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Re: Daniel, thread vs. fork question.


On Thu, Mar 11, 2004 at 01:08:40AM +0000, Michael Snyder wrote:
> Hey Daniel,
> 
> Got a question concerning the code in 
> linux-nat.c::linux_handle_extended_wait.
> 
> You've got a PTRACE_EVENT_FORK event, and now you're going to call 
> waitpid.  You pull a pid out of a list of stopped pids, and wait for
> it using waitpid.  In your comment, you explain that you don't have to
> worry about the pid being a clone, because you didn't ask for pids in
> the event mask.
> 
> But how is this affected by threads, especially NPTL threads?
> I've got a fairly simple test-case (modified from pthreads.c,
> I'll attach it), in which a child thread calls fork -- but gdb
> apparently tries to wait on the main thread (or perhaps the most
> recent event thread).  Since that's not the thread that called
> fork, waitpid returns -1 with "no child".  Gdb reports:
> 	waiting for new child: No child processes.
> 
> FWIW, I've tried this on both a single-processor and an SMP machine.

Actually, what happens is GDB tries to wait on pid 0.  Here's why:

      errno = 0;
      ret = ptrace (PTRACE_GETEVENTMSG, pid, 0, &new_pid);
      printf ("getevent: ret %d, errno %d, new_pid %d\n", ret, errno, new_pid);

getevent: ret 0, errno 0, new_pid 0
waiting for new child: No child processes.

So PTRACE_GETEVENTMSG did not return the correct message.  I can't see
why it doesn't.  My guess is that I overlooked something big in the
kernel-side implementation but I'm going to have to go digging to
figure out what it is.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer


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