Re: Thread exit error : gdb7.2 in FreeBSD (built from ports)

[Pedro Alves <pedro@codesourcery.com>]

On Thursday 22 September 2011 17:34:27, Jusctsch wrote:
> At this point, our inferior_ptid is the thread we planned on deleting, so we
> don't delete it and flag it as exited. It does do some cleanup however, and
> dips back into fbsd-threads.c a few times... Let's continue.
> 
> GDB on Application
> 
> infrun: target_wait (-1, status) =
> infrun:   1461 [Thread 831ff4900 (LWP 100269 BMC Event Log m)],
> infrun:   status->kind = stopped, signal = SIGTRAP
> infrun: infwait_normal_state
> infrun: TARGET_WAITKIND_STOPPED
> infrun: stop_pc = 0x801b169c1
> 
> GDB on GDB
> 
> Breakpoint 5, get_current_frame () at frame.c:1177
> 1177       error (_("Invalid selected thread."));
> 
> (gdb)
> (gdb) p inferior_ptid
> $6 = {pid = 1461, lwp = 0, tid = 100269}
> (gdb)

I still don't understand this.  If this thread has exited
before, then why is the backend reporting a
TARGET_WAITKIND_STOPPED for it now?  Did a new thread reappear later
out of nothing that reuses the same ID, description and all?

If by magic that's the case, then it's this bit in
infrun.c:handle_inferior_event:

  /* If it's a new process, add it to the thread database.  */

  ecs->new_thread_event = (!ptid_equal (ecs->ptid, inferior_ptid)
			   && !ptid_equal (ecs->ptid, minus_one_ptid)
			   && !in_thread_list (ecs->ptid));

  if (ecs->ws.kind != TARGET_WAITKIND_EXITED
      && ecs->ws.kind != TARGET_WAITKIND_SIGNALLED && ecs->new_thread_event)
    add_thread (ecs->ptid);

in_thread_list returns true for an exited thread, but we should end
up in add_thread as well if ecs->ptid is in the thread list marked exited.

-- 
Pedro Alves