
   after the swap partition has been configured with swapon(8). This
   is normally handled by setting the dumpdev variable in rc.conf(5)
   to the path of the swap device (the recommended way to extract a
   kernel dump) or AUTO to use the first configured swap device. The
   default for dumpdev is AUTO in HEAD, and changed to NO on RELENG_*
   branches (except for RELENG_7, which was left set to AUTO). On
   FreeBSD 9.0-RELEASE and later versions, bsdinstall will ask
   whether crash dumps should be enabled on the target system during
   the install process.

     Tip: Check /etc/fstab or swapinfo(8) for a list of swap devices.

     Important: Make sure the dumpdir specified in rc.conf(5) exists
     before a kernel crash!

 # mkdir /var/crash
 # chmod 700 /var/crash

     Also, remember that the contents of /var/crash is sensitive and
     very likely contains confidential information such as passwords.

     --------------------------------------------------------------

  10.1.2 Extracting a Kernel Dump

   Once a dump has been written to a dump device, the dump must be
   extracted before the swap device is mounted. To extract a dump
   from a dump device, use the savecore(8) program. If dumpdev has
   been set in rc.conf(5), savecore(8) will be called automatically
   on the first multi-user boot after the crash and before the swap
   device is mounted. The location of the extracted core is placed in
   the rc.conf(5) value dumpdir, by default /var/crash and will be
   named vmcore.0.

   In the event that there is already a file called vmcore.0 in
   /var/crash (or whatever dumpdir is set to), the kernel will
   increment the trailing number for every crash to avoid overwriting
   an existing vmcore (e.g., vmcore.1). While debugging, it is highly
   likely that you will want to use the highest version vmcore in
   /var/crash when searching for the right vmcore.

     Tip: If you are testing a new kernel but need to boot a
     different one in order to get your system up and running again,
     boot it only into single user mode using the -s flag at the boot
     prompt, and then perform the following steps:

 # fsck -p
 # mount -a -t ufs       # make sure /var/crash is writable
 # savecore /var/crash /dev/ad0s1b
 # exit                  # exit to multi-user

     This instructs savecore(8) to extract a kernel dump from
     /dev/ad0s1b and place the contents in /var/crash. Do not forget
     to make sure the destination directory /var/crash has enough
     space for the dump. Also, do not forget to specify the correct
     path to your swap device as it is likely different than
     /dev/ad0s1b!

     --------------------------------------------------------------

10.2 Debugging a Kernel Crash Dump with kgdb

     Note: This section covers kgdb(1) as found in FreeBSD 5.3 and
     later. In previous versions, one must use gdb -k to read a core
     dump file.

   Once a dump has been obtained, getting useful information out of
   the dump is relatively easy for simple problems. Before launching
   into the internals of kgdb(1) to debug the crash dump, locate the
   debug version of your kernel (normally called kernel.debug) and
   the path to the source files used to build your kernel (normally
   /usr/obj/usr/src/sys/KERNCONF, where KERNCONF is the ident
   specified in a kernel config(5)). With those two pieces of info,
   let the debugging commence!

   To enter into the debugger and begin getting information from the
   dump, the following steps are required at a minimum:

 # cd /usr/obj/usr/src/sys/KERNCONF
 # kgdb kernel.debug /var/crash/vmcore.0

   You can debug the crash dump using the kernel sources just like
   you can for any other program.

   This first dump is from a 5.2-BETA kernel and the crash comes from
   deep within the kernel. The output below has been modified to
   include line numbers on the left. This first trace inspects the
   instruction pointer and obtains a back trace. The address that is
   used on line 41 for the list command is the instruction pointer
   and can be found on line 17. Most developers will request having
   at least this information sent to them if you are unable to debug
   the problem yourself. If, however, you do solve the problem, make
   sure that your patch winds its way into the source tree via a
   problem report, mailing lists, or by being able to commit it!

  1:# cd /usr/obj/usr/src/sys/KERNCONF
  2:# kgdb kernel.debug /var/crash/vmcore.0
  3:GNU gdb 5.2.1 (FreeBSD)
  4:Copyright 2002 Free Software Foundation, Inc.
  5:GDB is free software, covered by the GNU General Public License, and you are
  6:welcome to change it and/or distribute copies of it under certain conditions.
  7:Type "show copying" to see the conditions.
  8:There is absolutely no warranty for GDB.  Type "show warranty" for details.
  9:This GDB was configured as "i386-undermydesk-freebsd"...
 10:panic: page fault
 11:panic messages:
 12:---
 13:Fatal trap 12: page fault while in kernel mode
 14:cpuid = 0; apic id = 00
 15:fault virtual address   = 0x300
 16:fault code:             = supervisor read, page not present
 17:instruction pointer     = 0x8:0xc0713860
 18:stack pointer           = 0x10:0xdc1d0b70
 19:frame pointer           = 0x10:0xdc1d0b7c
 20:code segment            = base 0x0, limit 0xfffff, type 0x1b
 21:                        = DPL 0, pres 1, def32 1, gran 1
 22:processor eflags        = resume, IOPL = 0
 23:current process         = 14394 (uname)
 24:trap number             = 12
 25:panic: page fault
 26      cpuid = 0;
 27:Stack backtrace:
 28
 29:syncing disks, buffers remaining... 2199 2199 panic: mi_switch: switch in a critical section
 30:cpuid = 0;
 31:Uptime: 2h43m19s
 32:Dumping 255 MB
 33: 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
 34:---
 35:Reading symbols from /boot/kernel/snd_maestro3.ko...done.
 36:Loaded symbols for /boot/kernel/snd_maestro3.ko
 37:Reading symbols from /boot/kernel/snd_pcm.ko...done.
 38:Loaded symbols for /boot/kernel/snd_pcm.ko
 39:#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:240
 40:240             dumping++;
 41:(kgdb) list *0xc0713860
 42:0xc0713860 is in lapic_ipi_wait (/usr/src/sys/i386/i386/local_apic.c:663).
 43:658                     incr = 0;
 44:659                     delay = 1;
 45:660             } else
 46:661                     incr = 1;
 47:662             for (x = 0; x < delay; x += incr) {
 48:663                     if ((lapic->icr_lo & APIC_DELSTAT_MASK) == APIC_DELSTAT_IDLE)
 49:664                             return (1);
 50:665                     ia32_pause();
 51:666             }
 52:667             return (0);
 53:(kgdb) backtrace
 54:#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:240
 55:#1  0xc055fd9b in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:372
 56:#2  0xc056019d in panic () at /usr/src/sys/kern/kern_shutdown.c:550
 57:#3  0xc0567ef5 in mi_switch () at /usr/src/sys/kern/kern_synch.c:470
 58:#4  0xc055fa87 in boot (howto=256) at /usr/src/sys/kern/kern_shutdown.c:312
 59:#5  0xc056019d in panic () at /usr/src/sys/kern/kern_shutdown.c:550
 60:#6  0xc0720c66 in trap_fatal (frame=0xdc1d0b30, eva=0)
 61:    at /usr/src/sys/i386/i386/trap.c:821
 62:#7  0xc07202b3 in trap (frame=
 63:      {tf_fs = -1065484264, tf_es = -1065484272, tf_ds = -1065484272, tf_edi = 1, tf_esi = 0, tf_ebp = -602076292, tf_isp = -602076324, tf_ebx = 0, tf_edx = 0, tf_ecx = 1000000, tf_eax = 243, tf_trapno = 12, tf_err = 0, tf_eip = -1066321824, tf_cs = 8, tf_eflags = 65671, tf_esp = 243, tf_ss = 0})
 64:    at /usr/src/sys/i386/i386/trap.c:250
 65:#8  0xc070c9f8 in calltrap () at {standard input}:94
 66:#9  0xc07139f3 in lapic_ipi_vectored (vector=0, dest=0)
 67:    at /usr/src/sys/i386/i386/local_apic.c:733
 68:#10 0xc0718b23 in ipi_selected (cpus=1, ipi=1)
 69:    at /usr/src/sys/i386/i386/mp_machdep.c:1115
 70:#11 0xc057473e in kseq_notify (ke=0xcc05e360, cpu=0)
 71:    at /usr/src/sys/kern/sched_ule.c:520
 72:#12 0xc0575cad in sched_add (td=0xcbcf5c80)
 73:    at /usr/src/sys/kern/sched_ule.c:1366
 74:#13 0xc05666c6 in setrunqueue (td=0xcc05e360)
 75:    at /usr/src/sys/kern/kern_switch.c:422
 76:#14 0xc05752f4 in sched_wakeup (td=0xcbcf5c80)
 77:    at /usr/src/sys/kern/sched_ule.c:999
 78:#15 0xc056816c in setrunnable (td=0xcbcf5c80)
 79:    at /usr/src/sys/kern/kern_synch.c:570
 80:#16 0xc0567d53 in wakeup (ident=0xcbcf5c80)
 81:    at /usr/src/sys/kern/kern_synch.c:411
 82:#17 0xc05490a8 in exit1 (td=0xcbcf5b40, rv=0)
 83:    at /usr/src/sys/kern/kern_exit.c:509
 84:#18 0xc0548011 in sys_exit () at /usr/src/sys/kern/kern_exit.c:102
 85:#19 0xc0720fd0 in syscall (frame=
 86:      {tf_fs = 47, tf_es = 47, tf_ds = 47, tf_edi = 0, tf_esi = -1, tf_ebp = -1077940712, tf_isp = -602075788, tf_ebx = 672411944, tf_edx = 10, tf_ecx = 672411600, tf_eax = 1, tf_trapno = 12, tf_err = 2, tf_eip = 671899563, tf_cs = 31, tf_eflags = 642, tf_esp = -1077940740, tf_ss = 47})
 87:    at /usr/src/sys/i386/i386/trap.c:1010
 88:#20 0xc070ca4d in Xint0x80_syscall () at {standard input}:136
 89:---Can't read userspace from dump, or kernel process---
 90:(kgdb) quit

   This next trace is an older dump from the FreeBSD 2 time frame,
   but is more involved and demonstrates more of the features of gdb.
   Long lines have been folded to improve readability, and the lines
   are numbered for reference. Despite this, it is a real-world error
   trace taken during the development of the pcvt console driver.

  1:Script started on Fri Dec 30 23:15:22 1994
  2:# cd /sys/compile/URIAH
  3:# gdb -k kernel /var/crash/vmcore.1
  4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel
 ...done.
  5:IdlePTD 1f3000
  6:panic: because you said to!
  7:current pcb at 1e3f70
  8:Reading in symbols for ../../i386/i386/machdep.c...done.
  9:(kgdb) backtrace
 10:#0  boot (arghowto=256) (../../i386/i386/machdep.c line 767)
 11:#1  0xf0115159 in panic ()
 12:#2  0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
 13:#3  0xf010185e in db_fncall ()
 14:#4  0xf0101586 in db_command (-266509132, -266509516, -267381073)
 15:#5  0xf0101711 in db_command_loop ()
 16:#6  0xf01040a0 in db_trap ()
 17:#7  0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
 18:#8  0xf019d2eb in trap_fatal (...)
 19:#9  0xf019ce60 in trap_pfault (...)
 20:#10 0xf019cb2f in trap (...)
 21:#11 0xf01932a1 in exception:calltrap ()
 22:#12 0xf0191503 in cnopen (...)
 23:#13 0xf0132c34 in spec_open ()
 24:#14 0xf012d014 in vn_open ()
 25:#15 0xf012a183 in open ()
 26:#16 0xf019d4eb in syscall (...)
 27:(kgdb) up 10
 28:Reading in symbols for ../../i386/i386/trap.c...done.
 29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
 30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
 31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
 32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
 33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
 34:ss = -266427884}) (../../i386/i386/trap.c line 283)
 35:283                             (void) trap_pfault(&frame, FALSE);
 36:(kgdb) frame frame->tf_ebp frame->tf_eip
 37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
 38:#0  0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
 39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
 40:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
 41:(kgdb) list
 42:398
 43:399             tp->t_state |= TS_CARR_ON;
 44:400             tp->t_cflag |= CLOCAL;  /* cannot be a modem (:-) */
 45:401
 46:402     #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
 47:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
 48:404     #else
 49:405             return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
 50:406     #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
 51:407     }
 52:(kgdb) print tp
 53:Reading in symbols for ../../i386/i386/cons.c...done.
 54:$1 = (struct tty *) 0x1bae
 55:(kgdb) print tp->t_line
 56:$2 = 1767990816
 57:(kgdb) up
 58:#1  0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
 59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
 60:       return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
 61:(kgdb) up
 62:#2  0xf0132c34 in spec_open ()
 63:(kgdb) up
 64:#3  0xf012d014 in vn_open ()
 65:(kgdb) up
 66:#4  0xf012a183 in open ()
 67:(kgdb) up
 68:#5  0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
 69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
 70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
 71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
 72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
 73:673             error = (*callp->sy_call)(p, args, rval);
 74:(kgdb) up
 75:Initial frame selected; you cannot go up.
 76:(kgdb) quit

   Comments to the above script:

   line 6:

           This is a dump taken from within DDB (see below), hence
           the panic comment "because you said to!", and a rather
           long stack trace; the initial reason for going into DDB
           has been a page fault trap though.

   line 20:

           This is the location of function trap() in the stack
           trace.

   line 36:

           Force usage of a new stack frame; this is no longer
           necessary. The stack frames are supposed to point to the
           right locations now, even in case of a trap. From looking
           at the code in source line 403, there is a high
           probability that either the pointer access for "tp" was
           messed up, or the array access was out of bounds.

   line 52:

           The pointer looks suspicious, but happens to be a valid
           address.

   line 56:

           However, it obviously points to garbage, so we have found
           our error! (For those unfamiliar with that particular
           piece of code: tp->t_line refers to the line discipline of
           the console device here, which must be a rather small
           integer number.)

     Tip: If your system is crashing regularly and you are running
     out of disk space, deleting old vmcore files in /var/crash could
     save a considerable amount of disk space!

     --------------------------------------------------------------

10.3 Debugging a Crash Dump with DDD

   Examining a kernel crash dump with a graphical debugger like ddd
   is also possible (you will need to install the devel/ddd port in
   order to use the ddd debugger). Add the -k option to the ddd
   command line you would use normally. For example;

 # ddd --debugger kgdb kernel.debug /var/crash/vmcore.0

   You should then be able to go about looking at the crash dump
   using ddd's graphical interface.

     --------------------------------------------------------------

10.4 On-Line Kernel Debugging Using DDB

   While kgdb as an off-line debugger provides a very high level of
   user interface, there are some things it cannot do. The most
   important ones being breakpointing and single-stepping kernel
   code.

   If you need to do low-level debugging on your kernel, there is an
   on-line debugger available called DDB. It allows setting of
   breakpoints, single-stepping kernel functions, examining and
   changing kernel variables, etc. However, it cannot access kernel
   source files, and only has access to the global and static
   symbols, not to the full debug information like gdb does.

   To configure your kernel to include DDB, add the options

 options KDB

 options DDB

   to your config file, and rebuild. (See The FreeBSD Handbook for
   details on configuring the FreeBSD kernel).

     Note: If you have an older version of the boot blocks, your
     debugger symbols might not be loaded at all. Update the boot
     blocks; the recent ones load the DDB symbols automatically.

   Once your DDB kernel is running, there are several ways to enter
   DDB. The first, and earliest way is to type the boot flag -d right
   at the boot prompt. The kernel will start up in debug mode and
   enter DDB prior to any device probing. Hence you can even debug
   the device probe/attach functions. Users of FreeBSD-CURRENT will
   need to use the boot menu option, six, to escape to a command
   prompt.

   The second scenario is to drop to the debugger once the system has
   booted. There are two simple ways to accomplish this. If you would
   like to break to the debugger from the command prompt, simply type
   the command:

 # sysctl debug.kdb.enter=1

     Note: To force a panic on the fly, issue the following command:

 # sysctl debug.kdb.panic=1

   Alternatively, if you are at the system console, you may use a
   hot-key on the keyboard. The default break-to-debugger sequence is
   Ctrl+Alt+ESC. For syscons, this sequence can be remapped and some
   of the distributed maps out there do this, so check to make sure
   you know the right sequence to use. There is an option available
   for serial consoles that allows the use of a serial line BREAK on
   the console line to enter DDB (options BREAK_TO_DEBUGGER in the
   kernel config file). It is not the default since there are a lot
   of serial adapters around that gratuitously generate a BREAK
   condition, for example when pulling the cable.

   The third way is that any panic condition will branch to DDB if
   the kernel is configured to use it. For this reason, it is not
   wise to configure a kernel with DDB for a machine running
   unattended.

   To obtain the unattended functionality, add:

 options KDB_UNATTENDED

   to the kernel configuration file and rebuild/reinstall.

   The DDB commands roughly resemble some gdb commands. The first
   thing you probably need to do is to set a breakpoint:

 break function-name address

   Numbers are taken hexadecimal by default, but to make them
   distinct from symbol names; hexadecimal numbers starting with the
   letters a-f need to be preceded with 0x (this is optional for
   other numbers). Simple expressions are allowed, for example:
   function-name + 0x103.

   To exit the debugger and continue execution, type:

 continue

   To get a stack trace, use:

 trace

     Note: Note that when entering DDB via a hot-key, the kernel is
     currently servicing an interrupt, so the stack trace might be
     not of much use to you.

   If you want to remove a breakpoint, use

 del
 del address-expression

   The first form will be accepted immediately after a breakpoint
   hit, and deletes the current breakpoint. The second form can
   remove any breakpoint, but you need to specify the exact address;
   this can be obtained from:

 show b

   or:

 show break

   To single-step the kernel, try:

 s

   This will step into functions, but you can make DDB trace them
   until the matching return statement is reached by:

 n

     Note: This is different from gdb's next statement; it is like
     gdb's finish. Pressing n more than once will cause a continue.

   To examine data from memory, use (for example):

 x/wx 0xf0133fe0,40
 x/hd db_symtab_space
 x/bc termbuf,10
 x/s stringbuf

   for word/halfword/byte access, and hexadecimal/decimal/character/
   string display. The number after the comma is the object count. To
   display the next 0x10 items, simply use:

 x ,10

   Similarly, use

 x/ia foofunc,10

   to disassemble the first 0x10 instructions of foofunc, and display
   them along with their offset from the beginning of foofunc.

   To modify memory, use the write command:

 w/b termbuf 0xa 0xb 0
 w/w 0xf0010030 0 0

   The command modifier (b/h/w) specifies the size of the data to be
   written, the first following expression is the address to write to
   and the remainder is interpreted as data to write to successive
   memory locations.

   If you need to know the current registers, use:

 show reg

   Alternatively, you can display a single register value by e.g.

 p $eax

   and modify it by:

 set $eax new-value

   Should you need to call some kernel functions from DDB, simply
   say:

 call func(arg1, arg2, ...)

   The return value will be printed.

   For a ps(1) style summary of all running processes, use:

 ps

   Now you have examined why your kernel failed, and you wish to
   reboot. Remember that, depending on the severity of previous
   malfunctioning, not all parts of the kernel might still be working
   as expected. Perform one of the following actions to shut down and
   reboot your system:

 panic

   This will cause your kernel to dump core and reboot, so you can
   later analyze the core on a higher level with gdb. This command
   usually must be followed by another continue statement.

 call boot(0)

   Might be a good way to cleanly shut down the running system,
   sync() all disks, and finally, in some cases, reboot. As long as
   the disk and filesystem interfaces of the kernel are not damaged,
   this could be a good way for an almost clean shutdown.

 call cpu_reset()

   This is the final way out of disaster and almost the same as
   hitting the Big Red Button.

   If you need a short command summary, simply type:

 help

   It is highly recommended to have a printed copy of the ddb(4)
   manual page ready for a debugging session. Remember that it is
   hard to read the on-line manual while single-stepping the kernel.

     --------------------------------------------------------------

10.5 On-Line Kernel Debugging Using Remote GDB

   This feature has been supported since FreeBSD 2.2, and it is
   actually a very neat one.

