This appears to be an oversight on the developers of gdb's part, but
direct from the info page:
GDB has no special support for debugging programs which create
additional processes using the `fork' function. When a program forks,
GDB will continue to debug the parent process and the child process
will run unimpeded. If you have set a breakpoint in any code which the
child then executes, the child will get a `SIGTRAP' signal which
(unless it catches the signal) will cause it to terminate.
However, if you want to debug the child process there is a workaround
which isn't too painful. Put a call to `sleep' in the code which the
child process executes after the fork. It may be useful to sleep only
if a certain environment variable is set, or a certain file exists, so
that the delay need not occur when you don't want to run GDB on the
child. While the child is sleeping, use the `ps' program to get its
process ID. Then tell GDB (a new invocation of GDB if you are also
debugging the parent process) to attach to the child process (see *Note
Attach::). From that point on you can debug the child process just
like any other process which you attached to.
On Wed, 8 Sep 1999, Zhihui Zhang wrote:
> On Wed, 8 Sep 1999, Kip Macy wrote:
>
> > You need to detach from your current process and attach to the spawned
> > process. It might make it easier to attach in a timely fashion if you put
> > a 3 second sleep in right after the fork. This would all be easiest using
> > something like DDD where DDD will tell you what other processes are
> > running with the same name, and allow you to attach to them through the
> > GUI.
>
> In dbx on a Sun workstation, all I need to do to follow a child process
> after fork() is to use the following command in advance:
>
> (dbx)dbxenv follow_fork_mode child
>
> Your response suggests that I can not achieve the same result simply by
> using (I am using gdb 4.18):
>
> (gdb)set follow-fork-mode child
>
> I have to use attach and dettach to do so. Does that mean I have to
> display the pid of the new process in order to follow it. And I have to
> modify the child process so that it can wait until I can attach to it.
> That will not be as easy.
>
> -Zhihui
>
>
> >
> >
> >
> > On Wed, 8 Sep 1999, Zhihui Zhang wrote:
> >
> > >
> > > I am using gdb 4.18 on FreeBSD-current. The program being debugged
> > > consists of two small files: test1.c and test2.c. The main() in test1.c
> > > has a call to fork() and for the child process case, it will call a
> > > routine, say test(), in test2.c.
> > >
> > > I use "set follow-fork-mode child", "break fork", "step" command trying to
> > > access the source in test2.c without success. The program is compiled
> > > with "cc -g test1.c test2.c" and I run gdb with "gdb a.out".
> > >
> > > If there is no fork(), a call from test1.c to a routine in test2.c will
> > > bring up the source of test2.c if I step that routine. Why it does not
> > > work with fork()? Am I missing something?
> > >
>
>
>
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