robert robert -4 years ago 220
Python Question

Python: How to prevent subprocesses from receiving CTRL-C / Control-C / SIGINT

I am currently working on a wrapper for a dedicated server running in the shell. The wrapper spawns the server process via subprocess and observes and reacts to its output.

The dedicated server must be explicitly given a command to shut down gracefully. Thus, CTRL-C must not reach the server process.

If I capture the KeyboardInterrupt exception or overwrite the SIGINT-handler in python, the server process still receives the CTRL-C and stops immediately.

So my question is:
How to prevent subprocesses from receiving CTRL-C / Control-C / SIGINT?

Answer Source

Somebody in the #python IRC-Channel (Freenode) helped me by pointing out the preexec_fn parameter of subprocess.Popen(...):

If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (Unix only)

Thus, the following code solves the problem (UNIX only):

import subprocess
import signal

def preexec_function():
    # Ignore the SIGINT signal by setting the handler to the standard
    # signal handler SIG_IGN.
    signal.signal(signal.SIGINT, signal.SIG_IGN)

my_process = subprocess.Popen(
    preexec_fn = preexec_function

Note: The signal is actually not prevented from reaching the subprocess. Instead, the preexec_fn above overwrites the signal's default handler so that the signal is ignored. Thus, this solution may not work if the subprocess overwrites the SIGINT handler again.

Another note: This solution works for all sorts of subprocesses, i.e. it is not restricted to subprocesses written in Python, too. For example the dedicated server I am writing my wrapper for is in fact written in Java.

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