   GDB has already supported remote debugging for a long time. This
   is done using a very simple protocol along a serial line. Unlike
   the other methods described above, you will need two machines for
   doing this. One is the host providing the debugging environment,
   including all the sources, and a copy of the kernel binary with
   all the symbols in it, and the other one is the target machine
   that simply runs a similar copy of the very same kernel (but
   stripped of the debugging information).

   You should configure the kernel in question with config -g if
   building the "traditional" way. If building the "new" way, make
   sure that makeoptions DEBUG=-g is in the configuration. In both
   cases, include DDB in the configuration, and compile it as usual.
   This gives a large binary, due to the debugging information. Copy
   this kernel to the target machine, strip the debugging symbols off
   with strip -x, and boot it using the -d boot option. Connect the
   serial line of the target machine that has "flags 080" set on its
   uart device to any serial line of the debugging host. See uart(4)
   for information on how to set the flags on an uart device. Now, on
   the debugging machine, go to the compile directory of the target
   kernel, and start gdb:

 % kgdb kernel
 GDB is free software and you are welcome to distribute copies of it
  under certain conditions; type "show copying" to see the conditions.
 There is absolutely no warranty for GDB; type "show warranty" for details.
 GDB 4.16 (i386-unknown-freebsd),
 Copyright 1996 Free Software Foundation, Inc...
 (kgdb)

   Initialize the remote debugging session (assuming the first serial
   port is being used) by:

 (kgdb) target remote /dev/cuau0

   Now, on the target host (the one that entered DDB right before
   even starting the device probe), type:

 Debugger("Boot flags requested debugger")
 Stopped at Debugger+0x35: movb  $0, edata+0x51bc
 db> gdb

   DDB will respond with:

 Next trap will enter GDB remote protocol mode

   Every time you type gdb, the mode will be toggled between remote
   GDB and local DDB. In order to force a next trap immediately,
   simply type s (step). Your hosting GDB will now gain control over
   the target kernel:

 Remote debugging using /dev/cuau0
 Debugger (msg=0xf01b0383 "Boot flags requested debugger")
     at ../../i386/i386/db_interface.c:257
 (kgdb)

   You can use this session almost as any other GDB session,
   including full access to the source, running it in gud-mode inside
   an Emacs window (which gives you an automatic source code display
   in another Emacs window), etc.

     --------------------------------------------------------------

10.6 Debugging a Console Driver

   Since you need a console driver to run DDB on, things are more
   complicated if the console driver itself is failing. You might
   remember the use of a serial console (either with modified boot
   blocks, or by specifying -h at the Boot: prompt), and hook up a
   standard terminal onto your first serial port. DDB works on any
   configured console driver, including a serial console.

     --------------------------------------------------------------

10.7 Debugging Deadlocks

   You may experience so called deadlocks, the situation where a
   system stops doing useful work. To provide a helpful bug report in
   this situation, use ddb(4) as described above. Include the output
   of ps and trace for suspected processes in the report.

   If possible, consider doing further investigation. The receipt
   below is especially useful if you suspect that a deadlock occurs
   in the VFS layer. Add the following options

 makeoptions             DEBUG=-g
         options         INVARIANTS
         options         INVARIANT_SUPPORT
         options         WITNESS
         options         DEBUG_LOCKS
         options         DEBUG_VFS_LOCKS
         options         DIAGNOSTIC

   to the kernel configuration file. When a deadlock occurs, in
   addition to the output of the ps command, provide information from
   the show pcpu, show allpcpu, show locks, show alllocks, show
   lockedvnods and alltrace.

   To obtain meaningful backtraces for threaded processes, use thread
   thread-id to switch to the thread stack, and do a backtrace with
   where.

     --------------------------------------------------------------

10.8 Kernel debugging with Dcons

   dcons(4) is a very simple console driver that is not directly
   connected with any physical devices. It just reads and writes
   characters from and to a buffer in a kernel or loader. Due to its
   simple nature, it is very useful for kernel debugging, especially
   with a FireWire(R) device. Currently, FreeBSD provides two ways to
   interact with the buffer from outside of the kernel using
   dconschat(8).

     --------------------------------------------------------------

  10.8.1 Dcons over FireWire(R)

   Most FireWire (IEEE1394) host controllers are based on the OHCI
   specification that supports physical access to the host memory.
   This means that once the host controller is initialized, we can
   access the host memory without the help of software (kernel). We
   can exploit this facility for interaction with dcons(4). dcons(4)
   provides similar functionality as a serial console. It emulates
   two serial ports, one for the console and DDB, the other for GDB.
   Because remote memory access is fully handled by the hardware, the
   dcons(4) buffer is accessible even when the system crashes.

   FireWire devices are not limited to those integrated into
   motherboards. PCI cards exist for desktops, and a cardbus
   interface can be purchased for laptops.

     --------------------------------------------------------------

    10.8.1.1 Enabling FireWire and Dcons support on the target machine

   To enable FireWire and Dcons support in the kernel of the target
   machine:

     * Make sure your kernel supports dcons, dcons_crom and firewire.
       Dcons should be statically linked with the kernel. For
       dcons_crom and firewire, modules should be OK.

     * Make sure physical DMA is enabled. You may need to add
       hw.firewire.phydma_enable=1 to /boot/loader.conf.

     * Add options for debugging.

     * Add dcons_gdb=1 in /boot/loader.conf if you use GDB over
       FireWire.

     * Enable dcons in /etc/ttys.

     * Optionally, to force dcons to be the high-level console, add
       hw.firewire.dcons_crom.force_console=1 to loader.conf.

   To enable FireWire and Dcons support in loader(8) on i386 or
   amd64:

   Add LOADER_FIREWIRE_SUPPORT=YES in /etc/make.conf and rebuild
   loader(8):

 # cd /sys/boot/i386 && make clean && make && make install

   To enable dcons(4) as an active low-level console, add
   boot_multicons="YES" to /boot/loader.conf.

   Here are a few configuration examples. A sample kernel
   configuration file would contain:

 device dcons
 device dcons_crom
 options KDB
 options DDB
 options GDB
 options ALT_BREAK_TO_DEBUGGER

   And a sample /boot/loader.conf would contain:

 dcons_crom_load="YES"
 dcons_gdb=1
 boot_multicons="YES"
 hw.firewire.phydma_enable=1
 hw.firewire.dcons_crom.force_console=1

     --------------------------------------------------------------

    10.8.1.2 Enabling FireWire and Dcons support on the host machine

   To enable FireWire support in the kernel on the host machine:

 # kldload firewire

   Find out the EUI64 (the unique 64 bit identifier) of the FireWire
   host controller, and use fwcontrol(8) or dmesg to find the EUI64
   of the target machine.

   Run dconschat(8), with:

 # dconschat -e \# -br -G 12345 -t 00-11-22-33-44-55-66-77

   The following key combinations can be used once dconschat(8) is
   running:

   +----------------------------------------------------------------+
   | ~ .                       | Disconnect                         |
   |---------------------------+------------------------------------|
   | ~ Ctrl+B                  | ALT BREAK                          |
   |---------------------------+------------------------------------|
   | ~ Ctrl+R                  | RESET target                       |
   |---------------------------+------------------------------------|
   | ~ Ctrl+Z                  | Suspend dconschat                  |
   +----------------------------------------------------------------+

   Attach remote GDB by starting kgdb(1) with a remote debugging
   session:

 kgdb -r :12345 kernel

     --------------------------------------------------------------

    10.8.1.3 Some general tips

   Here are some general tips:

   To take full advantage of the speed of FireWire, disable other
   slow console drivers:

 # conscontrol delete ttyd0           # serial console
 # conscontrol delete consolectl # video/keyboard

   There exists a GDB mode for emacs(1); this is what you will need
   to add to your .emacs:

 (setq gud-gdba-command-name "kgdb -a -a -a -r :12345")
 (setq gdb-many-windows t)
 (xterm-mouse-mode 1)
 M-x gdba

   And for DDD (devel/ddd):

 # remote serial protocol
 LANG=C ddd --debugger kgdb -r :12345 kernel
 # live core debug
 LANG=C ddd --debugger kgdb kernel /dev/fwmem0.2

     --------------------------------------------------------------

  10.8.2 Dcons with KVM

   We can directly read the dcons(4) buffer via /dev/mem for live
   systems, and in the core dump for crashed systems. These give you
   similar output to dmesg -a, but the dcons(4) buffer includes more
   information.

     --------------------------------------------------------------

    10.8.2.1 Using Dcons with KVM

   To use dcons(4) with KVM:

   Dump a dcons(4) buffer of a live system:

 # dconschat -1

   Dump a dcons(4) buffer of a crash dump:

 # dconschat -1 -M vmcore.XX

   Live core debugging can be done via:

 # fwcontrol -m target_eui64
 # kgdb kernel /dev/fwmem0.2

     --------------------------------------------------------------

10.9 Glossary of Kernel Options for Debugging

   This section provides a brief glossary of compile-time kernel
   options used for debugging:

     * options KDB: compiles in the kernel debugger framework.
       Required for options DDB and options GDB. Little or no
       performance overhead. By default, the debugger will be entered
       on panic instead of an automatic reboot.

     * options KDB_UNATTENDED: change the default value of the
       debug.debugger_on_panic sysctl to 0, which controls whether
       the debugger is entered on panic. When options KDB is not
       compiled into the kernel, the behavior is to automatically
       reboot on panic; when it is compiled into the kernel, the
       default behavior is to drop into the debugger unless options
       KDB_UNATTENDED is compiled in. If you want to leave the kernel
       debugger compiled into the kernel but want the system to come
       back up unless you're on-hand to use the debugger for
       diagnostics, use this option.

     * options KDB_TRACE: change the default value of the
       debug.trace_on_panic sysctl to 1, which controls whether the
       debugger automatically prints a stack trace on panic.
       Especially if running with options KDB_UNATTENDED, this can be
       helpful to gather basic debugging information on the serial or
       firewire console while still rebooting to recover.

     * options DDB: compile in support for the console debugger, DDB.
       This interactive debugger runs on whatever the active
       low-level console of the system is, which includes the video
       console, serial console, or firewire console. It provides
       basic integrated debugging facilities, such as stack tracing,
       process and thread listing, dumping of lock state, VM state,
       file system state, and kernel memory management. DDB does not
       require software running on a second machine or being able to
       generate a core dump or full debugging kernel symbols, and
       provides detailed diagnostics of the kernel at run-time. Many
       bugs can be fully diagnosed using only DDB output. This option
       depends on options KDB.

     * options GDB: compile in support for the remote debugger, GDB,
       which can operate over serial cable or firewire. When the
       debugger is entered, GDB may be attached to inspect structure
       contents, generate stack traces, etc. Some kernel state is
       more awkward to access than in DDB, which is able to generate
       useful summaries of kernel state automatically, such as
       automatically walking lock debugging or kernel memory
       management structures, and a second machine running the
       debugger is required. On the other hand, GDB combines
       information from the kernel source and full debugging symbols,
       and is aware of full data structure definitions, local
       variables, and is scriptable. This option is not required to
       run GDB on a kernel core dump. This option depends on options
       KDB.

     * options BREAK_TO_DEBUGGER, options ALT_BREAK_TO_DEBUGGER:
       allow a break signal or alternative signal on the console to
       enter the debugger. If the system hangs without a panic, this
       is a useful way to reach the debugger. Due to the current
       kernel locking, a break signal generated on a serial console
       is significantly more reliable at getting into the debugger,
       and is generally recommended. This option has little or no
       performance impact.

     * options INVARIANTS: compile into the kernel a large number of
       run-time assertion checks and tests, which constantly test the
       integrity of kernel data structures and the invariants of
       kernel algorithms. These tests can be expensive, so are not
       compiled in by default, but help provide useful "fail stop"
       behavior, in which certain classes of undesired behavior enter
       the debugger before kernel data corruption occurs, making them
       easier to debug. Tests include memory scrubbing and
       use-after-free testing, which is one of the more significant
       sources of overhead. This option depends on options
       INVARIANT_SUPPORT.

     * options INVARIANT_SUPPORT: many of the tests present in
       options INVARIANTS require modified data structures or
       additional kernel symbols to be defined.

     * options WITNESS: this option enables run-time lock order
       tracking and verification, and is an invaluable tool for
       deadlock diagnosis. WITNESS maintains a graph of acquired lock
       orders by lock type, and checks the graph at each acquire for
       cycles (implicit or explicit). If a cycle is detected, a
       warning and stack trace are generated to the console,
       indicating that a potential deadlock might have occurred.
       WITNESS is required in order to use the show locks, show
       witness and show alllocks DDB commands. This debug option has
       significant performance overhead, which may be somewhat
       mitigated through the use of options WITNESS_SKIPSPIN.
       Detailed documentation may be found in witness(4).

     * options WITNESS_SKIPSPIN: disable run-time checking of
       spinlock lock order with WITNESS. As spin locks are acquired
       most frequently in the scheduler, and scheduler events occur
       often, this option can significantly speed up systems running
       with WITNESS. This option depends on options WITNESS.

     * options WITNESS_KDB: change the default value of the
       debug.witness.kdb sysctl to 1, which causes WITNESS to enter
       the debugger when a lock order violation is detected, rather
       than simply printing a warning. This option depends on options
       WITNESS.

     * options SOCKBUF_DEBUG: perform extensive run-time consistency
       checking on socket buffers, which can be useful for debugging
       both socket bugs and race conditions in protocols and device
       drivers that interact with sockets. This option significantly
       impacts network performance, and may change the timing in
       device driver races.

     * options DEBUG_VFS_LOCKS: track lock acquisition points for
       lockmgr/vnode locks, expanding the amount of information
       displayed by show lockedvnods in DDB. This option has a
       measurable performance impact.

     * options DEBUG_MEMGUARD: a replacement for the malloc(9) kernel
       memory allocator that uses the VM system to detect reads or
       writes from allocated memory after free. Details may be found
       in memguard(9). This option has a significant performance
       impact, but can be very helpful in debugging kernel memory
       corruption bugs.

     * options DIAGNOSTIC: enable additional, more expensive
       diagnostic tests along the lines of options INVARIANTS.

                           IV. Architectures

   Table of Contents

   11 x86 Assembly Language Programming

     --------------------------------------------------------------

              Chapter 11 x86 Assembly Language Programming

   This chapter was written by G. Adam Stanislav
   <adam@redprince.net>.

     --------------------------------------------------------------

11.1 Synopsis

   Assembly language programming under UNIX is highly undocumented.
   It is generally assumed that no one would ever want to use it
   because various UNIX systems run on different microprocessors, so
   everything should be written in C for portability.

   In reality, C portability is quite a myth. Even C programs need to
   be modified when ported from one UNIX to another, regardless of
   what processor each runs on. Typically, such a program is full of
   conditional statements depending on the system it is compiled for.

   Even if we believe that all of UNIX software should be written in
   C, or some other high-level language, we still need assembly
   language programmers: Who else would write the section of C
   library that accesses the kernel?

   In this chapter I will attempt to show you how you can use
   assembly language writing UNIX programs, specifically under
   FreeBSD.

   This chapter does not explain the basics of assembly language.
   There are enough resources about that (for a complete online
   course in assembly language, see Randall Hyde's Art of Assembly
   Language; or if you prefer a printed book, take a look at Jeff
   Duntemann's Assembly Language Step-by-Step). However, once the
   chapter is finished, any assembly language programmer will be able
   to write programs for FreeBSD quickly and efficiently.

   Copyright (c) 2000-2001 G. Adam Stanislav. All rights reserved.

     --------------------------------------------------------------

11.2 The Tools

  11.2.1 The Assembler

   The most important tool for assembly language programming is the
   assembler, the software that converts assembly language code into
   machine language.

   Two very different assemblers are available for FreeBSD. One is
   as(1), which uses the traditional UNIX assembly language syntax.
   It comes with the system.

   The other is /usr/ports/devel/nasm. It uses the Intel syntax. Its
   main advantage is that it can assemble code for many operating
   systems. It needs to be installed separately, but is completely
   free.

   This chapter uses nasm syntax because most assembly language
   programmers coming to FreeBSD from other operating systems will
   find it easier to understand. And, because, quite frankly, that is
   what I am used to.

     --------------------------------------------------------------

  11.2.2 The Linker

   The output of the assembler, like that of any compiler, needs to
   be linked to form an executable file.

   The standard ld(1) linker comes with FreeBSD. It works with the
   code assembled with either assembler.

     --------------------------------------------------------------

11.3 System Calls

  11.3.1 Default Calling Convention

   By default, the FreeBSD kernel uses the C calling convention.
   Further, although the kernel is accessed using int 80h, it is
   assumed the program will call a function that issues int 80h,
   rather than issuing int 80h directly.

   This convention is very convenient, and quite superior to the
   Microsoft convention used by MS-DOS. Why? Because the UNIX
   convention allows any program written in any language to access
   the kernel.

   An assembly language program can do that as well. For example, we
   could open a file:

 kernel:
         int     80h     ; Call kernel
         ret

 open:
         push    dword mode
         push    dword flags
         push    dword path
         mov     eax, 5
         call    kernel
         add     esp, byte 12
         ret

   This is a very clean and portable way of coding. If you need to
   port the code to a UNIX system which uses a different interrupt,
   or a different way of passing parameters, all you need to change
   is the kernel procedure.

   But assembly language programmers like to shave off cycles. The
   above example requires a call/ret combination. We can eliminate it
   by pushing an extra dword:

 open:
         push    dword mode
         push    dword flags
         push    dword path
         mov     eax, 5
         push    eax             ; Or any other dword
         int     80h
         add     esp, byte 16

   The 5 that we have placed in EAX identifies the kernel function,
   in this case open.

     --------------------------------------------------------------

  11.3.2 Alternate Calling Convention

   FreeBSD is an extremely flexible system. It offers other ways of
   calling the kernel. For it to work, however, the system must have
   Linux emulation installed.

   Linux is a UNIX like system. However, its kernel uses the same
   system-call convention of passing parameters in registers MS-DOS
   does. As with the UNIX convention, the function number is placed
   in EAX. The parameters, however, are not passed on the stack but
   in EBX, ECX, EDX, ESI, EDI, EBP:

 open:
         mov     eax, 5
         mov     ebx, path
         mov     ecx, flags
         mov     edx, mode
         int     80h

   This convention has a great disadvantage over the UNIX way, at
   least as far as assembly language programming is concerned: Every
   time you make a kernel call you must push the registers, then pop
   them later. This makes your code bulkier and slower. Nevertheless,
   FreeBSD gives you a choice.

   If you do choose the Linux convention, you must let the system
   know about it. After your program is assembled and linked, you
   need to brand the executable:

 % brandelf -t Linux filename

     --------------------------------------------------------------

  11.3.3 Which Convention Should You Use?

   If you are coding specifically for FreeBSD, you should always use
   the UNIX convention: It is faster, you can store global variables
   in registers, you do not have to brand the executable, and you do
   not impose the installation of the Linux emulation package on the
   target system.

   If you want to create portable code that can also run on Linux,
   you will probably still want to give the FreeBSD users as
   efficient a code as possible. I will show you how you can
   accomplish that after I have explained the basics.

     --------------------------------------------------------------

  11.3.4 Call Numbers

   To tell the kernel which system service you are calling, place its
   number in EAX. Of course, you need to know what the number is.

     --------------------------------------------------------------

    11.3.4.1 The syscalls File

   The numbers are listed in syscalls. locate syscalls finds this
   file in several different formats, all produced automatically from
   syscalls.master.

   You can find the master file for the default UNIX calling
   convention in /usr/src/sys/kern/syscalls.master. If you need to
   use the other convention implemented in the Linux emulation mode,
   read /usr/src/sys/i386/linux/syscalls.master.

