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Re: Problems debugging forked processes

On Fri, Apr 19, 2002 at 09:45:49AM +0200, Frank Schaefer wrote:
> Hi there,
> I've written a network daemon and client. The daemon forks to serve a
> client request.
> Everything runs fine outside of the debugger, but I want to trace the
> code at least once, before I release it.
> Here goes my problem:
> # gdb daemon
> GNU gdb 5.0
> Copyright 2000 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public License, and you
> are
> welcome to change it and/or distribute copies of it under certain
> conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show warranty" for
> details.
> This GDB was configured as "i686-pc-linux-gnu"...
> (gdb) b 13
> Breakpoint 1 at 0x8048899: file daemon.c, line 13.
> (gdb) show follow-fork-mode
> Debugger response to a program call of fork or vfork is "parent".
> (gdb) set follow-fork-mode child
> (gdb) show follow-fork-mode
> Debugger response to a program call of fork or vfork is "child".

This variable doesn't actually affect GNU/Linux.  Yeah, I know, I know. 
We need a documentation patch to address this; please file a PR
(, Bugs).

> If I use the attach method from a second instance of gdb ( there's the
> sleep() above for ), the child process exits on exceptions ( mostly
> segmentation fault and sometimes invalid operation ) on different
> (random???) lines in the code. This happens everytime in the fixup()
> from

Because the breakpoints which were inserted when the application forked
turn into traps.  There's no clear solution to this without actually
implementing fork-following - which should not be hard, but no one has
stepped up to do it.

Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer

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