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

This is the converse of using assertions for program validation; in that situation, you want to stop
when the assertion is violated―that is, when the condition is false. In C, if you want to test an
assertion expressed by the condition assert, you should set the condition '! assert' on the appropriate
breakpoint.
Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is
inspecting the value of an expression anyhow―but it might be simpler, say, to just set a watchpoint
on a variable name, and specify a condition that tests whether the new value is an interesting one.
Break conditions can have side effects, and may even call functions in your program. This can be
useful, for example, to activate functions that log program progress, or to use your own print
functions to format special data structures. The effects are completely predictable unless there is
another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint
first and stop your program without checking the condition of this one.) Note that breakpoint
commands are usually more convenient and flexible than break conditions for the purpose of
performing side effects when a breakpoint is reached (see “Breakpoint command lists” (page 43)).
Break conditions can be specified when a breakpoint is set, by using 'if' in the arguments to the
break command. See “Setting breakpoints” (page 36). They can also be changed at any time
with the condition command.
You can also use the if keyword with the watch command. The catch command does not
recognize the if keyword; condition is the only way to impose a further condition on a catchpoint.
condition bnum expression Specify expression as the break condition for breakpoint,
watchpoint, or catchpoint number bnum. After you set a
condition, breakpoint bnum stops your program only if the
value of expression is true (nonzero, in C). When you
use condition, GDB checks expression immediately
for syntactic correctness, and to determine whether symbols
in it have referents in the context of your breakpoint. If
expression uses symbols not referenced in the context of
the breakpoint, GDB prints an error message:
No symbol "foo" in current context.
GDB does not actually evaluate expression at the time the
condition command (or a command that sets a breakpoint
with a condition, like break if ...) is given, however. See
“Expressions” (page 58).
condition bnum Remove the condition from breakpoint number bnum. It
becomes an ordinary unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the breakpoint has been reached a
certain number of times. This is so useful that there is a special way to do it, using the ignore
count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the
time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint
whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one
and continues. As a result, if the ignore count value is n, the breakpoint does not stop the next n
times your program reaches it.
ignore bnum count Set the ignore count of breakpoint number bnum to count. The next
count times the breakpoint is reached, your program's execution
does not stop; other than to decrement the ignore count, GDB takes
no action.
To make the breakpoint stop the next time it is reached, specify a count
of zero.
When you use continue to resume execution of your program from
a breakpoint, you can specify an ignore count directly as an argument
42 Stopping and Continuing