Question

On Linux I have an executable I wish to run via Python script. The executable divides by zero, and I can’t seem to handle the signal from subprocess.
I read around and it seem that the preexec_fn should handle SIGFPE, but no luck so far.

I’m using Python 2.7

My code:

# b.py
import os
import subprocess
import signal
import sys

def pref_fun():
    signal.signal(signal.SIGFPE,foo)

def foo(signal,frame):
    print "Caught signal!"
    sys.exit(0)

sub = subprocess.Popen(["a.out"], preexec_fn=pref_fun)
sub.wait()
v = sub.returncode
print "value: ", v

and my child:

a.c
#include <stdio.h>
#include <stdlib.h>
int main() {
    printf("Now dividing by zero\n");
    fflush(stdout);
    double x = 5;
    x= 5/0;
    printf("oh no\n");
    return 0;

}

The expected output is “Caught signal!”, but I don’t seem to get it.

Answer 1

I am afraid that we can’t do it in such way. Though the signal handler was installed in the forked child, after ./a.out was exec()ed, the image which has the signal handler was replaced by the latter one — so there’s no handlers in the child process any more. Yet we can check whether the child process was terminated by a signal by using os.WIFSIGNALED(v), if True, we can use os.WTERMSIG(v) to get which signal it was, after this, we can do something in parent.

However, I got very odd results from os.WTERMSIG(v) on my machine, and found that the real signal number seemed to be the negative of v (I’d checked SIGSEGV and SIGFPE), I don’t know whether if it is my machine’s issue, anyway, hope this helps :).

import os
import subprocess
import signal
import sys 

def pref_fun():
    signal.signal(signal.SIGFPE,foo)

def foo(signal,frame):
    print "Caught signal!"
    sys.exit(0)

sub = subprocess.Popen(["./a.out"], preexec_fn=pref_fun)
sub.wait()
v = sub.returncode
print "value: ", v
print os.WIFSIGNALED(v)
print "signal:", os.WTERMSIG(v)
print "SIGFPE", signal.SIGFPE    

which outputs:

Now dividing by zero
value:  -8
True
signal: 120
SIGFPE 8