     Note: Not only do FreeBSD and Linux use different calling
     conventions, they sometimes use different numbers for the same
     functions.

   syscalls.master describes how the call is to be made:

 0       STD     NOHIDE  { int nosys(void); } syscall nosys_args int
 1       STD     NOHIDE  { void exit(int rval); } exit rexit_args void
 2       STD     POSIX   { int fork(void); }
 3       STD     POSIX   { ssize_t read(int fd, void *buf, size_t nbyte); }
 4       STD     POSIX   { ssize_t write(int fd, const void *buf, size_t nbyte); }
 5       STD     POSIX   { int open(char *path, int flags, int mode); }
 6       STD     POSIX   { int close(int fd); }
 etc...

   It is the leftmost column that tells us the number to place in
   EAX.

   The rightmost column tells us what parameters to push. They are
   pushed from right to left.

   For example, to open a file, we need to push the mode first, then
   flags, then the address at which the path is stored.

     --------------------------------------------------------------

11.4 Return Values

   A system call would not be useful most of the time if it did not
   return some kind of a value: The file descriptor of an open file,
   the number of bytes read to a buffer, the system time, etc.

   Additionally, the system needs to inform us if an error occurs: A
   file does not exist, system resources are exhausted, we passed an
   invalid parameter, etc.

     --------------------------------------------------------------

  11.4.1 Man Pages

   The traditional place to look for information about various system
   calls under UNIX systems are the manual pages. FreeBSD describes
   its system calls in section 2, sometimes in section 3.

   For example, open(2) says:

     If successful, open() returns a non-negative integer, termed a
     file descriptor. It returns -1 on failure, and sets errno to
     indicate the error.

   The assembly language programmer new to UNIX and FreeBSD will
   immediately ask the puzzling question: Where is errno and how do I
   get to it?

     Note: The information presented in the manual pages applies to C
     programs. The assembly language programmer needs additional
     information.

     --------------------------------------------------------------

  11.4.2 Where Are the Return Values?

   Unfortunately, it depends... For most system calls it is in EAX,
   but not for all. A good rule of thumb, when working with a system
   call for the first time, is to look for the return value in EAX.
   If it is not there, you need further research.

     Note: I am aware of one system call that returns the value in
     EDX: SYS_fork. All others I have worked with use EAX. But I have
     not worked with them all yet.

     Tip: If you cannot find the answer here or anywhere else, study
     libc source code and see how it interfaces with the kernel.

     --------------------------------------------------------------

  11.4.3 Where Is errno?

   Actually, nowhere...

   errno is part of the C language, not the UNIX kernel. When
   accessing kernel services directly, the error code is returned in
   EAX, the same register the proper return value generally ends up
   in.

   This makes perfect sense. If there is no error, there is no error
   code. If there is an error, there is no return value. One register
   can contain either.

     --------------------------------------------------------------

  11.4.4 Determining an Error Occurred

   When using the standard FreeBSD calling convention, the carry flag
   is cleared upon success, set upon failure.

   When using the Linux emulation mode, the signed value in EAX is
   non-negative upon success, and contains the return value. In case
   of an error, the value is negative, i.e., -errno.

     --------------------------------------------------------------

11.5 Creating Portable Code

   Portability is generally not one of the strengths of assembly
   language. Yet, writing assembly language programs for different
   platforms is possible, especially with nasm. I have written
   assembly language libraries that can be assembled for such
   different operating systems as Windows and FreeBSD.

   It is all the more possible when you want your code to run on two
   platforms which, while different, are based on similar
   architectures.

   For example, FreeBSD is UNIX, Linux is UNIX like. I only mentioned
   three differences between them (from an assembly language
   programmer's perspective): The calling convention, the function
   numbers, and the way of returning values.

     --------------------------------------------------------------

  11.5.1 Dealing with Function Numbers

   In many cases the function numbers are the same. However, even
   when they are not, the problem is easy to deal with: Instead of
   using numbers in your code, use constants which you have declared
   differently depending on the target architecture:

 %ifdef  LINUX
 %define SYS_execve      11
 %else
 %define SYS_execve      59
 %endif

     --------------------------------------------------------------

  11.5.2 Dealing with Conventions

   Both, the calling convention, and the return value (the errno
   problem) can be resolved with macros:

 %ifdef  LINUX

 %macro  system  0
         call    kernel
 %endmacro

 align 4
 kernel:
         push    ebx
         push    ecx
         push    edx
         push    esi
         push    edi
         push    ebp

         mov     ebx, [esp+32]
         mov     ecx, [esp+36]
         mov     edx, [esp+40]
         mov     esi, [esp+44]
         mov     ebp, [esp+48]
         int     80h

         pop     ebp
         pop     edi
         pop     esi
         pop     edx
         pop     ecx
         pop     ebx

         or      eax, eax
         js      .errno
         clc
         ret

 .errno:
         neg     eax
         stc
         ret

 %else

 %macro  system  0
         int     80h
 %endmacro

 %endif

     --------------------------------------------------------------

  11.5.3 Dealing with Other Portability Issues

   The above solutions can handle most cases of writing code portable
   between FreeBSD and Linux. Nevertheless, with some kernel services
   the differences are deeper.

   In that case, you need to write two different handlers for those
   particular system calls, and use conditional assembly. Luckily,
   most of your code does something other than calling the kernel, so
   usually you will only need a few such conditional sections in your
   code.

     --------------------------------------------------------------

  11.5.4 Using a Library

   You can avoid portability issues in your main code altogether by
   writing a library of system calls. Create a separate library for
   FreeBSD, a different one for Linux, and yet other libraries for
   more operating systems.

   In your library, write a separate function (or procedure, if you
   prefer the traditional assembly language terminology) for each
   system call. Use the C calling convention of passing parameters.
   But still use EAX to pass the call number in. In that case, your
   FreeBSD library can be very simple, as many seemingly different
   functions can be just labels to the same code:

 sys.open:
 sys.close:
 [etc...]
         int     80h
         ret

   Your Linux library will require more different functions. But even
   here you can group system calls using the same number of
   parameters:

 sys.exit:
 sys.close:
 [etc... one-parameter functions]
         push    ebx
         mov     ebx, [esp+12]
         int     80h
         pop     ebx
         jmp     sys.return

 ...

 sys.return:
         or      eax, eax
         js      sys.err
         clc
         ret

 sys.err:
         neg     eax
         stc
         ret

   The library approach may seem inconvenient at first because it
   requires you to produce a separate file your code depends on. But
   it has many advantages: For one, you only need to write it once
   and can use it for all your programs. You can even let other
   assembly language programmers use it, or perhaps use one written
   by someone else. But perhaps the greatest advantage of the library
   is that your code can be ported to other systems, even by other
   programmers, by simply writing a new library without any changes
   to your code.

   If you do not like the idea of having a library, you can at least
   place all your system calls in a separate assembly language file
   and link it with your main program. Here, again, all porters have
   to do is create a new object file to link with your main program.

     --------------------------------------------------------------

  11.5.5 Using an Include File

   If you are releasing your software as (or with) source code, you
   can use macros and place them in a separate file, which you
   include in your code.

   Porters of your software will simply write a new include file. No
   library or external object file is necessary, yet your code is
   portable without any need to edit the code.

     Note: This is the approach we will use throughout this chapter.
     We will name our include file system.inc, and add to it whenever
     we deal with a new system call.

   We can start our system.inc by declaring the standard file
   descriptors:

 %define stdin   0
 %define stdout  1
 %define stderr  2

   Next, we create a symbolic name for each system call:

 %define SYS_nosys       0
 %define SYS_exit        1
 %define SYS_fork        2
 %define SYS_read        3
 %define SYS_write       4
 ; [etc...]

   We add a short, non-global procedure with a long name, so we do
   not accidentally reuse the name in our code:

 section .text
 align 4
 access.the.bsd.kernel:
         int     80h
         ret

   We create a macro which takes one argument, the syscall number:

 %macro  system  1
         mov     eax, %1
         call    access.the.bsd.kernel
 %endmacro

   Finally, we create macros for each syscall. These macros take no
   arguments.

 %macro  sys.exit        0
         system  SYS_exit
 %endmacro

 %macro  sys.fork        0
         system  SYS_fork
 %endmacro

 %macro  sys.read        0
         system  SYS_read
 %endmacro

 %macro  sys.write       0
         system  SYS_write
 %endmacro

 ; [etc...]

   Go ahead, enter it into your editor and save it as system.inc. We
   will add more to it as we discuss more syscalls.

     --------------------------------------------------------------

11.6 Our First Program

   We are now ready for our first program, the mandatory Hello,
   World!

  1:     %include        'system.inc'
  2:
  3:     section .data
  4:     hello   db      'Hello, World!', 0Ah
  5:     hbytes  equ     $-hello
  6:
  7:     section .text
  8:     global  _start
  9:     _start:
 10:     push    dword hbytes
 11:     push    dword hello
 12:     push    dword stdout
 13:     sys.write
 14:
 15:     push    dword 0
 16:     sys.exit

   Here is what it does: Line 1 includes the defines, the macros, and
   the code from system.inc.

   Lines 3-5 are the data: Line 3 starts the data section/segment.
   Line 4 contains the string "Hello, World!" followed by a new line
   (0Ah). Line 5 creates a constant that contains the length of the
   string from line 4 in bytes.

   Lines 7-16 contain the code. Note that FreeBSD uses the elf file
   format for its executables, which requires every program to start
   at the point labeled _start (or, more precisely, the linker
   expects that). This label has to be global.

   Lines 10-13 ask the system to write hbytes bytes of the hello
   string to stdout.

   Lines 15-16 ask the system to end the program with the return
   value of 0. The SYS_exit syscall never returns, so the code ends
   there.

     Note: If you have come to UNIX from MS-DOS assembly language
     background, you may be used to writing directly to the video
     hardware. You will never have to worry about this in FreeBSD, or
     any other flavor of UNIX. As far as you are concerned, you are
     writing to a file known as stdout. This can be the video screen,
     or a telnet terminal, or an actual file, or even the input of
     another program. Which one it is, is for the system to figure
     out.

     --------------------------------------------------------------

  11.6.1 Assembling the Code

   Type the code (except the line numbers) in an editor, and save it
   in a file named hello.asm. You need nasm to assemble it.

     --------------------------------------------------------------

    11.6.1.1 Installing nasm

   If you do not have nasm, type:

 % su
 Password:your root password
 # cd /usr/ports/devel/nasm
 # make install
 # exit
 %

   You may type make install clean instead of just make install if
   you do not want to keep nasm source code.

   Either way, FreeBSD will automatically download nasm from the
   Internet, compile it, and install it on your system.

     Note: If your system is not FreeBSD, you need to get nasm from
     its home page. You can still use it to assemble FreeBSD code.

   Now you can assemble, link, and run the code:

 % nasm -f elf hello.asm
 % ld -s -o hello hello.o
 % ./hello
 Hello, World!
 %

     --------------------------------------------------------------

11.7 Writing UNIX(R) Filters

   A common type of UNIX application is a filter-a program that reads
   data from the stdin, processes it somehow, then writes the result
   to stdout.

   In this chapter, we shall develop a simple filter, and learn how
   to read from stdin and write to stdout. This filter will convert
   each byte of its input into a hexadecimal number followed by a
   blank space.

 %include        'system.inc'

 section .data
 hex     db      '0123456789ABCDEF'
 buffer  db      0, 0, ' '

 section .text
 global  _start
 _start:
         ; read a byte from stdin
         push    dword 1
         push    dword buffer
         push    dword stdin
         sys.read
         add     esp, byte 12
         or      eax, eax
         je      .done

         ; convert it to hex
         movzx   eax, byte [buffer]
         mov     edx, eax
         shr     dl, 4
         mov     dl, [hex+edx]
         mov     [buffer], dl
         and     al, 0Fh
         mov     al, [hex+eax]
         mov     [buffer+1], al

         ; print it
         push    dword 3
         push    dword buffer
         push    dword stdout
         sys.write
         add     esp, byte 12
         jmp     short _start

 .done:
         push    dword 0
         sys.exit

   In the data section we create an array called hex. It contains the
   16 hexadecimal digits in ascending order. The array is followed by
   a buffer which we will use for both input and output. The first
   two bytes of the buffer are initially set to 0. This is where we
   will write the two hexadecimal digits (the first byte also is
   where we will read the input). The third byte is a space.

   The code section consists of four parts: Reading the byte,
   converting it to a hexadecimal number, writing the result, and
   eventually exiting the program.

   To read the byte, we ask the system to read one byte from stdin,
   and store it in the first byte of the buffer. The system returns
   the number of bytes read in EAX. This will be 1 while data is
   coming, or 0, when no more input data is available. Therefore, we
   check the value of EAX. If it is 0, we jump to .done, otherwise we
   continue.

     Note: For simplicity sake, we are ignoring the possibility of an
     error condition at this time.

   The hexadecimal conversion reads the byte from the buffer into
   EAX, or actually just AL, while clearing the remaining bits of EAX
   to zeros. We also copy the byte to EDX because we need to convert
   the upper four bits (nibble) separately from the lower four bits.
   We store the result in the first two bytes of the buffer.

   Next, we ask the system to write the three bytes of the buffer,
   i.e., the two hexadecimal digits and the blank space, to stdout.
   We then jump back to the beginning of the program and process the
   next byte.

   Once there is no more input left, we ask the system to exit our
   program, returning a zero, which is the traditional value meaning
   the program was successful.

   Go ahead, and save the code in a file named hex.asm, then type the
   following (the ^D means press the control key and type D while
   holding the control key down):

 % nasm -f elf hex.asm
 % ld -s -o hex hex.o
 % ./hex
 Hello, World!
 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come!
 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D %

     Note: If you are migrating to UNIX from MS-DOS, you may be
     wondering why each line ends with 0A instead of 0D 0A. This is
     because UNIX does not use the cr/lf convention, but a "new line"
     convention, which is 0A in hexadecimal.

   Can we improve this? Well, for one, it is a bit confusing because
   once we have converted a line of text, our input no longer starts
   at the beginning of the line. We can modify it to print a new line
   instead of a space after each 0A:

 %include        'system.inc'

 section .data
 hex     db      '0123456789ABCDEF'
 buffer  db      0, 0, ' '

 section .text
 global  _start
 _start:
         mov     cl, ' '

 .loop:
         ; read a byte from stdin
         push    dword 1
         push    dword buffer
         push    dword stdin
         sys.read
         add     esp, byte 12
         or      eax, eax
         je      .done

         ; convert it to hex
         movzx   eax, byte [buffer]
         mov     [buffer+2], cl
         cmp     al, 0Ah
         jne     .hex
         mov     [buffer+2], al

 .hex:
         mov     edx, eax
         shr     dl, 4
         mov     dl, [hex+edx]
         mov     [buffer], dl
         and     al, 0Fh
         mov     al, [hex+eax]
         mov     [buffer+1], al

         ; print it
         push    dword 3
         push    dword buffer
         push    dword stdout
         sys.write
         add     esp, byte 12
         jmp     short .loop

 .done:
         push    dword 0
         sys.exit

   We have stored the space in the CL register. We can do this safely
   because, unlike Microsoft Windows, UNIX system calls do not modify
   the value of any register they do not use to return a value in.

   That means we only need to set CL once. We have, therefore, added
   a new label .loop and jump to it for the next byte instead of
   jumping at _start. We have also added the .hex label so we can
   either have a blank space or a new line as the third byte of the
   buffer.

   Once you have changed hex.asm to reflect these changes, type:

 % nasm -f elf hex.asm
 % ld -s -o hex hex.o
 % ./hex
 Hello, World!
 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
 Here I come!
 48 65 72 65 20 49 20 63 6F 6D 65 21 0A
 ^D %

   That looks better. But this code is quite inefficient! We are
   making a system call for every single byte twice (once to read it,
   another time to write the output).

     --------------------------------------------------------------

11.8 Buffered Input and Output

   We can improve the efficiency of our code by buffering our input
   and output. We create an input buffer and read a whole sequence of
   bytes at one time. Then we fetch them one by one from the buffer.

   We also create an output buffer. We store our output in it until
   it is full. At that time we ask the kernel to write the contents
   of the buffer to stdout.

   The program ends when there is no more input. But we still need to
   ask the kernel to write the contents of our output buffer to
   stdout one last time, otherwise some of our output would make it
   to the output buffer, but never be sent out. Do not forget that,
   or you will be wondering why some of your output is missing.

 %include        'system.inc'

 %define BUFSIZE 2048

 section .data
 hex     db      '0123456789ABCDEF'

 section .bss
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE

 section .text
 global  _start
 _start:
         sub     eax, eax
         sub     ebx, ebx
         sub     ecx, ecx
         mov     edi, obuffer

 .loop:
         ; read a byte from stdin
         call    getchar

         ; convert it to hex
         mov     dl, al
         shr     al, 4
         mov     al, [hex+eax]
         call    putchar

         mov     al, dl
         and     al, 0Fh
         mov     al, [hex+eax]
         call    putchar

         mov     al, ' '
         cmp     dl, 0Ah
         jne     .put
         mov     al, dl

 .put:
         call    putchar
         jmp     short .loop

 align 4
 getchar:
         or      ebx, ebx
         jne     .fetch

         call    read

 .fetch:
         lodsb
         dec     ebx
         ret

 read:
         push    dword BUFSIZE
         mov     esi, ibuffer
         push    esi
         push    dword stdin
         sys.read
         add     esp, byte 12
         mov     ebx, eax
         or      eax, eax
         je      .done
         sub     eax, eax
         ret

 align 4
 .done:
         call    write           ; flush output buffer
         push    dword 0
         sys.exit

 align 4
 putchar:
         stosb
         inc     ecx
         cmp     ecx, BUFSIZE
         je      write
         ret

 align 4
 write:
         sub     edi, ecx        ; start of buffer
         push    ecx
         push    edi
         push    dword stdout
         sys.write
         add     esp, byte 12
         sub     eax, eax
         sub     ecx, ecx        ; buffer is empty now
         ret

   We now have a third section in the source code, named .bss. This
   section is not included in our executable file, and, therefore,
   cannot be initialized. We use resb instead of db. It simply
   reserves the requested size of uninitialized memory for our use.

   We take advantage of the fact that the system does not modify the
   registers: We use registers for what, otherwise, would have to be
   global variables stored in the .data section. This is also why the
   UNIX convention of passing parameters to system calls on the stack
   is superior to the Microsoft convention of passing them in the
   registers: We can keep the registers for our own use.

   We use EDI and ESI as pointers to the next byte to be read from or
   written to. We use EBX and ECX to keep count of the number of
   bytes in the two buffers, so we know when to dump the output to,
   or read more input from, the system.

   Let us see how it works now:

 % nasm -f elf hex.asm
 % ld -s -o hex hex.o
 % ./hex
 Hello, World!
 Here I come!
 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
 48 65 72 65 20 49 20 63 6F 6D 65 21 0A
 ^D %

   Not what you expected? The program did not print the output until
   we pressed ^D. That is easy to fix by inserting three lines of
   code to write the output every time we have converted a new line
   to 0A. I have marked the three lines with > (do not copy the > in
   your hex.asm).

 %include        'system.inc'

 %define BUFSIZE 2048

 section .data
 hex     db      '0123456789ABCDEF'

 section .bss
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE

 section .text
 global  _start
 _start:
         sub     eax, eax
         sub     ebx, ebx
         sub     ecx, ecx
         mov     edi, obuffer

 .loop:
         ; read a byte from stdin
         call    getchar

         ; convert it to hex
         mov     dl, al
         shr     al, 4
         mov     al, [hex+eax]
         call    putchar

         mov     al, dl
         and     al, 0Fh
         mov     al, [hex+eax]
         call    putchar

         mov     al, ' '
         cmp     dl, 0Ah
         jne     .put
         mov     al, dl

 .put:
         call    putchar
 >       cmp     al, 0Ah
 >       jne     .loop
 >       call    write
         jmp     short .loop

 align 4
 getchar:
         or      ebx, ebx
         jne     .fetch

         call    read

 .fetch:
         lodsb
         dec     ebx
         ret

 read:
         push    dword BUFSIZE
         mov     esi, ibuffer
         push    esi
         push    dword stdin
         sys.read
         add     esp, byte 12
         mov     ebx, eax
         or      eax, eax
         je      .done
         sub     eax, eax
         ret

 align 4
 .done:
         call    write           ; flush output buffer
         push    dword 0
         sys.exit

 align 4
 putchar:
         stosb
         inc     ecx
         cmp     ecx, BUFSIZE
         je      write
         ret

 align 4
 write:
         sub     edi, ecx        ; start of buffer
         push    ecx
         push    edi
         push    dword stdout
         sys.write
         add     esp, byte 12
         sub     eax, eax
         sub     ecx, ecx        ; buffer is empty now
         ret

   Now, let us see how it works:

 % nasm -f elf hex.asm
 % ld -s -o hex hex.o
 % ./hex
 Hello, World!
 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
 Here I come!
 48 65 72 65 20 49 20 63 6F 6D 65 21 0A
 ^D %

   Not bad for a 644-byte executable, is it!

