You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.
… command-list …
Specify a list of commands for the given breakpoints. The commands
themselves appear on the following lines. Type a line containing just
end to terminate the commands.
To remove all commands from a breakpoint, type
follow it immediately with
end; that is, give no commands.
With no argument,
commands refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered). If the most recent breakpoints were set with a single
command, then the
commands will apply to all the breakpoints
set by that command. This applies to breakpoints set by
rbreak, and also applies when a single
creates multiple breakpoints (see Ambiguous
Pressing RET as a means of repeating the last GDB command is disabled within a command-list.
You can use breakpoint commands to start your program up again. Simply
continue command, or
step, or any other command
that resumes execution.
Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple
step), you may encounter
another breakpoint—which could have its own command list, leading to
ambiguities about which list to execute.
If the first command you specify in a command list is
usual message about stopping at a breakpoint is not printed. This may
be desirable for breakpoints that are to print a specific message and
then continue. If none of the remaining commands print anything, you
see no sign that the breakpoint was reached.
meaningful only at the beginning of a breakpoint command list.
printf allow you to
print precisely controlled output, and are often useful in silent
breakpoints. See Commands for Controlled Output.
For example, here is how you could use breakpoint commands to print the
x at entry to
x is positive.
break foo if x>0 commands silent printf "x is %d\n",x cont end
One application for breakpoint commands is to compensate for one bug so
you can test for another. Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them. End with the
so that your program does not stop, and start with the
command so that no output is produced. Here is an example:
break 403 commands silent set x = y + 4 cont end