Prevent exit(1) from exiting Python when executing C-function from Python

Question

I'm executing a C function from python using ctypes. Under certain conditions, the C function I'm calling exits using exit(1), and I am not in a position where I can change this behaviour. I wish for my Python program to keep running even after this exit signal. I've attempted using threading to solve the issue; however, my program still exits on the exit signal.

my_lib.c

#include <stdio.h>
#include <stdlib.h>
void c_func(int i)
{
    if(i == 3)
        exit(1);
    printf("%d", i);
}

my_program.py

from ctypes import *
import threading
lib = CDLL("my_lib.so")
for i in range(8):
    thread = threading.Thread(target=lib.c_func, args=(i))
    thread.start()

Desired output on running my_program.py:
0124567

Actual output:
012

You could avoid calling it if `i==3`, but I'm guessing you won't find that a very satisfying answer. — Scott Hunter, Jul 21 '22 at 13:51
I don't think you can do this. `exit` is terminating the process and there is nothing you can do about it, except running it in a different process. — Eugene Sh., Jul 21 '22 at 13:53
Yep. Threading won't help, you need a separate process. Oh, and don't forget to file a bug ticket, because libraries should generally refrain from calling `exit()`. — Ulrich Eckhardt, Jul 21 '22 at 14:07
@veysel I assume the *actual* function the OP has no control over and this is a toy example. — Mark Tolonen, Jul 21 '22 at 16:46

score 2 · Accepted Answer · answered Jul 21 '22 at 16:56

A multiprocessing solution works:

import ctypes as ct
import multiprocessing as mp

dll = ct.CDLL('./test')
dll.c_func.argtypes = ct.c_int,
dll.c_func.restype = None

def func(i):
    dll.c_func(i)

if __name__ == '__main__':
    ps = [mp.Process(target=func, args=(i,)) for i in range(8)]
    for p in ps:
        p.start()
    for p in ps:
        p.join()

Output from running in parallel so output varies. Put the join inside the first for to run serially:

score 1 · Answer 2 · answered Jul 21 '22 at 14:20

Running the function in a subprocess is a solution that suited my needs:

my_lib.c

#include <stdio.h>
#include <stdlib.h>
void c_func(int i)
{
    if(i == 3)
        exit(1);
    printf("%d", i);
}

my_program.py

import subprocess
for i in range(8):
    subprocess.run(['python3', f'my_func_call.py {i}'])

my_func_call.py

import sys
from ctypes import *
lib = CDLL("my_lib.so")

if __name__ == "__main__":
    lib.c_func(sys.argv[1]) # c_func(i)

Output:
0124567

Prevent exit(1) from exiting Python when executing C-function from Python

2 Answers2

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