Re: GDB hangs on kill or quit (after following a fork child, not detaching from the parent)

[Pedro Alves <pedro@codesourcery.com>]

On Thursday 18 December 2008 21:03:41, Pedro Alves wrote:

> Yeah, I should have mentioned it before:  At first I also
> thought it was checkpoints related, then I noticed that
> when 'set follow-fork-mode' is child, checkpoints are broken
> for other reasons.  It may well be that it always was (broken):
> 
> gdb-6.8:
> 
>  (top-gdb) set follow-fork-mode child
>  (top-gdb) start
>  Breakpoint 3 at 0x4509a7: file ../../src/gdb/gdb.c, line 28.
>  Starting program: /home/pedro/gdb/baseline/build/gdb/gdb
>  [Thread debugging using libthread_db enabled]
>  [New Thread 0x7ffff7fd36e0 (LWP 24392)]
>  [Switching to Thread 0x7ffff7fd36e0 (LWP 24392)]
>  main (argc=1, argv=0x7fffffffe3f8) at ../../src/gdb/gdb.c:28
>  28        memset (&args, 0, sizeof args);
>  (top-gdb) checkpoint
>  warning: Can't attach process 24400: Operation not permitted
>  /build/buildd/gdb-6.8/gdb/linux-thread-db.c:302: internal-error: thread_get_info_callback: Assertion `thread_info != NULL' failed.
>  A problem internal to GDB has been detected,
>  further debugging may prove unreliable.
>  Quit this debugging session? (y or n)  
> 

That was a thread-db related issue, which you could claim is a bit
undefined for the current checkpoints support.

For completeness, here's a non-threaded example showing how
following the child behaved:

>gdb /home/pedro/gdb/tests/test
GNU gdb 6.8-debian
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu"...
Setting up the environment for debugging gdb.
Function "internal_error" not defined.
Make breakpoint pending on future shared library load? (y or [n]) [answered N; input not from terminal]
Function "info_command" not defined.
Make breakpoint pending on future shared library load? (y or [n]) [answered N; input not from terminal]
/home/pedro/gdb/baseline/build/gdb/.gdbinit:8: Error in sourced command file:
No breakpoint number 0.
(gdb) start
Breakpoint 1 at 0x40044c: file main.c, line 5.
Starting program: /home/pedro/gdb/tests/test
main () at main.c:5
5         return 0;
(gdb) maint print target-stack
The current target stack is:
  - child (Unix child process)
  - exec (Local exec file)
  - None (None)
(gdb) set follow-fork-mode child
(gdb) checkpoint
[Switching to process 28696]
checkpoint: fork returned pid 0.
Failed to find new fork
(gdb)  

-- 
Pedro Alves