Trapping the segfault of a subprocess.Popen

Nobody nobody at
Thu Apr 7 01:58:33 CEST 2011

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 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):
        os.kill(, 0)
    except OSError, e:
        if e.errno == errno.ESRCH:
            return False
    return True

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

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