|Debugging with GDB||www.imodulo.com · 2003-04-05|
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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 (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 (Breakpoint command lists).
Break conditions can be specified when a breakpoint is set, by using
if in the arguments to the
break command. Setting breakpoints. They can also be changed at any time with the
You can also use the
if keyword with the
watch command. The
catch command does not recognize the
condition is the only way to impose a further condition on a catchpoint.
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. Expressions.
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.
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. 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. Convenience variables.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
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