Debugging fork()'ing processes with gdb(1)
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Lukas Fleischer <info at cryptocrack dot de>

I ran into the issue of debugging fork()'ing programs today when helping
someone to debug a helper program of the GNOME Session Manager that crashed
unexpectedly, printing "Failed to execute child process: [...]" to stderr.
Digging deeper into the code, it emerged that the error occured in a Glib2
function called fork_exec_with_pipes() that uses a double fork() and
subsequently execv()'s in the child process. As everything seemed to originate
from the execv() syscall, we needed to find a way to debug those child
processes. To sum up, there's actually two obvious ways of achieving this:

1. "attach $pid" to the new process. Not feasible if you need to debug the
   child process just in time.

2. "set follow-fork-mode child" makes gdb follow child processes whenever a
   program fork()s. Use "set follow-fork-mode parent" to switch back to the
   default behaviour. "set follow-fork-mode ask" can be used if you want to be
   prompted on every single fork().