anton.c

/** Demonstrate that kill(pid, 0) == 0 for zombie processes.

    $ make anton && ./anton

See https://ru.stackoverflow.com/q/725572/23044
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
  pid_t pid = fork();
  if ( pid == 0)
  {
    int pid = getpid();
    int ppid = getppid();

    printf("Child process: pid: %d \n", pid);
    printf("Child process: ppid: %d \n", ppid);
    int res = execvp("", NULL);
    printf("Child process: res: %d \n", res);
    _exit(72);
  }
  sleep(1); // allow the child to exit

  printf("child pid: %d\n", pid);

  { // show zombie (<defunct>)
    const char *format = "ps -ef | grep %d";
    const int n = snprintf(NULL, 0, format, pid);
    assert(n > 0);
    char command[n+1];
    int c = snprintf(command, n+1, format, pid);
    assert(command[n] == '\0');
    assert(c == n);
    system(command);
  }

  if ( kill(pid, 0) == 0 ) // zombie "exists"
  {
    printf("kill returned 0 \n");
  }
  else
  {
    exit(EXIT_FAILURE);
  }


  { // reap the child process
    int status = 0;
    if (waitpid(pid, &status, WNOHANG) && WIFEXITED(status))
    {
      printf("child exited with status %d\n", WEXITSTATUS(status));
    }
    if ( kill(pid, 0) == 0 ) // can't be
    {
        exit(EXIT_FAILURE);
    }
  }
  return 0;
}

kill-is-zero-for-zombie-process.py

#!/usr/bin/env python3
"""Show that `kill(pid, 0)` is successful for zombies (no ESRCH)."""
from os import *

pid = fork()
if pid == 0:  # child process
    _exit(42)

# parent process
# wait until the child dies (becomes zombie)
while system(fr"ps -ef | grep {pid} | grep defunct\> >{devnull}"):
    pass

kill(pid, 0)  # no exception: zombie "exists"

w_pid, status = waitpid(-1, WNOHANG)  # reap zombie
assert w_pid == pid and WIFEXITED(status)
assert WEXITSTATUS(status) == 42, status
try:
    kill(pid, 0)
except ProcessLookupError:  # errno.ESRCH
    pass
else:
    assert 0, "can't be"