| Debugging with GDB: Conditions |
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The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language (see Expressions). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.
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).
Breakpoint conditions can also be evaluated on the target’s side if the target supports it. Instead of evaluating the conditions locally, GDB encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDB. Global variables become raw memory locations, locals become stack accesses, and so forth.
In this case, GDB will only be notified of a breakpoint trigger when its condition evaluates to true. This mechanism may provide faster response times depending on the performance characteristics of the target since it does not need to keep GDB informed about every breakpoint trigger, even those with false conditions.
Break conditions can be specified when a breakpoint is set, by using
‘if’ in the arguments to the break command. See Setting Breakpoints. 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.
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 to
continue, rather than using ignore. See Continuing and Stepping.
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.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
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