systemd/man/event-quick-child.c

/* SPDX-License-Identifier: MIT-0 */

#define _GNU_SOURCE 1
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <systemd/sd-event.h>

int main(int argc, char **argv) {
  pid_t pid = fork();
  assert(pid >= 0);

  /* SIGCHLD signal must be blocked for sd_event_add_child to work */
  sigset_t ss;
  sigemptyset(&ss);
  sigaddset(&ss, SIGCHLD);
  sigprocmask(SIG_BLOCK, &ss, NULL);

  if (pid == 0)  /* child */
    sleep(1);

  else {         /* parent */
    sd_event *e = NULL;
    int r;

    /* Create the default event loop */
    sd_event_default(&e);
    assert(e);

    /* We create a floating child event source (attached to 'e').
     * The default handler will be called with 666 as userdata, which
     * will become the exit value of the loop. */
    r = sd_event_add_child(e, NULL, pid, WEXITED, NULL, (void*) 666);
    assert(r >= 0);

    r = sd_event_loop(e);
    assert(r == 666);

    sd_event_unref(e);
  }

  return 0;
}