     Note: This approach to buffered input/output still contains a
     hidden danger. I will discuss-and fix-it later, when I talk
     about the dark side of buffering.

     --------------------------------------------------------------

  11.8.1 How to Unread a Character

     Warning: This may be a somewhat advanced topic, mostly of
     interest to programmers familiar with the theory of compilers.
     If you wish, you may skip to the next section, and perhaps read
     this later.

   While our sample program does not require it, more sophisticated
   filters often need to look ahead. In other words, they may need to
   see what the next character is (or even several characters). If
   the next character is of a certain value, it is part of the token
   currently being processed. Otherwise, it is not.

   For example, you may be parsing the input stream for a textual
   string (e.g., when implementing a language compiler): If a
   character is followed by another character, or perhaps a digit, it
   is part of the token you are processing. If it is followed by
   white space, or some other value, then it is not part of the
   current token.

   This presents an interesting problem: How to return the next
   character back to the input stream, so it can be read again later?

   One possible solution is to store it in a character variable, then
   set a flag. We can modify getchar to check the flag, and if it is
   set, fetch the byte from that variable instead of the input
   buffer, and reset the flag. But, of course, that slows us down.

   The C language has an ungetc() function, just for that purpose. Is
   there a quick way to implement it in our code? I would like you to
   scroll back up and take a look at the getchar procedure and see if
   you can find a nice and fast solution before reading the next
   paragraph. Then come back here and see my own solution.

   The key to returning a character back to the stream is in how we
   are getting the characters to start with:

   First we check if the buffer is empty by testing the value of EBX.
   If it is zero, we call the read procedure.

   If we do have a character available, we use lodsb, then decrease
   the value of EBX. The lodsb instruction is effectively identical
   to:

         mov     al, [esi]
         inc     esi

   The byte we have fetched remains in the buffer until the next time
   read is called. We do not know when that happens, but we do know
   it will not happen until the next call to getchar. Hence, to
   "return" the last-read byte back to the stream, all we have to do
   is decrease the value of ESI and increase the value of EBX:

 ungetc:
         dec     esi
         inc     ebx
         ret

   But, be careful! We are perfectly safe doing this if our
   look-ahead is at most one character at a time. If we are examining
   more than one upcoming character and call ungetc several times in
   a row, it will work most of the time, but not all the time (and
   will be tough to debug). Why?

   Because as long as getchar does not have to call read, all of the
   pre-read bytes are still in the buffer, and our ungetc works
   without a glitch. But the moment getchar calls read, the contents
   of the buffer change.

   We can always rely on ungetc working properly on the last
   character we have read with getchar, but not on anything we have
   read before that.

   If your program reads more than one byte ahead, you have at least
   two choices:

   If possible, modify the program so it only reads one byte ahead.
   This is the simplest solution.

   If that option is not available, first of all determine the
   maximum number of characters your program needs to return to the
   input stream at one time. Increase that number slightly, just to
   be sure, preferably to a multiple of 16-so it aligns nicely. Then
   modify the .bss section of your code, and create a small "spare"
   buffer right before your input buffer, something like this:

 section .bss
         resb    16      ; or whatever the value you came up with
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE

   You also need to modify your ungetc to pass the value of the byte
   to unget in AL:

 ungetc:
         dec     esi
         inc     ebx
         mov     [esi], al
         ret

   With this modification, you can call ungetc up to 17 times in a
   row safely (the first call will still be within the buffer, the
   remaining 16 may be either within the buffer or within the
   "spare").

     --------------------------------------------------------------

11.9 Command Line Arguments

   Our hex program will be more useful if it can read the names of an
   input and output file from its command line, i.e., if it can
   process the command line arguments. But... Where are they?

   Before a UNIX system starts a program, it pushes some data on the
   stack, then jumps at the _start label of the program. Yes, I said
   jumps, not calls. That means the data can be accessed by reading
   [esp+offset], or by simply popping it.

   The value at the top of the stack contains the number of command
   line arguments. It is traditionally called argc, for "argument
   count."

   Command line arguments follow next, all argc of them. These are
   typically referred to as argv, for "argument value(s)." That is,
   we get argv[0], argv[1], ..., argv[argc-1]. These are not the
   actual arguments, but pointers to arguments, i.e., memory
   addresses of the actual arguments. The arguments themselves are
   NUL-terminated character strings.

   The argv list is followed by a NULL pointer, which is simply a 0.
   There is more, but this is enough for our purposes right now.

     Note: If you have come from the MS-DOS programming environment,
     the main difference is that each argument is in a separate
     string. The second difference is that there is no practical
     limit on how many arguments there can be.

   Armed with this knowledge, we are almost ready for the next
   version of hex.asm. First, however, we need to add a few lines to
   system.inc:

   First, we need to add two new entries to our list of system call
   numbers:

 %define SYS_open        5
 %define SYS_close       6

   Then we add two new macros at the end of the file:

 %macro  sys.open        0
         system  SYS_open
 %endmacro

 %macro  sys.close       0
         system  SYS_close
 %endmacro

   Here, then, is our modified source code:

 %include        'system.inc'

 %define BUFSIZE 2048

 section .data
 fd.in   dd      stdin
 fd.out  dd      stdout
 hex     db      '0123456789ABCDEF'

 section .bss
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE

 section .text
 align 4
 err:
         push    dword 1         ; return failure
         sys.exit

 align 4
 global  _start
 _start:
         add     esp, byte 8     ; discard argc and argv[0]

         pop     ecx
         jecxz   .init           ; no more arguments

         ; ECX contains the path to input file
         push    dword 0         ; O_RDONLY
         push    ecx
         sys.open
         jc      err             ; open failed

         add     esp, byte 8
         mov     [fd.in], eax

         pop     ecx
         jecxz   .init           ; no more arguments

         ; ECX contains the path to output file
         push    dword 420       ; file mode (644 octal)
         push    dword 0200h | 0400h | 01h
         ; O_CREAT | O_TRUNC | O_WRONLY
         push    ecx
         sys.open
         jc      err

         add     esp, byte 12
         mov     [fd.out], eax

 .init:
         sub     eax, eax
         sub     ebx, ebx
         sub     ecx, ecx
         mov     edi, obuffer

 .loop:
         ; read a byte from input file or stdin
         call    getchar

         ; convert it to hex
         mov     dl, al
         shr     al, 4
         mov     al, [hex+eax]
         call    putchar

         mov     al, dl
         and     al, 0Fh
         mov     al, [hex+eax]
         call    putchar

         mov     al, ' '
         cmp     dl, 0Ah
         jne     .put
         mov     al, dl

 .put:
         call    putchar
         cmp     al, dl
         jne     .loop
         call    write
         jmp     short .loop

 align 4
 getchar:
         or      ebx, ebx
         jne     .fetch

         call    read

 .fetch:
         lodsb
         dec     ebx
         ret

 read:
         push    dword BUFSIZE
         mov     esi, ibuffer
         push    esi
         push    dword [fd.in]
         sys.read
         add     esp, byte 12
         mov     ebx, eax
         or      eax, eax
         je      .done
         sub     eax, eax
         ret

 align 4
 .done:
         call    write           ; flush output buffer

         ; close files
         push    dword [fd.in]
         sys.close

         push    dword [fd.out]
         sys.close

         ; return success
         push    dword 0
         sys.exit

 align 4
 putchar:
         stosb
         inc     ecx
         cmp     ecx, BUFSIZE
         je      write
         ret

 align 4
 write:
         sub     edi, ecx        ; start of buffer
         push    ecx
         push    edi
         push    dword [fd.out]
         sys.write
         add     esp, byte 12
         sub     eax, eax
         sub     ecx, ecx        ; buffer is empty now
         ret

   In our .data section we now have two new variables, fd.in and
   fd.out. We store the input and output file descriptors here.

   In the .text section we have replaced the references to stdin and
   stdout with [fd.in] and [fd.out].

   The .text section now starts with a simple error handler, which
   does nothing but exit the program with a return value of 1. The
   error handler is before _start so we are within a short distance
   from where the errors occur.

   Naturally, the program execution still begins at _start. First, we
   remove argc and argv[0] from the stack: They are of no interest to
   us (in this program, that is).

   We pop argv[1] to ECX. This register is particularly suited for
   pointers, as we can handle NULL pointers with jecxz. If argv[1] is
   not NULL, we try to open the file named in the first argument.
   Otherwise, we continue the program as before: Reading from stdin,
   writing to stdout. If we fail to open the input file (e.g., it
   does not exist), we jump to the error handler and quit.

   If all went well, we now check for the second argument. If it is
   there, we open the output file. Otherwise, we send the output to
   stdout. If we fail to open the output file (e.g., it exists and we
   do not have the write permission), we, again, jump to the error
   handler.

   The rest of the code is the same as before, except we close the
   input and output files before exiting, and, as mentioned, we use
   [fd.in] and [fd.out].

   Our executable is now a whopping 768 bytes long.

   Can we still improve it? Of course! Every program can be improved.
   Here are a few ideas of what we could do:

     * Have our error handler print a message to stderr.

     * Add error handlers to the read and write functions.

     * Close stdin when we open an input file, stdout when we open an
       output file.

     * Add command line switches, such as -i and -o, so we can list
       the input and output files in any order, or perhaps read from
       stdin and write to a file.

     * Print a usage message if command line arguments are incorrect.

   I shall leave these enhancements as an exercise to the reader: You
   already know everything you need to know to implement them.

     --------------------------------------------------------------

11.10 UNIX Environment

   An important UNIX concept is the environment, which is defined by
   environment variables. Some are set by the system, others by you,
   yet others by the shell, or any program that loads another
   program.

     --------------------------------------------------------------

  11.10.1 How to Find Environment Variables

   I said earlier that when a program starts executing, the stack
   contains argc followed by the NULL-terminated argv array, followed
   by something else. The "something else" is the environment, or, to
   be more precise, a NULL-terminated array of pointers to
   environment variables. This is often referred to as env.

   The structure of env is the same as that of argv, a list of memory
   addresses followed by a NULL (0). In this case, there is no
   "envc"-we figure out where the array ends by searching for the
   final NULL.

   The variables usually come in the name=value format, but sometimes
   the =value part may be missing. We need to account for that
   possibility.

     --------------------------------------------------------------

  11.10.2 webvars

   I could just show you some code that prints the environment the
   same way the UNIX env command does. But I thought it would be more
   interesting to write a simple assembly language CGI utility.

     --------------------------------------------------------------

    11.10.2.1 CGI: A Quick Overview

   I have a detailed CGI tutorial on my web site, but here is a very
   quick overview of CGI:

     * The web server communicates with the CGI program by setting
       environment variables.

     * The CGI program sends its output to stdout. The web server
       reads it from there.

     * It must start with an HTTP header followed by two blank lines.

     * It then prints the HTML code, or whatever other type of data
       it is producing.

     Note: While certain environment variables use standard names,
     others vary, depending on the web server. That makes webvars
     quite a useful diagnostic tool.

     --------------------------------------------------------------

    11.10.2.2 The Code

   Our webvars program, then, must send out the HTTP header followed
   by some HTML mark-up. It then must read the environment variables
   one by one and send them out as part of the HTML page.

   The code follows. I placed comments and explanations right inside
   the code:

 ;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;
 ; Copyright (c) 2000 G. Adam Stanislav
 ; All rights reserved.
 ;
 ; Redistribution and use in source and binary forms, with or without
 ; modification, are permitted provided that the following conditions
 ; are met:
 ; 1. Redistributions of source code must retain the above copyright
 ;    notice, this list of conditions and the following disclaimer.
 ; 2. Redistributions in binary form must reproduce the above copyright
 ;    notice, this list of conditions and the following disclaimer in the
 ;    documentation and/or other materials provided with the distribution.
 ;
 ; THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 ; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 ; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 ; ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 ; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 ; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 ; OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 ; HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 ; LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 ; OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 ; SUCH DAMAGE.
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;
 ; Version 1.0
 ;
 ; Started:       8-Dec-2000
 ; Updated:       8-Dec-2000
 ;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 %include        'system.inc'

 section .data
 http    db      'Content-type: text/html', 0Ah, 0Ah
         db      '<?xml version="1.0" encoding="utf-8"?>', 0Ah
         db      '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" '
         db      '"DTD/xhtml1-strict.dtd">', 0Ah
         db      '<html xmlns="http://www.w3.org/1999/xhtml" '
         db      'xml.lang="en" lang="en">', 0Ah
         db      '<head>', 0Ah
         db      '<title>Web Environment</title>', 0Ah
         db      '<meta name="author" content="G. Adam Stanislav" />', 0Ah
         db      '</head>', 0Ah, 0Ah
         db      '<body bgcolor="#ffffff" text="#000000" link="#0000ff" '
         db      'vlink="#840084" alink="#0000ff">', 0Ah
         db      '<div class="webvars">', 0Ah
         db      '<h1>Web Environment</h1>', 0Ah
         db      '<p>The following <b>environment variables</b> are defined '
         db      'on this web server:</p>', 0Ah, 0Ah
         db      '<table align="center" width="80" border="0" cellpadding="10" '
         db      'cellspacing="0" class="webvars">', 0Ah
 httplen equ     $-http
 left    db      '<tr>', 0Ah
         db      '<td class="name"><tt>'
 leftlen equ     $-left
 middle  db      '</tt></td>', 0Ah
         db      '<td class="value"><tt><b>'
 midlen  equ     $-middle
 undef   db      '<i>(undefined)</i>'
 undeflen        equ     $-undef
 right   db      '</b></tt></td>', 0Ah
         db      '</tr>', 0Ah
 rightlen        equ     $-right
 wrap    db      '</table>', 0Ah
         db      '</div>', 0Ah
         db      '</body>', 0Ah
         db      '</html>', 0Ah, 0Ah
 wraplen equ     $-wrap

 section .text
 global  _start
 _start:
         ; First, send out all the http and xhtml stuff that is
         ; needed before we start showing the environment
         push    dword httplen
         push    dword http
         push    dword stdout
         sys.write

         ; Now find how far on the stack the environment pointers
         ; are. We have 12 bytes we have pushed before "argc"
         mov     eax, [esp+12]

         ; We need to remove the following from the stack:
         ;
         ;       The 12 bytes we pushed for sys.write
         ;       The  4 bytes of argc
         ;       The EAX*4 bytes of argv
         ;       The  4 bytes of the NULL after argv
         ;
         ; Total:
         ;       20 + eax * 4
         ;
         ; Because stack grows down, we need to ADD that many bytes
         ; to ESP.
         lea     esp, [esp+20+eax*4]
         cld             ; This should already be the case, but let's be sure.

         ; Loop through the environment, printing it out
 .loop:
         pop     edi
         or      edi, edi        ; Done yet?
         je      near .wrap

         ; Print the left part of HTML
         push    dword leftlen
         push    dword left
         push    dword stdout
         sys.write

         ; It may be tempting to search for the '=' in the env string next.
         ; But it is possible there is no '=', so we search for the
         ; terminating NUL first.
         mov     esi, edi        ; Save start of string
         sub     ecx, ecx
         not     ecx             ; ECX = FFFFFFFF
         sub     eax, eax
 repne   scasb
         not     ecx             ; ECX = string length + 1
         mov     ebx, ecx        ; Save it in EBX

         ; Now is the time to find '='
         mov     edi, esi        ; Start of string
         mov     al, '='
 repne   scasb
         not     ecx
         add     ecx, ebx        ; Length of name

         push    ecx
         push    esi
         push    dword stdout
         sys.write

         ; Print the middle part of HTML table code
         push    dword midlen
         push    dword middle
         push    dword stdout
         sys.write

         ; Find the length of the value
         not     ecx
         lea     ebx, [ebx+ecx-1]

         ; Print "undefined" if 0
         or      ebx, ebx
         jne     .value

         mov     ebx, undeflen
         mov     edi, undef

 .value:
         push    ebx
         push    edi
         push    dword stdout
         sys.write

         ; Print the right part of the table row
         push    dword rightlen
         push    dword right
         push    dword stdout
         sys.write

         ; Get rid of the 60 bytes we have pushed
         add     esp, byte 60

         ; Get the next variable
         jmp     .loop

 .wrap:
         ; Print the rest of HTML
         push    dword wraplen
         push    dword wrap
         push    dword stdout
         sys.write

         ; Return success
         push    dword 0
         sys.exit

   This code produces a 1,396-byte executable. Most of it is data,
   i.e., the HTML mark-up we need to send out.

   Assemble and link it as usual:

 % nasm -f elf webvars.asm
 % ld -s -o webvars webvars.o

   To use it, you need to upload webvars to your web server.
   Depending on how your web server is set up, you may have to store
   it in a special cgi-bin directory, or perhaps rename it with a
   .cgi extension.

   Then you need to use your browser to view its output. To see its
   output on my web server, please go to
   http://www.int80h.org/webvars/. If curious about the additional
   environment variables present in a password protected web
   directory, go to http://www.int80h.org/private/, using the name
   asm and password programmer.

     --------------------------------------------------------------

11.11 Working with Files

   We have already done some basic file work: We know how to open and
   close them, how to read and write them using buffers. But UNIX
   offers much more functionality when it comes to files. We will
   examine some of it in this section, and end up with a nice file
   conversion utility.

   Indeed, let us start at the end, that is, with the file conversion
   utility. It always makes programming easier when we know from the
   start what the end product is supposed to do.

   One of the first programs I wrote for UNIX was tuc, a text-to-UNIX
   file converter. It converts a text file from other operating
   systems to a UNIX text file. In other words, it changes from
   different kind of line endings to the newline convention of UNIX.
   It saves the output in a different file. Optionally, it converts a
   UNIX text file to a DOS text file.

   I have used tuc extensively, but always only to convert from some
   other OS to UNIX, never the other way. I have always wished it
   would just overwrite the file instead of me having to send the
   output to a different file. Most of the time, I end up using it
   like this:

 % tuc myfile tempfile
 % mv tempfile myfile

   It would be nice to have a ftuc, i.e., fast tuc, and use it like
   this:

 % ftuc myfile

   In this chapter, then, we will write ftuc in assembly language
   (the original tuc is in C), and study various file-oriented kernel
   services in the process.

   At first sight, such a file conversion is very simple: All you
   have to do is strip the carriage returns, right?

   If you answered yes, think again: That approach will work most of
   the time (at least with MS DOS text files), but will fail
   occasionally.

   The problem is that not all non UNIX text files end their line
   with the carriage return / line feed sequence. Some use carriage
   returns without line feeds. Others combine several blank lines
   into a single carriage return followed by several line feeds. And
   so on.

   A text file converter, then, must be able to handle any possible
   line endings:

     * carriage return / line feed

     * carriage return

     * line feed / carriage return

     * line feed

   It should also handle files that use some kind of a combination of
   the above (e.g., carriage return followed by several line feeds).

     --------------------------------------------------------------

  11.11.1 Finite State Machine

   The problem is easily solved by the use of a technique called
   finite state machine, originally developed by the designers of
   digital electronic circuits. A finite state machine is a digital
   circuit whose output is dependent not only on its input but on its
   previous input, i.e., on its state. The microprocessor is an
   example of a finite state machine: Our assembly language code is
   assembled to machine language in which some assembly language code
   produces a single byte of machine language, while others produce
   several bytes. As the microprocessor fetches the bytes from the
   memory one by one, some of them simply change its state rather
   than produce some output. When all the bytes of the op code are
   fetched, the microprocessor produces some output, or changes the
   value of a register, etc.

