(gdb) break *0x972

Debugging, GNU± Linux and WebHosting and ... and ...

Break if outside of section

Just a little example of how to use gdb.py to create a breakpoint that only fires if called outside of a give section :

int main() {

  for (int i = 0; i < 100; i++) {
    if (rand() > THRESHOLD) do_in_section(0);


Functions start() and stop() delimit the section: it is okay to call do_in_section() inside the section. However, every once in a while, do_in_section() is called from outside of this section. (Forget about that boolean flag parameter, it's just for logging and consistency checks. Real-life codes won't have such an easy way to discriminate outside from inside call ... otherwise you don't need an advanced breakpoint!)

(gdb) break do_in_section

will lead to many useless stops, as most of the call to do_in_section() are legal, so what should do is:

  • set an internal breakpoint on start(), where we set a flag and continue
  • set an internal breakpoint on stop()` where we unset the flag and continue
  • set a breakpoint on do_in_section() that checks that flag, and only stop if outside of the section

which gives in Python:

import gdb

in_section = False

class StartStopBreakpoint(gdb.Breakpoint):
    def __init__(self, loc, is_start):
        gdb.Breakpoint.__init__(self, loc, internal=True)
        self.silent = True

        self.is_start = is_start

    def stop(self):
        global in_section

        if self.is_start: # I know that this 'if' is not necessary ...
            assert not in_section
            in_section = True
            assert in_section
            in_section = False

        return False # never stop here

class SectionBreakpoint(gdb.Breakpoint):
    def __init__(self, location):
        gdb.Breakpoint.__init__(self, location, internal=True)
        self.silent = True

    def stop(self):
        if in_section:
            print("ignore hit in section")
            assert gdb.parse_and_eval("inside") == 1
            return False

            print("Section breakpoint hit outside of section")
            assert gdb.parse_and_eval("inside") == 0
            return True

StartStopBreakpoint("start", is_start=True)
StartStopBreakpoint("stop", is_start=False)

Just source that file, or input it in GDB python commandline. The three last line instantiate the breakpoints:

$ gdb -ex "source test.py" a.out
GNU gdb (GDB)
(gdb) run
Starting program: /home/kevin/a.out 
ignore ....

Section breakpoint hit outside of section

(gdb) print inside
$1 = 0

Et voila!