Delegate signal handling to a child process in python

Question

How can I run a command from a python script and delegate to it signals like Ctrl+C?

I mean when I run e.g:

from subprocess import call
call(["child_proc"])

I want child_proc to handle Ctrl+C

Cormac

Can you be a bit more clear on what you want to accomplish? What do you want the child process to do? – Moises

@Cormac Do you want signals sent to the parent process to be forwarded to child, or do you just want to be able to send signals to the child from the parent? – Cracksman

I want signals sent to the parent to be forwarded to child. – Cormac

Answer 1

I'm guessing that your problem is that you want the subprocess to receive Ctrl-C and not have the parent Python process terminate? If your child process initialises its own signal handler for Ctrl-C (SIGINT) then this might do the trick:

import signal, subprocess

old_action = signal.signal(signal.SIGINT, signal.SIG_IGN)
subprocess.call(['less', '/etc/passwd'])
signal.signal(signal.SIGINT, old_action)         # restore original signal handler

Now you can hit Ctrl-C (which generates SIGINT), Python will ignore it but less will still see it.

However this only works if the child sets its signal handlers up properly (otherwise these are inherited from the parent).

Youngstown

you should probably set: preexec_fn=restore_signal, where restore_signal restores SIGINT to SIG_DFL. preexec_fn is run in the child (after fork(), before exec*()). – Warr

"otherwise these [signal handlers] are inherited from the parent" -- is this a python thing or a unix thing? Also, any links for further reading on process signaling and inheritance? – Minetta

@bennlich: inheritance of signal disposition is certainly a Unix thing - I'm not sure about its implementation on Windows. Use the Linux man pages man 7 signal for reference, and search for additional resources if you need. – Youngstown