Debugging with GDB Manual The GNU Source-Level Debugger (769148-001, March 2014)

to continue, rather than using ignore. See “Continuing and
stepping” (page 45).
If a breakpoint has a positive ignore count and a condition, the
condition is not checked. Once the ignore count reaches zero, GDB
resumes checking the condition.
You could achieve the effect of the ignore count with a condition such
as '$foo-- <= 0' using a debugger convenience variable that is
decremented each time. See “Convenience variables” (page 68).
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
Breakpoint command lists
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.
commands [bnum], ...
command-list ..., end
Specify a list of commands for breakpoint number bnum. 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 commands and
follow it immediately with end; that is, give no commands.
With no bnum argument, commands refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered).
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 use the 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 next or step), you may
encounter another breakpoint―which could have its own command list, leading to ambiguities
about which list to execute. On Itanium systems, if there is a return command in the command
list, any command after the return command is ignored.
If the first command you specify in a command list is silent, the 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. silent is meaningful only at the beginning of a breakpoint
command list.
The commands echo, output, and printf allow you to print precisely controlled output, and
are often useful in silent breakpoints. See “Commands for controlled output” (page 222).
For example, here is how you could use breakpoint commands to print the value of x at entry to
foo whenever 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 continue command so that your program does not stop, and start
with the silent command so that no output is produced. Here is an example:
Breakpoints 43