   Because of that, all software is essentially a sequence of state
   instructions for the microprocessor. Nevertheless, the concept of
   finite state machine is useful in software design as well.

   Our text file converter can be designed as a finite state machine
   with three possible states. We could call them states 0-2, but it
   will make our life easier if we give them symbolic names:

     * ordinary

     * cr

     * lf

   Our program will start in the ordinary state. During this state,
   the program action depends on its input as follows:

     * If the input is anything other than a carriage return or line
       feed, the input is simply passed on to the output. The state
       remains unchanged.

     * If the input is a carriage return, the state is changed to cr.
       The input is then discarded, i.e., no output is made.

     * If the input is a line feed, the state is changed to lf. The
       input is then discarded.

   Whenever we are in the cr state, it is because the last input was
   a carriage return, which was unprocessed. What our software does
   in this state again depends on the current input:

     * If the input is anything other than a carriage return or line
       feed, output a line feed, then output the input, then change
       the state to ordinary.

     * If the input is a carriage return, we have received two (or
       more) carriage returns in a row. We discard the input, we
       output a line feed, and leave the state unchanged.

     * If the input is a line feed, we output the line feed and
       change the state to ordinary. Note that this is not the same
       as the first case above - if we tried to combine them, we
       would be outputting two line feeds instead of one.

   Finally, we are in the lf state after we have received a line feed
   that was not preceded by a carriage return. This will happen when
   our file already is in UNIX format, or whenever several lines in a
   row are expressed by a single carriage return followed by several
   line feeds, or when line ends with a line feed / carriage return
   sequence. Here is how we need to handle our input in this state:

     * If the input is anything other than a carriage return or line
       feed, we output a line feed, then output the input, then
       change the state to ordinary. This is exactly the same action
       as in the cr state upon receiving the same kind of input.

     * If the input is a carriage return, we discard the input, we
       output a line feed, then change the state to ordinary.

     * If the input is a line feed, we output the line feed, and
       leave the state unchanged.

     --------------------------------------------------------------

    11.11.1.1 The Final State

   The above finite state machine works for the entire file, but
   leaves the possibility that the final line end will be ignored.
   That will happen whenever the file ends with a single carriage
   return or a single line feed. I did not think of it when I wrote
   tuc, just to discover that occasionally it strips the last line
   ending.

   This problem is easily fixed by checking the state after the
   entire file was processed. If the state is not ordinary, we simply
   need to output one last line feed.

     Note: Now that we have expressed our algorithm as a finite state
     machine, we could easily design a dedicated digital electronic
     circuit (a "chip") to do the conversion for us. Of course, doing
     so would be considerably more expensive than writing an assembly
     language program.

     --------------------------------------------------------------

    11.11.1.2 The Output Counter

   Because our file conversion program may be combining two
   characters into one, we need to use an output counter. We
   initialize it to 0, and increase it every time we send a character
   to the output. At the end of the program, the counter will tell us
   what size we need to set the file to.

     --------------------------------------------------------------

  11.11.2 Implementing FSM in Software

   The hardest part of working with a finite state machine is
   analyzing the problem and expressing it as a finite state machine.
   That accomplished, the software almost writes itself.

   In a high-level language, such as C, there are several main
   approaches. One is to use a switch statement which chooses what
   function should be run. For example,

         switch (state) {
         default:
         case REGULAR:
                 regular(inputchar);
                 break;
         case CR:
                 cr(inputchar);
                 break;
         case LF:
                 lf(inputchar);
                 break;
         }

   Another approach is by using an array of function pointers,
   something like this:

         (output[state])(inputchar);

   Yet another is to have state be a function pointer, set to point
   at the appropriate function:

         (*state)(inputchar);

   This is the approach we will use in our program because it is very
   easy to do in assembly language, and very fast, too. We will
   simply keep the address of the right procedure in EBX, and then
   just issue:

         call    ebx

   This is possibly faster than hardcoding the address in the code
   because the microprocessor does not have to fetch the address from
   the memory-it is already stored in one of its registers. I said
   possibly because with the caching modern microprocessors do,
   either way may be equally fast.

     --------------------------------------------------------------

  11.11.3 Memory Mapped Files

   Because our program works on a single file, we cannot use the
   approach that worked for us before, i.e., to read from an input
   file and to write to an output file.

   UNIX allows us to map a file, or a section of a file, into memory.
   To do that, we first need to open the file with the appropriate
   read/write flags. Then we use the mmap system call to map it into
   the memory. One nice thing about mmap is that it automatically
   works with virtual memory: We can map more of the file into the
   memory than we have physical memory available, yet still access it
   through regular memory op codes, such as mov, lods, and stos.
   Whatever changes we make to the memory image of the file will be
   written to the file by the system. We do not even have to keep the
   file open: As long as it stays mapped, we can read from it and
   write to it.

   The 32-bit Intel microprocessors can access up to four gigabytes
   of memory - physical or virtual. The FreeBSD system allows us to
   use up to a half of it for file mapping.

   For simplicity sake, in this tutorial we will only convert files
   that can be mapped into the memory in their entirety. There are
   probably not too many text files that exceed two gigabytes in
   size. If our program encounters one, it will simply display a
   message suggesting we use the original tuc instead.

   If you examine your copy of syscalls.master, you will find two
   separate syscalls named mmap. This is because of evolution of
   UNIX: There was the traditional BSD mmap, syscall 71. That one was
   superseded by the POSIX mmap, syscall 197. The FreeBSD system
   supports both because older programs were written by using the
   original BSD version. But new software uses the POSIX version,
   which is what we will use.

   The syscalls.master file lists the POSIX version like this:

 197     STD     BSD     { caddr_t mmap(caddr_t addr, size_t len, int prot, \
                             int flags, int fd, long pad, off_t pos); }

   This differs slightly from what mmap(2) says. That is because
   mmap(2) describes the C version.

   The difference is in the long pad argument, which is not present
   in the C version. However, the FreeBSD syscalls add a 32-bit pad
   after pushing a 64-bit argument. In this case, off_t is a 64-bit
   value.

   When we are finished working with a memory-mapped file, we unmap
   it with the munmap syscall:

     Tip: For an in-depth treatment of mmap, see W. Richard Stevens'
     Unix Network Programming, Volume 2, Chapter 12.

     --------------------------------------------------------------

  11.11.4 Determining File Size

   Because we need to tell mmap how many bytes of the file to map
   into the memory, and because we want to map the entire file, we
   need to determine the size of the file.

   We can use the fstat syscall to get all the information about an
   open file that the system can give us. That includes the file
   size.

   Again, syscalls.master lists two versions of fstat, a traditional
   one (syscall 62), and a POSIX one (syscall 189). Naturally, we
   will use the POSIX version:

 189     STD     POSIX   { int fstat(int fd, struct stat *sb); }

   This is a very straightforward call: We pass to it the address of
   a stat structure and the descriptor of an open file. It will fill
   out the contents of the stat structure.

   I do, however, have to say that I tried to declare the stat
   structure in the .bss section, and fstat did not like it: It set
   the carry flag indicating an error. After I changed the code to
   allocate the structure on the stack, everything was working fine.

     --------------------------------------------------------------

  11.11.5 Changing the File Size

   Because our program may combine carriage return / line feed
   sequences into straight line feeds, our output may be smaller than
   our input. However, since we are placing our output into the same
   file we read the input from, we may have to change the size of the
   file.

   The ftruncate system call allows us to do just that. Despite its
   somewhat misleading name, the ftruncate system call can be used to
   both truncate the file (make it smaller) and to grow it.

   And yes, we will find two versions of ftruncate in
   syscalls.master, an older one (130), and a newer one (201). We
   will use the newer one:

 201     STD     BSD     { int ftruncate(int fd, int pad, off_t length); }

   Please note that this one contains a int pad again.

     --------------------------------------------------------------

  11.11.6 ftuc

   We now know everything we need to write ftuc. We start by adding
   some new lines in system.inc. First, we define some constants and
   structures, somewhere at or near the beginning of the file:

 ;;;;;;; open flags
 %define O_RDONLY        0
 %define O_WRONLY        1
 %define O_RDWR  2

 ;;;;;;; mmap flags
 %define PROT_NONE       0
 %define PROT_READ       1
 %define PROT_WRITE      2
 %define PROT_EXEC       4
 ;;
 %define MAP_SHARED      0001h
 %define MAP_PRIVATE     0002h

 ;;;;;;; stat structure
 struc   stat
 st_dev          resd    1       ; = 0
 st_ino          resd    1       ; = 4
 st_mode         resw    1       ; = 8, size is 16 bits
 st_nlink        resw    1       ; = 10, ditto
 st_uid          resd    1       ; = 12
 st_gid          resd    1       ; = 16
 st_rdev         resd    1       ; = 20
 st_atime        resd    1       ; = 24
 st_atimensec    resd    1       ; = 28
 st_mtime        resd    1       ; = 32
 st_mtimensec    resd    1       ; = 36
 st_ctime        resd    1       ; = 40
 st_ctimensec    resd    1       ; = 44
 st_size         resd    2       ; = 48, size is 64 bits
 st_blocks       resd    2       ; = 56, ditto
 st_blksize      resd    1       ; = 64
 st_flags        resd    1       ; = 68
 st_gen          resd    1       ; = 72
 st_lspare       resd    1       ; = 76
 st_qspare       resd    4       ; = 80
 endstruc

   We define the new syscalls:

 %define SYS_mmap        197
 %define SYS_munmap      73
 %define SYS_fstat       189
 %define SYS_ftruncate   201

   We add the macros for their use:

 %macro  sys.mmap        0
         system  SYS_mmap
 %endmacro

 %macro  sys.munmap      0
         system  SYS_munmap
 %endmacro

 %macro  sys.ftruncate   0
         system  SYS_ftruncate
 %endmacro

 %macro  sys.fstat       0
         system  SYS_fstat
 %endmacro

   And here is our code:

 ;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; Started:     21-Dec-2000
 ;; Updated:     22-Dec-2000
 ;;
 ;; Copyright 2000 G. Adam Stanislav.
 ;; All rights reserved.
 ;;
 ;;;;;;; v.1 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 %include        'system.inc'

 section .data
         db      'Copyright 2000 G. Adam Stanislav.', 0Ah
         db      'All rights reserved.', 0Ah
 usg     db      'Usage: ftuc filename', 0Ah
 usglen  equ     $-usg
 co      db      "ftuc: Can't open file.", 0Ah
 colen   equ     $-co
 fae     db      'ftuc: File access error.', 0Ah
 faelen  equ     $-fae
 ftl     db      'ftuc: File too long, use regular tuc instead.', 0Ah
 ftllen  equ     $-ftl
 mae     db      'ftuc: Memory allocation error.', 0Ah
 maelen  equ     $-mae

 section .text

 align 4
 memerr:
         push    dword maelen
         push    dword mae
         jmp     short error

 align 4
 toolong:
         push    dword ftllen
         push    dword ftl
         jmp     short error

 align 4
 facerr:
         push    dword faelen
         push    dword fae
         jmp     short error

 align 4
 cantopen:
         push    dword colen
         push    dword co
         jmp     short error

 align 4
 usage:
         push    dword usglen
         push    dword usg

 error:
         push    dword stderr
         sys.write

         push    dword 1
         sys.exit

 align 4
 global  _start
 _start:
         pop     eax             ; argc
         pop     eax             ; program name
         pop     ecx             ; file to convert
         jecxz   usage

         pop     eax
         or      eax, eax        ; Too many arguments?
         jne     usage

         ; Open the file
         push    dword O_RDWR
         push    ecx
         sys.open
         jc      cantopen

         mov     ebp, eax        ; Save fd

         sub     esp, byte stat_size
         mov     ebx, esp

         ; Find file size
         push    ebx
         push    ebp             ; fd
         sys.fstat
         jc      facerr

         mov     edx, [ebx + st_size + 4]

         ; File is too long if EDX != 0 ...
         or      edx, edx
         jne     near toolong
         mov     ecx, [ebx + st_size]
         ; ... or if it is above 2 GB
         or      ecx, ecx
         js      near toolong

         ; Do nothing if the file is 0 bytes in size
         jecxz   .quit

         ; Map the entire file in memory
         push    edx
         push    edx             ; starting at offset 0
         push    edx             ; pad
         push    ebp             ; fd
         push    dword MAP_SHARED
         push    dword PROT_READ | PROT_WRITE
         push    ecx             ; entire file size
         push    edx             ; let system decide on the address
         sys.mmap
         jc      near memerr

         mov     edi, eax
         mov     esi, eax
         push    ecx             ; for SYS_munmap
         push    edi

         ; Use EBX for state machine
         mov     ebx, ordinary
         mov     ah, 0Ah
         cld

 .loop:
         lodsb
         call    ebx
         loop    .loop

         cmp     ebx, ordinary
         je      .filesize

         ; Output final lf
         mov     al, ah
         stosb
         inc     edx

 .filesize:
         ; truncate file to new size
         push    dword 0         ; high dword
         push    edx             ; low dword
         push    eax             ; pad
         push    ebp
         sys.ftruncate

         ; close it (ebp still pushed)
         sys.close

         add     esp, byte 16
         sys.munmap

 .quit:
         push    dword 0
         sys.exit

 align 4
 ordinary:
         cmp     al, 0Dh
         je      .cr

         cmp     al, ah
         je      .lf

         stosb
         inc     edx
         ret

 align 4
 .cr:
         mov     ebx, cr
         ret

 align 4
 .lf:
         mov     ebx, lf
         ret

 align 4
 cr:
         cmp     al, 0Dh
         je      .cr

         cmp     al, ah
         je      .lf

         xchg    al, ah
         stosb
         inc     edx

         xchg    al, ah
         ; fall through

 .lf:
         stosb
         inc     edx
         mov     ebx, ordinary
         ret

 align 4
 .cr:
         mov     al, ah
         stosb
         inc     edx
         ret

 align 4
 lf:
         cmp     al, ah
         je      .lf

         cmp     al, 0Dh
         je      .cr

         xchg    al, ah
         stosb
         inc     edx

         xchg    al, ah
         stosb
         inc     edx
         mov     ebx, ordinary
         ret

 align 4
 .cr:
         mov     ebx, ordinary
         mov     al, ah
         ; fall through

 .lf:
         stosb
         inc     edx
         ret

     Warning: Do not use this program on files stored on a disk
     formatted by MS-DOS or Windows. There seems to be a subtle bug
     in the FreeBSD code when using mmap on these drives mounted
     under FreeBSD: If the file is over a certain size, mmap will
     just fill the memory with zeros, and then copy them to the file
     overwriting its contents.

     --------------------------------------------------------------

11.12 One-Pointed Mind

   As a student of Zen, I like the idea of a one-pointed mind: Do one
   thing at a time, and do it well.

   This, indeed, is very much how UNIX works as well. While a typical
   Windows application is attempting to do everything imaginable (and
   is, therefore, riddled with bugs), a typical UNIX program does
   only one thing, and it does it well.

   The typical UNIX user then essentially assembles his own
   applications by writing a shell script which combines the various
   existing programs by piping the output of one program to the input
   of another.

   When writing your own UNIX software, it is generally a good idea
   to see what parts of the problem you need to solve can be handled
   by existing programs, and only write your own programs for that
   part of the problem that you do not have an existing solution for.

     --------------------------------------------------------------

  11.12.1 CSV

   I will illustrate this principle with a specific real-life example
   I was faced with recently:

   I needed to extract the 11th field of each record from a database
   I downloaded from a web site. The database was a CSV file, i.e., a
   list of comma-separated values. That is quite a standard format
   for sharing data among people who may be using different database
   software.

   The first line of the file contains the list of various fields
   separated by commas. The rest of the file contains the data listed
   line by line, with values separated by commas.

   I tried awk, using the comma as a separator. But because several
   lines contained a quoted comma, awk was extracting the wrong field
   from those lines.

   Therefore, I needed to write my own software to extract the 11th
   field from the CSV file. However, going with the UNIX spirit, I
   only needed to write a simple filter that would do the following:

     * Remove the first line from the file;

     * Change all unquoted commas to a different character;

     * Remove all quotation marks.

   Strictly speaking, I could use sed to remove the first line from
   the file, but doing so in my own program was very easy, so I
   decided to do it and reduce the size of the pipeline.

   At any rate, writing a program like this took me about 20 minutes.
   Writing a program that extracts the 11th field from the CSV file
   would take a lot longer, and I could not reuse it to extract some
   other field from some other database.

   This time I decided to let it do a little more work than a typical
   tutorial program would:

     * It parses its command line for options;

     * It displays proper usage if it finds wrong arguments;

     * It produces meaningful error messages.

   Here is its usage message:

 Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]

   All parameters are optional, and can appear in any order.

   The -t parameter declares what to replace the commas with. The tab
   is the default here. For example, -t; will replace all unquoted
   commas with semicolons.

   I did not need the -c option, but it may come in handy in the
   future. It lets me declare that I want a character other than a
   comma replaced with something else. For example, -c@ will replace
   all at signs (useful if you want to split a list of email
   addresses to their user names and domains).

   The -p option preserves the first line, i.e., it does not delete
   it. By default, we delete the first line because in a CSV file it
   contains the field names rather than data.

   The -i and -o options let me specify the input and the output
   files. Defaults are stdin and stdout, so this is a regular UNIX
   filter.

   I made sure that both -i filename and -ifilename are accepted. I
   also made sure that only one input and one output files may be
   specified.

   To get the 11th field of each record, I can now do:

 % csv '-t;' data.csv | awk '-F;' '{print $11}'

   The code stores the options (except for the file descriptors) in
   EDX: The comma in DH, the new separator in DL, and the flag for
   the -p option in the highest bit of EDX, so a check for its sign
   will give us a quick decision what to do.

