How to make a GDB breakpoint only break after the point is reached a given number times?

Question

I have a function that is called some large number of times, and eventually segfaults.

However, I don't want to set a breakpoint at this function and stop after every time it's called, because I will be here for years.

I've heard that I can set a counter in GDB for a breakpoint, and each time the breakpoint is hit, the counter is decremented, and only gets triggered when the counter = 0.

Is this accurate, and if so how do I do it? Please give the gdb code for setting such a breakpoint.

Answer 1

Read section 5.1.6 of the GDB manual. What you have to do is first set a breakpoint, then set an 'ignore count' for that breakpoint number, e.g. ignore 23 1000.

If you don't know how many times to ignore the breakpoint, and don't want to count manually, the following may help:

  ignore 23 1000000   # set ignore count very high.

  run                 # the program will SIGSEGV before reaching the ignore count.
                      # Once it stops with SIGSEGV:

  info break 23       # tells you how many times the breakpoint has been hit, 
                      # which is exactly the count you want

Answer 2

continue <n>

This is a convenient method that skips the last hit breakpoint n - 1 times:

gdb -n -q tmp.out
Reading symbols from tmp.out...done.
(gdb) l
1       #include <stdio.h>
2
3       int main(void) {
4           int i = 0;
5           while (1) {
6               i++;
7               printf("%d\n", i);
8           }
9       }
(gdb) start
Temporary breakpoint 1 at 0x6a8: file tmp.c, line 4.
Starting program: /home/ciro/bak/git/cpp-cheat/gdb/tmp.out

Temporary breakpoint 1, main () at tmp.c:4
4           int i = 0;
(gdb) b 6
Breakpoint 2 at 0x5555555546af: file tmp.c, line 6.
(gdb) c
Continuing.

Breakpoint 2, main () at tmp.c:6
6               i++;
(gdb) c 5
Will ignore next 4 crossings of breakpoint 2.  Continuing.
1
2
3
4
5

Breakpoint 2, main () at tmp.c:6
6               i++;
(gdb) p i
$1 = 5
(gdb)
(gdb) help c
Continue program being debugged, after signal or breakpoint.
Usage: continue [N]
If proceeding from breakpoint, a number N may be used as an argument,
which means to set the ignore count of that breakpoint to N - 1 (so that
the breakpoint won't break until the Nth time it is reached).