Trapping the segfault of a subprocess.Popen

Pierre GM pierregmcode at
Thu Apr 7 10:44:54 CEST 2011

On Apr 7, 1:58 am, Nobody <nob... at> wrote:
> On Wed, 06 Apr 2011 02:20:22 -0700, Pierre GM wrote:
> > I need to run a third-party binary from a python script and retrieve
> > its output (and its error messages). I use something like
> >>>> process = subprocess.Popen(options, stdout=subprocess.PIPE,
> > stderr=subprocess.PIPE)
> >>>> (info_out, info_err) = process.communicate()
> > That works fine, except that the third-party binary in question doesn't
> > behave very nicely and tend to segfaults without returning any error. In
> > that case, `process.communicate` hangs for ever.
> Odd. The .communicate method should return once both stdout and stderr
> report EOF and the process is no longer running. Whether it terminates
> normally or on a signal makes no difference.
> The only thing I can think of which would cause the situation which you
> describe is if the child process spawns a child of its own before
> terminating. In that case, stdout/stderr won't report EOF until any
> processes which inherited them have terminated.

I think you nailed it. Running the incriminating command line in a
terminal doesn't return to the prompt. In fact, a ps shows that the
process is sleeping in the foreground. Guess I should change the
subject of this thread...

> > I thought about calling a `threading.Timer` that would call
> > `process.terminate` if `process.wait` doesn't return after a given
> > time... But it's not really a solution: the process in question can
> > sometimes take a long time to run, and I wouldn't want to kill a
> > process still running.
> > I also thought about polling every x s and stopping when the result of
> > a subprocess.Popen(["ps","-p",str(],
> > stdout=subprocess.PIPE) becomes only the header line, but my script
> > needs to run on Windows as well (and no ps over there)...
> It should suffice to call .poll() on the process. In case that doesn't
> work, the usual alternative would be to send signal 0 to the process (this
> will fail with ESRCH if the process doesn't exist and do nothing
> otherwise), e.g.:
> import os
> import errno
> def exists(process):
>     try:
>         os.kill(, 0)
>     except OSError, e:
>         if e.errno == errno.ESRCH:
>             return False
>         raise
>     return True

OK, gonna try that, thx.

> You might need to take a different approach for Windows, but the above is
> preferable to trying to parse "ps" output. Note that this only tells you
> if /some/ process exists with the given PID, not whether the original
> process exists; that information can only be obtained from the Popen
> object.

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