   Here is the code:

 ;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;
 ; Convert a comma-separated file to a something-else separated file.
 ;
 ; Started:      31-May-2001
 ; Updated:       1-Jun-2001
 ;
 ; Copyright (c) 2001 G. Adam Stanislav
 ; All rights reserved.
 ;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

 %include        'system.inc'

 %define BUFSIZE 2048

 section .data
 fd.in   dd      stdin
 fd.out  dd      stdout
 usg     db      'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]', 0Ah
 usglen  equ     $-usg
 iemsg   db      "csv: Can't open input file", 0Ah
 iemlen  equ     $-iemsg
 oemsg   db      "csv: Can't create output file", 0Ah
 oemlen  equ     $-oemsg

 section .bss
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE

 section .text
 align 4
 ierr:
         push    dword iemlen
         push    dword iemsg
         push    dword stderr
         sys.write
         push    dword 1         ; return failure
         sys.exit

 align 4
 oerr:
         push    dword oemlen
         push    dword oemsg
         push    dword stderr
         sys.write
         push    dword 2
         sys.exit

 align 4
 usage:
         push    dword usglen
         push    dword usg
         push    dword stderr
         sys.write
         push    dword 3
         sys.exit

 align 4
 global  _start
 _start:
         add     esp, byte 8     ; discard argc and argv[0]
         mov     edx, (',' << 8) | 9

 .arg:
         pop     ecx
         or      ecx, ecx
         je      near .init              ; no more arguments

         ; ECX contains the pointer to an argument
         cmp     byte [ecx], '-'
         jne     usage

         inc     ecx
         mov     ax, [ecx]

 .o:
         cmp     al, 'o'
         jne     .i

         ; Make sure we are not asked for the output file twice
         cmp     dword [fd.out], stdout
         jne     usage

         ; Find the path to output file - it is either at [ECX+1],
         ; i.e., -ofile --
         ; or in the next argument,
         ; i.e., -o file

         inc     ecx
         or      ah, ah
         jne     .openoutput
         pop     ecx
         jecxz   usage

 .openoutput:
         push    dword 420       ; file mode (644 octal)
         push    dword 0200h | 0400h | 01h
         ; O_CREAT | O_TRUNC | O_WRONLY
         push    ecx
         sys.open
         jc      near oerr

         add     esp, byte 12
         mov     [fd.out], eax
         jmp     short .arg

 .i:
         cmp     al, 'i'
         jne     .p

         ; Make sure we are not asked twice
         cmp     dword [fd.in], stdin
         jne     near usage

         ; Find the path to the input file
         inc     ecx
         or      ah, ah
         jne     .openinput
         pop     ecx
         or      ecx, ecx
         je near usage

 .openinput:
         push    dword 0         ; O_RDONLY
         push    ecx
         sys.open
         jc      near ierr               ; open failed

         add     esp, byte 8
         mov     [fd.in], eax
         jmp     .arg

 .p:
         cmp     al, 'p'
         jne     .t
         or      ah, ah
         jne     near usage
         or      edx, 1 << 31
         jmp     .arg

 .t:
         cmp     al, 't'         ; redefine output delimiter
         jne     .c
         or      ah, ah
         je      near usage
         mov     dl, ah
         jmp     .arg

 .c:
         cmp     al, 'c'
         jne     near usage
         or      ah, ah
         je      near usage
         mov     dh, ah
         jmp     .arg

 align 4
 .init:
         sub     eax, eax
         sub     ebx, ebx
         sub     ecx, ecx
         mov     edi, obuffer

         ; See if we are to preserve the first line
         or      edx, edx
         js      .loop

 .firstline:
         ; get rid of the first line
         call    getchar
         cmp     al, 0Ah
         jne     .firstline

 .loop:
         ; read a byte from stdin
         call    getchar

         ; is it a comma (or whatever the user asked for)?
         cmp     al, dh
         jne     .quote

         ; Replace the comma with a tab (or whatever the user wants)
         mov     al, dl

 .put:
         call    putchar
         jmp     short .loop

 .quote:
         cmp     al, '"'
         jne     .put

         ; Print everything until you get another quote or EOL. If it
         ; is a quote, skip it. If it is EOL, print it.
 .qloop:
         call    getchar
         cmp     al, '"'
         je      .loop

         cmp     al, 0Ah
         je      .put

         call    putchar
         jmp     short .qloop

 align 4
 getchar:
         or      ebx, ebx
         jne     .fetch

         call    read

 .fetch:
         lodsb
         dec     ebx
         ret

 read:
         jecxz   .read
         call    write

 .read:
         push    dword BUFSIZE
         mov     esi, ibuffer
         push    esi
         push    dword [fd.in]
         sys.read
         add     esp, byte 12
         mov     ebx, eax
         or      eax, eax
         je      .done
         sub     eax, eax
         ret

 align 4
 .done:
         call    write           ; flush output buffer

         ; close files
         push    dword [fd.in]
         sys.close

         push    dword [fd.out]
         sys.close

         ; return success
         push    dword 0
         sys.exit

 align 4
 putchar:
         stosb
         inc     ecx
         cmp     ecx, BUFSIZE
         je      write
         ret

 align 4
 write:
         jecxz   .ret    ; nothing to write
         sub     edi, ecx        ; start of buffer
         push    ecx
         push    edi
         push    dword [fd.out]
         sys.write
         add     esp, byte 12
         sub     eax, eax
         sub     ecx, ecx        ; buffer is empty now
 .ret:
         ret

   Much of it is taken from hex.asm above. But there is one important
   difference: I no longer call write whenever I am outputting a line
   feed. Yet, the code can be used interactively.

   I have found a better solution for the interactive problem since I
   first started writing this chapter. I wanted to make sure each
   line is printed out separately only when needed. After all, there
   is no need to flush out every line when used non-interactively.

   The new solution I use now is to call write every time I find the
   input buffer empty. That way, when running in the interactive
   mode, the program reads one line from the user's keyboard,
   processes it, and sees its input buffer is empty. It flushes its
   output and reads the next line.

     --------------------------------------------------------------

    11.12.1.1 The Dark Side of Buffering

   This change prevents a mysterious lockup in a very specific case.
   I refer to it as the dark side of buffering, mostly because it
   presents a danger that is not quite obvious.

   It is unlikely to happen with a program like the csv above, so let
   us consider yet another filter: In this case we expect our input
   to be raw data representing color values, such as the red, green,
   and blue intensities of a pixel. Our output will be the negative
   of our input.

   Such a filter would be very simple to write. Most of it would look
   just like all the other filters we have written so far, so I am
   only going to show you its inner loop:

 .loop:
         call    getchar
         not     al              ; Create a negative
         call    putchar
         jmp     short .loop

   Because this filter works with raw data, it is unlikely to be used
   interactively.

   But it could be called by image manipulation software. And, unless
   it calls write before each call to read, chances are it will lock
   up.

   Here is what might happen:

    1. The image editor will load our filter using the C function
       popen().

    2. It will read the first row of pixels from a bitmap or pixmap.

    3. It will write the first row of pixels to the pipe leading to
       the fd.in of our filter.

    4. Our filter will read each pixel from its input, turn it to a
       negative, and write it to its output buffer.

    5. Our filter will call getchar to fetch the next pixel.

    6. getchar will find an empty input buffer, so it will call read.

    7. read will call the SYS_read system call.

    8. The kernel will suspend our filter until the image editor
       sends more data to the pipe.

    9. The image editor will read from the other pipe, connected to
       the fd.out of our filter so it can set the first row of the
       output image before it sends us the second row of the input.

   10. The kernel suspends the image editor until it receives some
       output from our filter, so it can pass it on to the image
       editor.

   At this point our filter waits for the image editor to send it
   more data to process, while the image editor is waiting for our
   filter to send it the result of the processing of the first row.
   But the result sits in our output buffer.

   The filter and the image editor will continue waiting for each
   other forever (or, at least, until they are killed). Our software
   has just entered a race condition.

   This problem does not exist if our filter flushes its output
   buffer before asking the kernel for more input data.

     --------------------------------------------------------------

11.13 Using the FPU

   Strangely enough, most of assembly language literature does not
   even mention the existence of the FPU, or floating point unit, let
   alone discuss programming it.

   Yet, never does assembly language shine more than when we create
   highly optimized FPU code by doing things that can be done only in
   assembly language.

     --------------------------------------------------------------

  11.13.1 Organization of the FPU

   The FPU consists of 8 80-bit floating-point registers. These are
   organized in a stack fashion-you can push a value on TOS (top of
   stack) and you can pop it.

   That said, the assembly language op codes are not push and pop
   because those are already taken.

   You can push a value on TOS by using fld, fild, and fbld. Several
   other op codes let you push many common constants-such as pi-on
   the TOS.

   Similarly, you can pop a value by using fst, fstp, fist, fistp,
   and fbstp. Actually, only the op codes that end with a p will
   literally pop the value, the rest will store it somewhere else
   without removing it from the TOS.

   We can transfer the data between the TOS and the computer memory
   either as a 32-bit, 64-bit, or 80-bit real, a 16-bit, 32-bit, or
   64-bit integer, or an 80-bit packed decimal.

   The 80-bit packed decimal is a special case of binary coded
   decimal which is very convenient when converting between the ASCII
   representation of data and the internal data of the FPU. It allows
   us to use 18 significant digits.

   No matter how we represent data in the memory, the FPU always
   stores it in the 80-bit real format in its registers.

   Its internal precision is at least 19 decimal digits, so even if
   we choose to display results as ASCII in the full 18-digit
   precision, we are still showing correct results.

   We can perform mathematical operations on the TOS: We can
   calculate its sine, we can scale it (i.e., we can multiply or
   divide it by a power of 2), we can calculate its base-2 logarithm,
   and many other things.

   We can also multiply or divide it by, add it to, or subtract it
   from, any of the FPU registers (including itself).

   The official Intel op code for the TOS is st, and for the
   registers st(0)-st(7). st and st(0), then, refer to the same
   register.

   For whatever reasons, the original author of nasm has decided to
   use different op codes, namely st0-st7. In other words, there are
   no parentheses, and the TOS is always st0, never just st.

     --------------------------------------------------------------

    11.13.1.1 The Packed Decimal Format

   The packed decimal format uses 10 bytes (80 bits) of memory to
   represent 18 digits. The number represented there is always an
   integer.

     Tip: You can use it to get decimal places by multiplying the TOS
     by a power of 10 first.

   The highest bit of the highest byte (byte 9) is the sign bit: If
   it is set, the number is negative, otherwise, it is positive. The
   rest of the bits of this byte are unused/ignored.

   The remaining 9 bytes store the 18 digits of the number: 2 digits
   per byte.

   The more significant digit is stored in the high nibble (4 bits),
   the less significant digit in the low nibble.

   That said, you might think that -1234567 would be stored in the
   memory like this (using hexadecimal notation):

 80 00 00 00 00 00 01 23 45 67

   Alas it is not! As with everything else of Intel make, even the
   packed decimal is little-endian.

   That means our -1234567 is stored like this:

 67 45 23 01 00 00 00 00 00 80

   Remember that, or you will be pulling your hair out in
   desperation!

     Note: The book to read-if you can find it-is Richard Startz'
     8087/80287/80387 for the IBM PC & Compatibles. Though it does
     seem to take the fact about the little-endian storage of the
     packed decimal for granted. I kid you not about the desperation
     of trying to figure out what was wrong with the filter I show
     below before it occurred to me I should try the little-endian
     order even for this type of data.

     --------------------------------------------------------------

  11.13.2 Excursion to Pinhole Photography

   To write meaningful software, we must not only understand our
   programming tools, but also the field we are creating software
   for.

   Our next filter will help us whenever we want to build a pinhole
   camera, so, we need some background in pinhole photography before
   we can continue.

     --------------------------------------------------------------

    11.13.2.1 The Camera

   The easiest way to describe any camera ever built is as some empty
   space enclosed in some lightproof material, with a small hole in
   the enclosure.

   The enclosure is usually sturdy (e.g., a box), though sometimes it
   is flexible (the bellows). It is quite dark inside the camera.
   However, the hole lets light rays in through a single point
   (though in some cases there may be several). These light rays form
   an image, a representation of whatever is outside the camera, in
   front of the hole.

   If some light sensitive material (such as film) is placed inside
   the camera, it can capture the image.

   The hole often contains a lens, or a lens assembly, often called
   the objective.

     --------------------------------------------------------------

    11.13.2.2 The Pinhole

   But, strictly speaking, the lens is not necessary: The original
   cameras did not use a lens but a pinhole. Even today, pinholes are
   used, both as a tool to study how cameras work, and to achieve a
   special kind of image.

   The image produced by the pinhole is all equally sharp. Or
   blurred. There is an ideal size for a pinhole: If it is either
   larger or smaller, the image loses its sharpness.

     --------------------------------------------------------------

    11.13.2.3 Focal Length

   This ideal pinhole diameter is a function of the square root of
   focal length, which is the distance of the pinhole from the film.

         D = PC * sqrt(FL)

   In here, D is the ideal diameter of the pinhole, FL is the focal
   length, and PC is a pinhole constant. According to Jay Bender, its
   value is 0.04, while Kenneth Connors has determined it to be
   0.037. Others have proposed other values. Plus, this value is for
   the daylight only: Other types of light will require a different
   constant, whose value can only be determined by experimentation.

     --------------------------------------------------------------

    11.13.2.4 The F-Number

   The f-number is a very useful measure of how much light reaches
   the film. A light meter can determine that, for example, to expose
   a film of specific sensitivity with f5.6 may require the exposure
   to last 1/1000 sec.

   It does not matter whether it is a 35-mm camera, or a 6x9cm
   camera, etc. As long as we know the f-number, we can determine the
   proper exposure.

   The f-number is easy to calculate:

         F = FL / D

   In other words, the f-number equals the focal length divided by
   the diameter of the pinhole. It also means a higher f-number
   either implies a smaller pinhole or a larger focal distance, or
   both. That, in turn, implies, the higher the f-number, the longer
   the exposure has to be.

   Furthermore, while pinhole diameter and focal distance are
   one-dimensional measurements, both, the film and the pinhole, are
   two-dimensional. That means that if you have measured the exposure
   at f-number A as t, then the exposure at f-number B is:

         t * (B / A)^2

     --------------------------------------------------------------

    11.13.2.5 Normalized F-Number

   While many modern cameras can change the diameter of their
   pinhole, and thus their f-number, quite smoothly and gradually,
   such was not always the case.

   To allow for different f-numbers, cameras typically contained a
   metal plate with several holes of different sizes drilled to them.

   Their sizes were chosen according to the above formula in such a
   way that the resultant f-number was one of standard f-numbers used
   on all cameras everywhere. For example, a very old Kodak Duaflex
   IV camera in my possession has three such holes for f-numbers 8,
   11, and 16.

   A more recently made camera may offer f-numbers of 2.8, 4, 5.6, 8,
   11, 16, 22, and 32 (as well as others). These numbers were not
   chosen arbitrarily: They all are powers of the square root of 2,
   though they may be rounded somewhat.

     --------------------------------------------------------------

    11.13.2.6 The F-Stop

   A typical camera is designed in such a way that setting any of the
   normalized f-numbers changes the feel of the dial. It will
   naturally stop in that position. Because of that, these positions
   of the dial are called f-stops.

   Since the f-numbers at each stop are powers of the square root of
   2, moving the dial by 1 stop will double the amount of light
   required for proper exposure. Moving it by 2 stops will quadruple
   the required exposure. Moving the dial by 3 stops will require the
   increase in exposure 8 times, etc.

     --------------------------------------------------------------

  11.13.3 Designing the Pinhole Software

   We are now ready to decide what exactly we want our pinhole
   software to do.

     --------------------------------------------------------------

    11.13.3.1 Processing Program Input

   Since its main purpose is to help us design a working pinhole
   camera, we will use the focal length as the input to the program.
   This is something we can determine without software: Proper focal
   length is determined by the size of the film and by the need to
   shoot "regular" pictures, wide angle pictures, or telephoto
   pictures.

   Most of the programs we have written so far worked with individual
   characters, or bytes, as their input: The hex program converted
   individual bytes into a hexadecimal number, the csv program either
   let a character through, or deleted it, or changed it to a
   different character, etc.

   One program, ftuc used the state machine to consider at most two
   input bytes at a time.

   But our pinhole program cannot just work with individual
   characters, it has to deal with larger syntactic units.

   For example, if we want the program to calculate the pinhole
   diameter (and other values we will discuss later) at the focal
   lengths of 100 mm, 150 mm, and 210 mm, we may want to enter
   something like this:

 100, 150, 210

   Our program needs to consider more than a single byte of input at
   a time. When it sees the first 1, it must understand it is seeing
   the first digit of a decimal number. When it sees the 0 and the
   other 0, it must know it is seeing more digits of the same number.

   When it encounters the first comma, it must know it is no longer
   receiving the digits of the first number. It must be able to
   convert the digits of the first number into the value of 100. And
   the digits of the second number into the value of 150. And, of
   course, the digits of the third number into the numeric value of
   210.

   We need to decide what delimiters to accept: Do the input numbers
   have to be separated by a comma? If so, how do we treat two
   numbers separated by something else?

   Personally, I like to keep it simple. Something either is a
   number, so I process it. Or it is not a number, so I discard it. I
   do not like the computer complaining about me typing in an extra
   character when it is obvious that it is an extra character. Duh!

   Plus, it allows me to break up the monotony of computing and type
   in a query instead of just a number:

 What is the best pinhole diameter for the focal length of 150?

   There is no reason for the computer to spit out a number of
   complaints:

 Syntax error: What
 Syntax error: is
 Syntax error: the
 Syntax error: best

   Et cetera, et cetera, et cetera.

   Secondly, I like the # character to denote the start of a comment
   which extends to the end of the line. This does not take too much
   effort to code, and lets me treat input files for my software as
   executable scripts.

   In our case, we also need to decide what units the input should
   come in: We choose millimeters because that is how most
   photographers measure the focus length.

   Finally, we need to decide whether to allow the use of the decimal
   point (in which case we must also consider the fact that much of
   the world uses a decimal comma).

   In our case allowing for the decimal point/comma would offer a
   false sense of precision: There is little if any noticeable
   difference between the focus lengths of 50 and 51, so allowing the
   user to input something like 50.5 is not a good idea. This is my
   opinion, mind you, but I am the one writing this program. You can
   make other choices in yours, of course.

     --------------------------------------------------------------

    11.13.3.2 Offering Options

   The most important thing we need to know when building a pinhole
   camera is the diameter of the pinhole. Since we want to shoot
   sharp images, we will use the above formula to calculate the
   pinhole diameter from focal length. As experts are offering
   several different values for the PC constant, we will need to have
   the choice.

   It is traditional in UNIX programming to have two main ways of
   choosing program parameters, plus to have a default for the time
   the user does not make a choice.

   Why have two ways of choosing?

   One is to allow a (relatively) permanent choice that applies
   automatically each time the software is run without us having to
   tell it over and over what we want it to do.

   The permanent choices may be stored in a configuration file,
   typically found in the user's home directory. The file usually has
   the same name as the application but is started with a dot. Often
   "rc" is added to the file name. So, ours could be ~/.pinhole or
   ~/.pinholerc. (The ~/ means current user's home directory.)

   The configuration file is used mostly by programs that have many
   configurable parameters. Those that have only one (or a few) often
   use a different method: They expect to find the parameter in an
   environment variable. In our case, we might look at an environment
   variable named PINHOLE.

   Usually, a program uses one or the other of the above methods.
   Otherwise, if a configuration file said one thing, but an
   environment variable another, the program might get confused (or
   just too complicated).

   Because we only need to choose one such parameter, we will go with
   the second method and search the environment for a variable named
   PINHOLE.

   The other way allows us to make ad hoc decisions: "Though I
   usually want you to use 0.039, this time I want 0.03872." In other
   words, it allows us to override the permanent choice.

   This type of choice is usually done with command line parameters.

   Finally, a program always needs a default. The user may not make
   any choices. Perhaps he does not know what to choose. Perhaps he
   is "just browsing." Preferably, the default will be the value most
   users would choose anyway. That way they do not need to choose.
   Or, rather, they can choose the default without an additional
   effort.

   Given this system, the program may find conflicting options, and
   handle them this way:

    1. If it finds an ad hoc choice (e.g., command line parameter),
       it should accept that choice. It must ignore any permanent
       choice and any default.

    2. Otherwise, if it finds a permanent option (e.g., an
       environment variable), it should accept it, and ignore the
       default.

    3. Otherwise, it should use the default.

   We also need to decide what format our PC option should have.

   At first site, it seems obvious to use the PINHOLE=0.04 format for
   the environment variable, and -p0.04 for the command line.

   Allowing that is actually a security risk. The PC constant is a
   very small number. Naturally, we will test our software using
   various small values of PC. But what will happen if someone runs
   the program choosing a huge value?

   It may crash the program because we have not designed it to handle
   huge numbers.

   Or, we may spend more time on the program so it can handle huge
   numbers. We might do that if we were writing commercial software
   for computer illiterate audience.

   Or, we might say, "Tough! The user should know better.""

   Or, we just may make it impossible for the user to enter a huge
   number. This is the approach we will take: We will use an implied
   0. prefix.

   In other words, if the user wants 0.04, we will expect him to type
   -p04, or set PINHOLE=04 in his environment. So, if he says
   -p9999999, we will interpret it as 0.9999999-still ridiculous but
   at least safer.

   Secondly, many users will just want to go with either Bender's
   constant or Connors' constant. To make it easier on them, we will
   interpret -b as identical to -p04, and -c as identical to -p037.

     --------------------------------------------------------------

    11.13.3.3 The Output

   We need to decide what we want our software to send to the output,
   and in what format.

   Since our input allows for an unspecified number of focal length
   entries, it makes sense to use a traditional database-style output
   of showing the result of the calculation for each focal length on
   a separate line, while separating all values on one line by a tab
   character.

