Debugging with GDB (September 2007)

40 Debugging with GDB

enable [breakpoints] delete range...

Enable the speciﬁed breakp oints to work once, then die. GDB deletes any of

these breakpoints as soon as your program stops there.

Except for a breakpoint set with tbreak (see Section 5.1.1 [Setting breakpoints],

page 33), breakpoints that you set are initially e nabled; subsequently, they become

disabled or enabled only when you use one of the commands above. (The command until

can set and delete a breakpoint of its own, but it does not change the state of your other

breakpoints; see Section 5.2 [Continuing and stepping], page 44.)

5.1.5 Break conditions

The simplest sort of breakp oint 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 Section 8.1 [Expressions], page 63). 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 expresse d by the condition assert, you should set the condition

‘! assert’ on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not nee d 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 Section 5.1.6 [Breakpoint command lists], page 41).

Break conditions can be speciﬁed when a breakpoint is set, by using ‘if’ in the arguments

to the break command. See Section 5.1.1 [Setting breakpoints], page 33. They can also be

changed at any time with the condition command.

You can also use the if keyword w ith 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 catch-

point 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