Break Commands (Debugging with GDB)
5.1.7 Breakpoint Command Lists
You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.
… command-list …
Specify a list of commands for the given breakpoints. The commands themselves appear on the following lines. Type a line containing just
end to terminate the commands.
To remove all commands from a breakpoint, type
commands and follow it immediately with
end; that is, give no commands.
With no argument,
commands refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered). If the most recent breakpoints were set with a single command, then the
commands will apply to all the breakpoints set by that command. This applies to breakpoints set by
rbreak, and also applies when a single
break command creates multiple breakpoints (see Ambiguous Expressions).
Pressing RET as a means of repeating the last GDB command is disabled within a
You can use breakpoint commands to start your program up again. Simply use the
continue command, or
step, or any other command that resumes execution.
Any other commands in the command list, after a command that resumes execution, are ignored. This is because any time you resume execution (even with a simple
step), you may encounter another breakpoint—which could have its own command list, leading to ambiguities about which list to execute.
If the first command you specify in a command list is
silent, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the remaining commands print anything, you see no sign that the breakpoint was reached.
silent is meaningful only at the beginning of a breakpoint command list.
printf allow you to print precisely controlled output, and are often useful in silent breakpoints. See Commands for Controlled Output.
For example, here is how you could use breakpoint commands to print the value of
x at entry to
x is positive.
break foo if x>0 commands silent printf "x is %d\n",x cont end
One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the
continue command so that your program does not stop, and start with the
silent command so that no output is produced. Here is an example:
break 403 commands silent set x = y + 4 cont end