   Optionally, we should also allow the user to specify the use of
   the CSV format we have studied earlier. In this case, we will
   print out a line of comma-separated names describing each field of
   every line, then show our results as before, but substituting a
   comma for the tab.

   We need a command line option for the CSV format. We cannot use -c
   because that already means use Connors' constant. For some strange
   reason, many web sites refer to CSV files as "Excel spreadsheet"
   (though the CSV format predates Excel). We will, therefore, use
   the -e switch to inform our software we want the output in the CSV
   format.

   We will start each line of the output with the focal length. This
   may sound repetitious at first, especially in the interactive
   mode: The user types in the focal length, and we are repeating it.

   But the user can type several focal lengths on one line. The input
   can also come in from a file or from the output of another
   program. In that case the user does not see the input at all.

   By the same token, the output can go to a file which we will want
   to examine later, or it could go to the printer, or become the
   input of another program.

   So, it makes perfect sense to start each line with the focal
   length as entered by the user.

   No, wait! Not as entered by the user. What if the user types in
   something like this:

 00000000150

   Clearly, we need to strip those leading zeros.

   So, we might consider reading the user input as is, converting it
   to binary inside the FPU, and printing it out from there.

   But...

   What if the user types something like this:

 17459765723452353453534535353530530534563507309676764423

   Ha! The packed decimal FPU format lets us input 18-digit numbers.
   But the user has entered more than 18 digits. How do we handle
   that?

   Well, we could modify our code to read the first 18 digits, enter
   it to the FPU, then read more, multiply what we already have on
   the TOS by 10 raised to the number of additional digits, then add
   to it.

   Yes, we could do that. But in this program it would be ridiculous
   (in a different one it may be just the thing to do): Even the
   circumference of the Earth expressed in millimeters only takes 11
   digits. Clearly, we cannot build a camera that large (not yet,
   anyway).

   So, if the user enters such a huge number, he is either bored, or
   testing us, or trying to break into the system, or playing
   games-doing anything but designing a pinhole camera.

   What will we do?

   We will slap him in the face, in a manner of speaking:

 17459765723452353453534535353530530534563507309676764423        ???     ???     ???     ???     ???

   To achieve that, we will simply ignore any leading zeros. Once we
   find a non-zero digit, we will initialize a counter to 0 and start
   taking three steps:

    1. Send the digit to the output.

    2. Append the digit to a buffer we will use later to produce the
       packed decimal we can send to the FPU.

    3. Increase the counter.

   Now, while we are taking these three steps, we also need to watch
   out for one of two conditions:

     * If the counter grows above 18, we stop appending to the
       buffer. We continue reading the digits and sending them to the
       output.

     * If, or rather when, the next input character is not a digit,
       we are done inputting for now.

       Incidentally, we can simply discard the non-digit, unless it
       is a #, which we must return to the input stream. It starts a
       comment, so we must see it after we are done producing output
       and start looking for more input.

   That still leaves one possibility uncovered: If all the user
   enters is a zero (or several zeros), we will never find a non-zero
   to display.

   We can determine this has happened whenever our counter stays at
   0. In that case we need to send 0 to the output, and perform
   another "slap in the face":

 0       ???     ???     ???     ???     ???

   Once we have displayed the focal length and determined it is valid
   (greater than 0 but not exceeding 18 digits), we can calculate the
   pinhole diameter.

   It is not by coincidence that pinhole contains the word pin.
   Indeed, many a pinhole literally is a pin hole, a hole carefully
   punched with the tip of a pin.

   That is because a typical pinhole is very small. Our formula gets
   the result in millimeters. We will multiply it by 1000, so we can
   output the result in microns.

   At this point we have yet another trap to face: Too much
   precision.

   Yes, the FPU was designed for high precision mathematics. But we
   are not dealing with high precision mathematics. We are dealing
   with physics (optics, specifically).

   Suppose we want to convert a truck into a pinhole camera (we would
   not be the first ones to do that!). Suppose its box is 12 meters
   long, so we have the focal length of 12000. Well, using Bender's
   constant, it gives us square root of 12000 multiplied by 0.04,
   which is 4.381780460 millimeters, or 4381.780460 microns.

   Put either way, the result is absurdly precise. Our truck is not
   exactly 12000 millimeters long. We did not measure its length with
   such a precision, so stating we need a pinhole with the diameter
   of 4.381780460 millimeters is, well, deceiving. 4.4 millimeters
   would do just fine.

     Note: I "only" used ten digits in the above example. Imagine the
     absurdity of going for all 18!

   We need to limit the number of significant digits of our result.
   One way of doing it is by using an integer representing microns.
   So, our truck would need a pinhole with the diameter of 4382
   microns. Looking at that number, we still decide that 4400
   microns, or 4.4 millimeters is close enough.

   Additionally, we can decide that no matter how big a result we
   get, we only want to display four significant digits (or any other
   number of them, of course). Alas, the FPU does not offer rounding
   to a specific number of digits (after all, it does not view the
   numbers as decimal but as binary).

   We, therefore, must devise an algorithm to reduce the number of
   significant digits.

   Here is mine (I think it is awkward-if you know a better one,
   please, let me know):

    1. Initialize a counter to 0.

    2. While the number is greater than or equal to 10000, divide it
       by 10 and increase the counter.

    3. Output the result.

    4. While the counter is greater than 0, output 0 and decrease the
       counter.

     Note: The 10000 is only good if you want four significant
     digits. For any other number of significant digits, replace
     10000 with 10 raised to the number of significant digits.

   We will, then, output the pinhole diameter in microns, rounded off
   to four significant digits.

   At this point, we know the focal length and the pinhole diameter.
   That means we have enough information to also calculate the
   f-number.

   We will display the f-number, rounded to four significant digits.
   Chances are the f-number will tell us very little. To make it more
   meaningful, we can find the nearest normalized f-number, i.e., the
   nearest power of the square root of 2.

   We do that by multiplying the actual f-number by itself, which, of
   course, will give us its square. We will then calculate its base-2
   logarithm, which is much easier to do than calculating the
   base-square-root-of-2 logarithm! We will round the result to the
   nearest integer. Next, we will raise 2 to the result. Actually,
   the FPU gives us a good shortcut to do that: We can use the fscale
   op code to "scale" 1, which is analogous to shifting an integer
   left. Finally, we calculate the square root of it all, and we have
   the nearest normalized f-number.

   If all that sounds overwhelming-or too much work, perhaps-it may
   become much clearer if you see the code. It takes 9 op codes
   altogether:

         fmul    st0, st0
         fld1
         fld     st1
         fyl2x
         frndint
         fld1
         fscale
         fsqrt
         fstp    st1

   The first line, fmul st0, st0, squares the contents of the TOS
   (top of the stack, same as st, called st0 by nasm). The fld1
   pushes 1 on the TOS.

   The next line, fld st1, pushes the square back to the TOS. At this
   point the square is both in st and st(2) (it will become clear why
   we leave a second copy on the stack in a moment). st(1) contains
   1.

   Next, fyl2x calculates base-2 logarithm of st multiplied by st(1).
   That is why we placed 1 on st(1) before.

   At this point, st contains the logarithm we have just calculated,
   st(1) contains the square of the actual f-number we saved for
   later.

   frndint rounds the TOS to the nearest integer. fld1 pushes a 1.
   fscale shifts the 1 we have on the TOS by the value in st(1),
   effectively raising 2 to st(1).

   Finally, fsqrt calculates the square root of the result, i.e., the
   nearest normalized f-number.

   We now have the nearest normalized f-number on the TOS, the base-2
   logarithm rounded to the nearest integer in st(1), and the square
   of the actual f-number in st(2). We are saving the value in st(2)
   for later.

   But we do not need the contents of st(1) anymore. The last line,
   fstp st1, places the contents of st to st(1), and pops. As a
   result, what was st(1) is now st, what was st(2) is now st(1),
   etc. The new st contains the normalized f-number. The new st(1)
   contains the square of the actual f-number we have stored there
   for posterity.

   At this point, we are ready to output the normalized f-number.
   Because it is normalized, we will not round it off to four
   significant digits, but will send it out in its full precision.

   The normalized f-number is useful as long as it is reasonably
   small and can be found on our light meter. Otherwise we need a
   different method of determining proper exposure.

   Earlier we have figured out the formula of calculating proper
   exposure at an arbitrary f-number from that measured at a
   different f-number.

   Every light meter I have ever seen can determine proper exposure
   at f5.6. We will, therefore, calculate an "f5.6 multiplier," i.e.,
   by how much we need to multiply the exposure measured at f5.6 to
   determine the proper exposure for our pinhole camera.

   From the above formula we know this factor can be calculated by
   dividing our f-number (the actual one, not the normalized one) by
   5.6, and squaring the result.

   Mathematically, dividing the square of our f-number by the square
   of 5.6 will give us the same result.

   Computationally, we do not want to square two numbers when we can
   only square one. So, the first solution seems better at first.

   But...

   5.6 is a constant. We do not have to have our FPU waste precious
   cycles. We can just tell it to divide the square of the f-number
   by whatever 5.6^2 equals to. Or we can divide the f-number by 5.6,
   and then square the result. The two ways now seem equal.

   But, they are not!

   Having studied the principles of photography above, we remember
   that the 5.6 is actually square root of 2 raised to the fifth
   power. An irrational number. The square of this number is exactly
   32.

   Not only is 32 an integer, it is a power of 2. We do not need to
   divide the square of the f-number by 32. We only need to use
   fscale to shift it right by five positions. In the FPU lingo it
   means we will fscale it with st(1) equal to -5. That is much
   faster than a division.

   So, now it has become clear why we have saved the square of the
   f-number on the top of the FPU stack. The calculation of the f5.6
   multiplier is the easiest calculation of this entire program! We
   will output it rounded to four significant digits.

   There is one more useful number we can calculate: The number of
   stops our f-number is from f5.6. This may help us if our f-number
   is just outside the range of our light meter, but we have a
   shutter which lets us set various speeds, and this shutter uses
   stops.

   Say, our f-number is 5 stops from f5.6, and the light meter says
   we should use 1/1000 sec. Then we can set our shutter speed to
   1/1000 first, then move the dial by 5 stops.

   This calculation is quite easy as well. All we have to do is to
   calculate the base-2 logarithm of the f5.6 multiplier we had just
   calculated (though we need its value from before we rounded it
   off). We then output the result rounded to the nearest integer. We
   do not need to worry about having more than four significant
   digits in this one: The result is most likely to have only one or
   two digits anyway.

     --------------------------------------------------------------

  11.13.4 FPU Optimizations

   In assembly language we can optimize the FPU code in ways
   impossible in high languages, including C.

   Whenever a C function needs to calculate a floating-point value,
   it loads all necessary variables and constants into FPU registers.
   It then does whatever calculation is required to get the correct
   result. Good C compilers can optimize that part of the code really
   well.

   It "returns" the value by leaving the result on the TOS. However,
   before it returns, it cleans up. Any variables and constants it
   used in its calculation are now gone from the FPU.

   It cannot do what we just did above: We calculated the square of
   the f-number and kept it on the stack for later use by another
   function.

   We knew we would need that value later on. We also knew we had
   enough room on the stack (which only has room for 8 numbers) to
   store it there.

   A C compiler has no way of knowing that a value it has on the
   stack will be required again in the very near future.

   Of course, the C programmer may know it. But the only recourse he
   has is to store the value in a memory variable.

   That means, for one, the value will be changed from the 80-bit
   precision used internally by the FPU to a C double (64 bits) or
   even single (32 bits).

   That also means that the value must be moved from the TOS into the
   memory, and then back again. Alas, of all FPU operations, the ones
   that access the computer memory are the slowest.

   So, whenever programming the FPU in assembly language, look for
   the ways of keeping intermediate results on the FPU stack.

   We can take that idea even further! In our program we are using a
   constant (the one we named PC).

   It does not matter how many pinhole diameters we are calculating:
   1, 10, 20, 1000, we are always using the same constant. Therefore,
   we can optimize our program by keeping the constant on the stack
   all the time.

   Early on in our program, we are calculating the value of the above
   constant. We need to divide our input by 10 for every digit in the
   constant.

   It is much faster to multiply than to divide. So, at the start of
   our program, we divide 10 into 1 to obtain 0.1, which we then keep
   on the stack: Instead of dividing the input by 10 for every digit,
   we multiply it by 0.1.

   By the way, we do not input 0.1 directly, even though we could. We
   have a reason for that: While 0.1 can be expressed with just one
   decimal place, we do not know how many binary places it takes. We,
   therefore, let the FPU calculate its binary value to its own high
   precision.

   We are using other constants: We multiply the pinhole diameter by
   1000 to convert it from millimeters to microns. We compare numbers
   to 10000 when we are rounding them off to four significant digits.
   So, we keep both, 1000 and 10000, on the stack. And, of course, we
   reuse the 0.1 when rounding off numbers to four digits.

   Last but not least, we keep -5 on the stack. We need it to scale
   the square of the f-number, instead of dividing it by 32. It is
   not by coincidence we load this constant last. That makes it the
   top of the stack when only the constants are on it. So, when the
   square of the f-number is being scaled, the -5 is at st(1),
   precisely where fscale expects it to be.

   It is common to create certain constants from scratch instead of
   loading them from the memory. That is what we are doing with -5:

         fld1                    ; TOS =  1
         fadd    st0, st0        ; TOS =  2
         fadd    st0, st0        ; TOS =  4
         fld1                    ; TOS =  1
         faddp   st1, st0        ; TOS =  5
         fchs                    ; TOS = -5

   We can generalize all these optimizations into one rule: Keep
   repeat values on the stack!

     Tip: PostScript(R) is a stack-oriented programming language.
     There are many more books available about PostScript than about
     the FPU assembly language: Mastering PostScript will help you
     master the FPU.

     --------------------------------------------------------------

  11.13.5 pinhole-The Code

 ;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;
 ; Find various parameters of a pinhole camera construction and use
 ;
 ; Started:       9-Jun-2001
 ; Updated:      10-Jun-2001
 ;
 ; Copyright (c) 2001 G. Adam Stanislav
 ; All rights reserved.
 ;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

 %include        'system.inc'

 %define BUFSIZE 2048

 section .data
 align 4
 ten     dd      10
 thousand        dd      1000
 tthou   dd      10000
 fd.in   dd      stdin
 fd.out  dd      stdout
 envar   db      'PINHOLE='      ; Exactly 8 bytes, or 2 dwords long
 pinhole db      '04,',          ; Bender's constant (0.04)
 connors db      '037', 0Ah      ; Connors' constant
 usg     db      'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i <infile>]', 0Ah
 usglen  equ     $-usg
 iemsg   db      "pinhole: Can't open input file", 0Ah
 iemlen  equ     $-iemsg
 oemsg   db      "pinhole: Can't create output file", 0Ah
 oemlen  equ     $-oemsg
 pinmsg  db      "pinhole: The PINHOLE constant must not be 0", 0Ah
 pinlen  equ     $-pinmsg
 toobig  db      "pinhole: The PINHOLE constant may not exceed 18 decimal places", 0Ah
 biglen  equ     $-toobig
 huhmsg  db      9, '???'
 separ   db      9, '???'
 sep2    db      9, '???'
 sep3    db      9, '???'
 sep4    db      9, '???', 0Ah
 huhlen  equ     $-huhmsg
 header  db      'focal length in millimeters,pinhole diameter in microns,'
         db      'F-number,normalized F-number,F-5.6 multiplier,stops '
         db      'from F-5.6', 0Ah
 headlen equ     $-header

 section .bss
 ibuffer resb    BUFSIZE
 obuffer resb    BUFSIZE
 dbuffer resb    20              ; decimal input buffer
 bbuffer resb    10              ; BCD buffer

 section .text
 align 4
 huh:
         call    write
         push    dword huhlen
         push    dword huhmsg
         push    dword [fd.out]
         sys.write
         add     esp, byte 12
         ret

 align 4
 perr:
         push    dword pinlen
         push    dword pinmsg
         push    dword stderr
         sys.write
         push    dword 4         ; return failure
         sys.exit

 align 4
 consttoobig:
         push    dword biglen
         push    dword toobig
         push    dword stderr
         sys.write
         push    dword 5         ; return failure
         sys.exit

 align 4
 ierr:
         push    dword iemlen
         push    dword iemsg
         push    dword stderr
         sys.write
         push    dword 1         ; return failure
         sys.exit

 align 4
 oerr:
         push    dword oemlen
         push    dword oemsg
         push    dword stderr
         sys.write
         push    dword 2
         sys.exit

 align 4
 usage:
         push    dword usglen
         push    dword usg
         push    dword stderr
         sys.write
         push    dword 3
         sys.exit

 align 4
 global  _start
 _start:
         add     esp, byte 8     ; discard argc and argv[0]
         sub     esi, esi

 .arg:
         pop     ecx
         or      ecx, ecx
         je      near .getenv            ; no more arguments

         ; ECX contains the pointer to an argument
         cmp     byte [ecx], '-'
         jne     usage

         inc     ecx
         mov     ax, [ecx]
         inc     ecx

 .o:
         cmp     al, 'o'
         jne     .i

         ; Make sure we are not asked for the output file twice
         cmp     dword [fd.out], stdout
         jne     usage

         ; Find the path to output file - it is either at [ECX+1],
         ; i.e., -ofile --
         ; or in the next argument,
         ; i.e., -o file

         or      ah, ah
         jne     .openoutput
         pop     ecx
         jecxz   usage

 .openoutput:
         push    dword 420       ; file mode (644 octal)
         push    dword 0200h | 0400h | 01h
         ; O_CREAT | O_TRUNC | O_WRONLY
         push    ecx
         sys.open
         jc      near oerr

         add     esp, byte 12
         mov     [fd.out], eax
         jmp     short .arg

 .i:
         cmp     al, 'i'
         jne     .p

         ; Make sure we are not asked twice
         cmp     dword [fd.in], stdin
         jne     near usage

         ; Find the path to the input file
         or      ah, ah
         jne     .openinput
         pop     ecx
         or      ecx, ecx
         je near usage

 .openinput:
         push    dword 0         ; O_RDONLY
         push    ecx
         sys.open
         jc      near ierr               ; open failed

         add     esp, byte 8
         mov     [fd.in], eax
         jmp     .arg

 .p:
         cmp     al, 'p'
         jne     .c
         or      ah, ah
         jne     .pcheck

         pop     ecx
         or      ecx, ecx
         je      near usage

         mov     ah, [ecx]

 .pcheck:
         cmp     ah, '0'
         jl      near usage
         cmp     ah, '9'
         ja      near usage
         mov     esi, ecx
         jmp     .arg

 .c:
         cmp     al, 'c'
         jne     .b
         or      ah, ah
         jne     near usage
         mov     esi, connors
         jmp     .arg

 .b:
         cmp     al, 'b'
         jne     .e
         or      ah, ah
         jne     near usage
         mov     esi, pinhole
         jmp     .arg

 .e:
         cmp     al, 'e'
         jne     near usage
         or      ah, ah
         jne     near usage
         mov     al, ','
         mov     [huhmsg], al
         mov     [separ], al
         mov     [sep2], al
         mov     [sep3], al
         mov     [sep4], al
         jmp     .arg

 align 4
 .getenv:
         ; If ESI = 0, we did not have a -p argument,
         ; and need to check the environment for "PINHOLE="
         or      esi, esi
         jne     .init

         sub     ecx, ecx

 .nextenv:
         pop     esi
         or      esi, esi
         je      .default        ; no PINHOLE envar found

         ; check if this envar starts with 'PINHOLE='
         mov     edi, envar
         mov     cl, 2           ; 'PINHOLE=' is 2 dwords long
 rep     cmpsd
         jne     .nextenv

         ; Check if it is followed by a digit
         mov     al, [esi]
         cmp     al, '0'
         jl      .default
         cmp     al, '9'
         jbe     .init
         ; fall through

 align 4
 .default:
         ; We got here because we had no -p argument,
         ; and did not find the PINHOLE envar.
         mov     esi, pinhole
         ; fall through

 align 4
 .init:
         sub     eax, eax
         sub     ebx, ebx
         sub     ecx, ecx
         sub     edx, edx
         mov     edi, dbuffer+1
         mov     byte [dbuffer], '0'

         ; Convert the pinhole constant to real
 .constloop:
         lodsb
         cmp     al, '9'
         ja      .setconst
         cmp     al, '0'
         je      .processconst
         jb      .setconst

         inc     dl

 .processconst:
         inc     cl
         cmp     cl, 18
         ja      near consttoobig
         stosb
         jmp     short .constloop

 align 4
 .setconst:
         or      dl, dl
         je      near perr

         finit
         fild    dword [tthou]

         fld1
         fild    dword [ten]
         fdivp   st1, st0

         fild    dword [thousand]
         mov     edi, obuffer

         mov     ebp, ecx
         call    bcdload

 .constdiv:
         fmul    st0, st2
         loop    .constdiv

         fld1
         fadd    st0, st0
         fadd    st0, st0
         fld1
         faddp   st1, st0
         fchs

         ; If we are creating a CSV file,
         ; print header
         cmp     byte [separ], ','
         jne     .bigloop

         push    dword headlen
         push    dword header
         push    dword [fd.out]
         sys.write

 .bigloop:
         call    getchar
         jc      near done

         ; Skip to the end of the line if you got '#'
         cmp     al, '#'
         jne     .num
         call    skiptoeol
         jmp     short .bigloop

 .num:
         ; See if you got a number
         cmp     al, '0'
         jl      .bigloop
         cmp     al, '9'
         ja      .bigloop

         ; Yes, we have a number
         sub     ebp, ebp
         sub     edx, edx

 .number:
         cmp     al, '0'
         je      .number0
         mov     dl, 1

 .number0:
         or      dl, dl          ; Skip leading 0's
         je      .nextnumber
         push    eax
         call    putchar
         pop     eax
         inc     ebp
         cmp     ebp, 19
         jae     .nextnumber
         mov     [dbuffer+ebp], al

 .nextnumber:
         call    getchar
         jc      .work
         cmp     al, '#'
         je      .ungetc
         cmp     al, '0'
         jl      .work
         cmp     al, '9'
         ja      .work
         jmp     short .number

 .ungetc:
         dec     esi
         inc     ebx

 .work:
         ; Now, do all the work
         or      dl, dl
         je      near .work0

         cmp     ebp, 19
         jae     near .toobig

         call    bcdload

         ; Calculate pinhole diameter

         fld     st0     ; save it
         fsqrt
         fmul    st0, st3
         fld     st0
         fmul    st5
         sub     ebp, ebp

         ; Round off to 4 significant digits
 .diameter:
         fcom    st0, st7
         fstsw   ax
         sahf
         jb      .printdiameter
         fmul    st0, st6
         inc     ebp
         jmp     short .diameter

 .printdiameter:
         call    printnumber     ; pinhole diameter

         ; Calculate F-number

         fdivp   st1, st0
         fld     st0

         sub     ebp, ebp

 .fnumber:
         fcom    st0, st6
         fstsw   ax
         sahf
         jb      .printfnumber
         fmul    st0, st5
         inc     ebp
         jmp     short .fnumber

 .printfnumber:
         call    printnumber     ; F number

         ; Calculate normalized F-number
         fmul    st0, st0
         fld1
         fld     st1
         fyl2x
         frndint
         fld1
         fscale
         fsqrt
         fstp    st1

         sub     ebp, ebp
         call    printnumber

         ; Calculate time multiplier from F-5.6

         fscale
         fld     st0

         ; Round off to 4 significant digits
 .fmul:
         fcom    st0, st6
         fstsw   ax
         sahf

         jb      .printfmul
         inc     ebp
         fmul    st0, st5
         jmp     short .fmul

 .printfmul:
         call    printnumber     ; F multiplier

         ; Calculate F-stops from 5.6

         fld1
         fxch    st1
         fyl2x

         sub     ebp, ebp
         call    printnumber

         mov     al, 0Ah
         call    putchar
         jmp     .bigloop

 .work0:
         mov     al, '0'
         call    putchar

 align 4
 .toobig:
         call    huh
         jmp     .bigloop

 align 4
 done:
         call    write           ; flush output buffer

         ; close files
         push    dword [fd.in]
         sys.close

         push    dword [fd.out]
         sys.close

         finit

         ; return success
         push    dword 0
         sys.exit

 align 4
 skiptoeol:
         ; Keep reading until you come to cr, lf, or eof
         call    getchar
         jc      done
         cmp     al, 0Ah
         jne     .cr
         ret

 .cr:
         cmp     al, 0Dh
         jne     skiptoeol
         ret

 align 4
 getchar:
         or      ebx, ebx
         jne     .fetch

         call    read

 .fetch:
         lodsb
         dec     ebx
         clc
         ret

 read:
         jecxz   .read
         call    write

 .read:
         push    dword BUFSIZE
         mov     esi, ibuffer
         push    esi
         push    dword [fd.in]
         sys.read
         add     esp, byte 12
         mov     ebx, eax
         or      eax, eax
         je      .empty
         sub     eax, eax
         ret

 align 4
 .empty:
         add     esp, byte 4
         stc
         ret

 align 4
 putchar:
         stosb
         inc     ecx
         cmp     ecx, BUFSIZE
         je      write
         ret

 align 4
 write:
         jecxz   .ret    ; nothing to write
         sub     edi, ecx        ; start of buffer
         push    ecx
         push    edi
         push    dword [fd.out]
         sys.write
         add     esp, byte 12
         sub     eax, eax
         sub     ecx, ecx        ; buffer is empty now
 .ret:
         ret

 align 4
 bcdload:
         ; EBP contains the number of chars in dbuffer
         push    ecx
         push    esi
         push    edi

         lea     ecx, [ebp+1]
         lea     esi, [dbuffer+ebp-1]
         shr     ecx, 1

         std

         mov     edi, bbuffer
         sub     eax, eax
         mov     [edi], eax
         mov     [edi+4], eax
         mov     [edi+2], ax

 .loop:
         lodsw
         sub     ax, 3030h
         shl     al, 4
         or      al, ah
         mov     [edi], al
         inc     edi
         loop    .loop

         fbld    [bbuffer]

         cld
         pop     edi
         pop     esi
         pop     ecx
         sub     eax, eax
         ret

 align 4
 printnumber:
         push    ebp
         mov     al, [separ]
         call    putchar

         ; Print the integer at the TOS
         mov     ebp, bbuffer+9
         fbstp   [bbuffer]

         ; Check the sign
         mov     al, [ebp]
         dec     ebp
         or      al, al
         jns     .leading

         ; We got a negative number (should never happen)
         mov     al, '-'
         call    putchar

 .leading:
         ; Skip leading zeros
         mov     al, [ebp]
         dec     ebp
         or      al, al
         jne     .first
         cmp     ebp, bbuffer
         jae     .leading

         ; We are here because the result was 0.
         ; Print '0' and return
         mov     al, '0'
         jmp     putchar

 .first:
         ; We have found the first non-zero.
         ; But it is still packed
         test    al, 0F0h
         jz      .second
         push    eax
         shr     al, 4
         add     al, '0'
         call    putchar
         pop     eax
         and     al, 0Fh

 .second:
         add     al, '0'
         call    putchar

 .next:
         cmp     ebp, bbuffer
         jb      .done

         mov     al, [ebp]
         push    eax
         shr     al, 4
         add     al, '0'
         call    putchar
         pop     eax
         and     al, 0Fh
         add     al, '0'
         call    putchar

         dec     ebp
         jmp     short .next

 .done:
         pop     ebp
         or      ebp, ebp
         je      .ret

 .zeros:
         mov     al, '0'
         call    putchar
         dec     ebp
         jne     .zeros

 .ret:
         ret

   The code follows the same format as all the other filters we have
   seen before, with one subtle exception:

     We are no longer assuming that the end of input implies the end
     of things to do, something we took for granted in the
     character-oriented filters.

     This filter does not process characters. It processes a language
     (albeit a very simple one, consisting only of numbers).

     When we have no more input, it can mean one of two things:

       * We are done and can quit. This is the same as before.

       * The last character we have read was a digit. We have stored
         it at the end of our ASCII-to-float conversion buffer. We
         now need to convert the contents of that buffer into a
         number and write the last line of our output.

     For that reason, we have modified our getchar and our read
     routines to return with the carry flag clear whenever we are
     fetching another character from the input, or the carry flag set
     whenever there is no more input.

     Of course, we are still using assembly language magic to do
     that! Take a good look at getchar. It always returns with the
     carry flag clear.

     Yet, our main code relies on the carry flag to tell it when to
     quit-and it works.

     The magic is in read. Whenever it receives more input from the
     system, it just returns to getchar, which fetches a character
     from the input buffer, clears the carry flag and returns.

     But when read receives no more input from the system, it does
     not return to getchar at all. Instead, the add esp, byte 4 op
     code adds 4 to ESP, sets the carry flag, and returns.

     So, where does it return to? Whenever a program uses the call op
     code, the microprocessor pushes the return address, i.e., it
     stores it on the top of the stack (not the FPU stack, the system
     stack, which is in the memory). When a program uses the ret op
     code, the microprocessor pops the return value from the stack,
     and jumps to the address that was stored there.

     But since we added 4 to ESP (which is the stack pointer
     register), we have effectively given the microprocessor a minor
     case of amnesia: It no longer remembers it was getchar that
     called read.

     And since getchar never pushed anything before calling read, the
     top of the stack now contains the return address to whatever or
     whoever called getchar. As far as that caller is concerned, he
     called getchar, which returned with the carry flag set!

   Other than that, the bcdload routine is caught up in the middle of
   a Lilliputian conflict between the Big-Endians and the
   Little-Endians.

   It is converting the text representation of a number into that
   number: The text is stored in the big-endian order, but the packed
   decimal is little-endian.

   To solve the conflict, we use the std op code early on. We cancel
   it with cld later on: It is quite important we do not call
   anything that may depend on the default setting of the direction
   flag while std is active.

   Everything else in this code should be quite clear, providing you
   have read the entire chapter that precedes it.

   It is a classical example of the adage that programming requires a
   lot of thought and only a little coding. Once we have thought
   through every tiny detail, the code almost writes itself.

     --------------------------------------------------------------

  11.13.6 Using pinhole

   Because we have decided to make the program ignore any input
   except for numbers (and even those inside a comment), we can
   actually perform textual queries. We do not have to, but we can.

   In my humble opinion, forming a textual query, instead of having
   to follow a very strict syntax, makes software much more user
   friendly.

   Suppose we want to build a pinhole camera to use the 4x5 inch
   film. The standard focal length for that film is about 150mm. We
   want to fine-tune our focal length so the pinhole diameter is as
   round a number as possible. Let us also suppose we are quite
   comfortable with cameras but somewhat intimidated by computers.
   Rather than just have to type in a bunch of numbers, we want to
   ask a couple of questions.

   Our session might look like this:

 % pinhole

 Computer,

 What size pinhole do I need for the focal length of 150?
 150     490     306     362     2930    12
 Hmmm... How about 160?
 160     506     316     362     3125    12
 Let's make it 155, please.
 155     498     311     362     3027    12
 Ah, let's try 157...
 157     501     313     362     3066    12
 156?
 156     500     312     362     3047    12
 That's it! Perfect! Thank you very much!
 ^D

   We have found that while for the focal length of 150, our pinhole
   diameter should be 490 microns, or 0.49 mm, if we go with the
   almost identical focal length of 156 mm, we can get away with a
   pinhole diameter of exactly one half of a millimeter.

     --------------------------------------------------------------

  11.13.7 Scripting

   Because we have chosen the # character to denote the start of a
   comment, we can treat our pinhole software as a scripting
   language.

   You have probably seen shell scripts that start with:

 #! /bin/sh

   ...or...

 #!/bin/sh

   ...because the blank space after the #! is optional.

   Whenever UNIX is asked to run an executable file which starts with
   the #!, it assumes the file is a script. It adds the command to
   the rest of the first line of the script, and tries to execute
   that.

   Suppose now that we have installed pinhole in /usr/local/bin/, we
   can now write a script to calculate various pinhole diameters
   suitable for various focal lengths commonly used with the 120
   film.

   The script might look something like this:

 #! /usr/local/bin/pinhole -b -i
 # Find the best pinhole diameter
 # for the 120 film

 ### Standard
 80

 ### Wide angle
 30, 40, 50, 60, 70

 ### Telephoto
 100, 120, 140

   Because 120 is a medium size film, we may name this file medium.

   We can set its permissions to execute, and run it as if it were a
   program:

 % chmod 755 medium
 % ./medium

   UNIX will interpret that last command as:

 % /usr/local/bin/pinhole -b -i ./medium

   It will run that command and display:

 80      358     224     256     1562    11
 30      219     137     128     586     9
 40      253     158     181     781     10
 50      283     177     181     977     10
 60      310     194     181     1172    10
 70      335     209     181     1367    10
 100     400     250     256     1953    11
 120     438     274     256     2344    11
 140     473     296     256     2734    11

   Now, let us enter:

 % ./medium -c

   UNIX will treat that as:

 % /usr/local/bin/pinhole -b -i ./medium -c

   That gives it two conflicting options: -b and -c (Use Bender's
   constant and use Connors' constant). We have programmed it so
   later options override early ones-our program will calculate
   everything using Connors' constant:

 80      331     242     256     1826    11
 30      203     148     128     685     9
 40      234     171     181     913     10
 50      262     191     181     1141    10
 60      287     209     181     1370    10
 70      310     226     256     1598    11
 100     370     270     256     2283    11
 120     405     296     256     2739    11
 140     438     320     362     3196    12

   We decide we want to go with Bender's constant after all. We want
   to save its values as a comma-separated file:

 % ./medium -b -e > bender
 % cat bender
 focal length in millimeters,pinhole diameter in microns,F-number,normalized F-number,F-5.6 multiplier,stops from F-5.6
 80,358,224,256,1562,11
 30,219,137,128,586,9
 40,253,158,181,781,10
 50,283,177,181,977,10
 60,310,194,181,1172,10
 70,335,209,181,1367,10
 100,400,250,256,1953,11
 120,438,274,256,2344,11
 140,473,296,256,2734,11
 %

     --------------------------------------------------------------

11.14 Caveats

   Assembly language programmers who "grew up" under MS-DOS and
   Windows often tend to take shortcuts. Reading the keyboard scan
   codes and writing directly to video memory are two classical
   examples of practices which, under MS-DOS are not frowned upon but
   considered the right thing to do.

   The reason? Both the PC BIOS and MS-DOS are notoriously slow when
   performing these operations.

   You may be tempted to continue similar practices in the UNIX
   environment. For example, I have seen a web site which explains
   how to access the keyboard scan codes on a popular UNIX clone.

   That is generally a very bad idea in UNIX environment! Let me
   explain why.

     --------------------------------------------------------------

  11.14.1 UNIX Is Protected

   For one thing, it may simply not be possible. UNIX runs in
   protected mode. Only the kernel and device drivers are allowed to
   access hardware directly. Perhaps a particular UNIX clone will let
   you read the keyboard scan codes, but chances are a real UNIX
   operating system will not. And even if one version may let you do
   it, the next one may not, so your carefully crafted software may
   become a dinosaur overnight.

     --------------------------------------------------------------

  11.14.2 UNIX Is an Abstraction

   But there is a much more important reason not to try accessing the
   hardware directly (unless, of course, you are writing a device
   driver), even on the UNIX like systems that let you do it:

   UNIX is an abstraction!

   There is a major difference in the philosophy of design between
   MS-DOS and UNIX. MS-DOS was designed as a single-user system. It
   is run on a computer with a keyboard and a video screen attached
   directly to that computer. User input is almost guaranteed to come
   from that keyboard. Your program's output virtually always ends up
   on that screen.

   This is NEVER guaranteed under UNIX. It is quite common for a UNIX
   user to pipe and redirect program input and output:

 % program1 | program2 | program3 > file1

   If you have written program2, your input does not come from the
   keyboard but from the output of program1. Similarly, your output
   does not go to the screen but becomes the input for program3 whose
   output, in turn, goes to file1.

   But there is more! Even if you made sure that your input comes
   from, and your output goes to, the terminal, there is no guarantee
   the terminal is a PC: It may not have its video memory where you
   expect it, nor may its keyboard be producing PC-style scan codes.
   It may be a Macintosh, or any other computer.

   Now you may be shaking your head: My software is in PC assembly
   language, how can it run on a Macintosh? But I did not say your
   software would be running on a Macintosh, only that its terminal
   may be a Macintosh.

   Under UNIX, the terminal does not have to be directly attached to
   the computer that runs your software, it can even be on another
   continent, or, for that matter, on another planet. It is perfectly
   possible that a Macintosh user in Australia connects to a UNIX
   system in North America (or anywhere else) via telnet. The
   software then runs on one computer, while the terminal is on a
   different computer: If you try to read the scan codes, you will
   get the wrong input!

   Same holds true about any other hardware: A file you are reading
   may be on a disk you have no direct access to. A camera you are
   reading images from may be on a space shuttle, connected to you
   via satellites.

   That is why under UNIX you must never make any assumptions about
   where your data is coming from and going to. Always let the system
   handle the physical access to the hardware.

     Note: These are caveats, not absolute rules. Exceptions are
     possible. For example, if a text editor has determined it is
     running on a local machine, it may want to read the scan codes
     directly for improved control. I am not mentioning these caveats
     to tell you what to do or what not to do, just to make you aware
     of certain pitfalls that await you if you have just arrived to
     UNIX form MS-DOS. Of course, creative people often break rules,
     and it is OK as long as they know they are breaking them and
     why.

     --------------------------------------------------------------

11.15 Acknowledgements

   This tutorial would never have been possible without the help of
   many experienced FreeBSD programmers from the FreeBSD technical
   discussions mailing list, many of whom have patiently answered my
   questions, and pointed me in the right direction in my attempts to
   explore the inner workings of UNIX system programming in general
   and FreeBSD in particular.

   Thomas M. Sommers opened the door for me. His How do I write
   "Hello, world" in FreeBSD assembler? web page was my first
   encounter with an example of assembly language programming under
   FreeBSD.

   Jake Burkholder has kept the door open by willingly answering all
   of my questions and supplying me with example assembly language
   source code.

   Copyright (c) 2000-2001 G. Adam Stanislav. All rights reserved.

                             V. Appendices

   Table of Contents

   Bibliography

  Bibliography

   [1] Dave A Patterson and John L Hennessy, 1998, 1-55860-428-6,
   Morgan Kaufmann Publishers, Inc., Computer Organization and
   Design: The Hardware / Software Interface, 1-2.

   [2] W. Richard Stevens, 1993, 0-201-56317-7, Addison Wesley
   Longman, Inc., Advanced Programming in the Unix Environment, 1-2.

   [3] Marshall Kirk McKusick and George Neville-Neil, 2004,
   0-201-70245-2, Addison-Wesley, The Design and Implementation of
   the FreeBSD Operating System, 1-2.

   [4] Aleph One, Phrack 49; "Smashing the Stack for Fun and Profit".

   [5] Chrispin Cowan, Calton Pu, and Dave Maier, StackGuard;
   Automatic Adaptive Detection and Prevention of Buffer-Overflow
   Attacks.

   [6] Todd Miller and Theo de Raadt, strlcpy and strlcat --
   consistent, safe string copy and concatenation..

  Notes

   [1] If you run it in the shell, you may get a core dump.           
   [2] In case you did not know, a binary sort is an efficient way of 
       sorting things into order and a bubble sort is not.            
   [3] The reasons for this are buried in the mists of history.       
   [4] Note, we did not use the -o flag to specify the executable     
       name, so we will get an executable called a.out. Producing a   
       debug version called foobar is left as an exercise for the     
       reader!                                                        
   [5] They do not use the MAKEFILE form as block capitals are often  
       used for documentation files like README.                      
   [6] Many Emacs users set their EDITOR environment to emacsclient   
       so this happens every time they need to edit a file.           

     --------------------------------------------------------------
