Re: process attaching gdb to itself

[Daniel Jacobowitz <drow@mvista.com>]

On Mon, Sep 29, 2003 at 09:21:55AM -0400, Andrew Cagney wrote:
> 
> >>works on BSD but fails on GNU/Linux.  When doing an attach, BSD always 
> >>generates something for wait4 to consume. GNU/Linux does not, leaving 
> >>GDB stuck in wait4 :-(
> >
> >
> >Yes, I've known about this problem for a long time.  We've [I, Roland,
> 
> This explains something.

I beg your pardon?

> >a couple of other people I can't recall] talked about changing it and
> >decided that, really, the current behavior makes more sense.
> 
> Not to me.
> 
> GDB sends a message to the kernel asking for the process to stop.  The 
> kernel sends a message back indicating that the request has completed.

The kernel generates a message at each change of the program's state. 
It isn't changing state; it was already stopped.  This behavior allows
the debugger to determine if the program was stopped before attach; I
can easily picture a multi-threaded daemon design that leaves parts of
itself SIGSTOP'd and would get confused if unexpectedly wakened.

> >It's not at all hard to make GDB work in the current system anyway. 
> >Just have to do it.  It goes something like:
> >  - attach
> >  - wait4 WNOHANG, break if succeeds (optimistic, not necessary)
> >  - check in /proc to make sure the process is in a stopped state
> >    - If it was:
> >      - wait4 WNOHANG
> >        - If we get a status, then the process was running when we attached
> >        - If no status is available then the process was stopped when
> >          we attached
> >    - If it wasn't:
> >      - The process was running when we attached and hasn't stopped yet
> >      - wait4 without WNOHANG
> 
> I feel ill.  What happens, for instance, if /proc isn't there?

At this point in its life, Linux can just assume the /proc filesystem
is available.  Linus has repeatedly refused to duplicate information
available via /proc through another interface.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer