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# 10.12. 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.

## 10.12.1. 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.

### 10.12.1.1. 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.

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