The debug facility permits interactively controlled execution of a program.
Changing the TRACE action to one with a prefix ? (for example, TRACE ?A or the TRACE built-in function) turns on interactive debug and indicates to the user that interactive debug is active. Further TRACE instructions in the program are ignored, and the language processor pauses after nearly all instructions that are traced at the console (see the following for the exceptions). When the language processor pauses, indicated by a READ in the lower corner of the screen, three debug actions are available:
Once the clause has been rerun, the language processor pauses again.
During execution of the string, no tracing takes place, except that nonzero return codes from host commands are displayed. Host commands are always run (that is, are not affected by the prefix ! on TRACE instructions), but the variable RC is not set.
Once the string has been processed, the language processor pauses again for further debug input unless a TRACE instruction was entered. In this latter case, the language processor immediately alters the tracing action (if necessary) and then continues executing until the next pause point (if any). To alter the tracing action (from All to Results, for example) and then rerun the instruction, you must use the built-in function TRACE (see page ***). For example, CALL TRACE I changes the trace action to I and allows re-execution of the statement after which the pause was made. Interactive debug is turned off, when it is in effect, if a TRACE instruction uses a prefix, or at any time when a TRACE O or TRACE with no options is entered.
You can use the numeric form of the TRACE instruction to allow sections of the program to be run without pause for debug input. TRACE n (that is, positive result) allows execution to continue, skipping the next n pauses (when interactive debug is or becomes active). TRACE -n (that is, negative result) allows execution to continue without pause and with tracing inhibited for n clauses that would otherwise be traced.
The trace action selected by a TRACE instruction is saved and restored across subroutine calls. This means that if you are stepping through a program (say after using TRACE ?R to trace Results) and then enter a subroutine in which you have no interest, you can enter TRACE O to turn tracing off. No further instructions in the subroutine are traced, but on return to the caller, tracing is restored.
Similarly, if you are interested only in a subroutine, you can put a TRACE ?R instruction at its start. Having traced the routine, the original status of tracing is restored and (if tracing was off on entry to the subroutine) tracing (and interactive debug) is turned off until the next entry to the subroutine.
Since any instructions may be run in interactive debug, you have considerable control over execution.
Say expr /* displays the result of evaluating the */
/* expression. */
name=expr /* alters the value of a variable. */
Trace O /* (or Trace with no options) turns off */
/* interactive debug and all tracing. */
Trace ?A /* turns off interactive debug but continues */
/* tracing all clauses. */
Trace L /* makes the language processor pause at labels */
/* only. This is similar to the traditional */
/* "breakpoint" function, except that you */
/* do not have to know the exact name and */
/* spelling of the labels in the program. */
exit /* stops execution of the program. */
Do i=1 to 10; say stem.i; end
/* displays ten elements of the array stem. */