process attaching gdb to itself

[Andrew Cagney]

>> 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.

> 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.

> 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?

Andrew