Question

I have a application which forks a child process.

Child process does some work and somewhere in the middle it gives Segmentation fault. I used GDB to debug this, I used:

set follow-fork-mode child

I have also set a breakpoint to a function within the child. But GDB doesn’t pause at my breakpoint.

Also the parent process handles the seg-fault so I had to ctrl-c to exit. Then when I use backtrace to print the stack all I got is

No stack

Why is the breakpoint not being set and why didn’t I get the stack?

Answer 1

Why is the breakpoint not being set

The breakpoint is being set, but it is not being hit because …

and why didn’t I get the stack?

… you are apparently debugging the wrong process.

With set follow-fork-mode child, GDB will follow the first child you create. Perhaps you create more than one?

One way to debug this is to establish a SIGSEGV handler using signal or sigaction.

In the handler, do this:

void handler(int signo)
{
  int i = 1;
  fprintf(stderr, "pid=%d, got signal=%d\n", getpid(), signo);
  while (i) { }
}

Once you see the message printed, in another window:

 gdb /proc/<pid>/exe <pid>
 (gdb) where