Re: g_spawn_sync and child PID

[Colomban Wendling <lists ban herbesfolles org>]

Le 15/03/2014 21:19, Allin Cottrell a écrit :
> I'm reasonably familiar with the usage of both g_spawn_sync and
> g_spawn_async, but recently I've found myself wanting to do domething
> that seems to fall between the two and I can't quite work out how to do it.
>
> Here's the task: I want to spawn an mpiexec process from my GTK app and,
> assuming all goes well, wait for the result (in itself that's a case of
> g_spawn_sync). But I also want to know the PID of mpiexec, so that I can
> give the user the option of killing that process "prematurely" if it
> seems to be going wrong. (This is based on the observation that an
> MPI_Bcast call, for example, can hang at 100% CPU if one or more of the
> processes launched by mpiexec bombs out at just the wrong moment.)
>
> Any suggestions on how to achieve this using g_spawn* (or judgements on
> whether it's even possible) would be gratefully received.

If you need any user interaction, you probably need an asynchronous
spawn anyway, so you need to use g_spawn_async() and use
g_child_watch_add() to get notified when the child quits.  But since
this is asynchronous, your program have to handle the case stuff happens
when the process is running (like click on a cancel button, but also
anything else).

Also, how complex it'd be will depend on whether you need to write to
the child's stdin and/or read from its stderr/stdout.
If you open the pipes, you *have* to make sure none ever gets full,
which would block the child.  This means you have to also asynchronously
read and write to the child.
If you don't communicate with the subprocess however, it's only a matter
of waiting for the child asynchronously to let the UI interact with the
user, and that's pretty easy as explained above.

BTW, GLib spawn API also lacks a portable way to kill the subprocess, so
one has to manually use platform-specific API like kill() or
TerminateProcess() -- not hard to do, but has to be done.


Anyway, since all this can be tricky, I suggest you to try and use the
brand new GIO GSubprocess API if you can, it does all the tricky job
itself (and has API to kill the subprocess :) -- but it's still
unreleased https://developer.gnome.org/gio/unstable/GSubprocess.html.

Regards,
Colomban