HP gnu source-level debugger 5992-4701 manual Stops your program only if the value

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 61)).

Break conditions can be specified when a breakpoint is set, by using 'if' in the arguments to the break command. See “Setting breakpoints” (page 52). 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 83).
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

60 Stopping and Continuing