shared memory fork example

shm_fork.c

#include <stdbool.h>
#include <stdio.h>
#include <sys/fcntl.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <unistd.h>

static volatile bool *running;

void handler(int sig) {
  (void)sig;
  *running = false;
}

void do_child_work(void) {
  while (*running)
    sleep(1);
}

const char *shmem_name = "shm_fork_test";

int main(void) {
  int ret = 0;

  if (signal(SIGINT, handler) == SIG_IGN)
    signal(SIGINT, SIG_IGN);
  if (signal(SIGHUP, handler) == SIG_IGN)
    signal(SIGHUP, SIG_IGN);
  if (signal(SIGTERM, handler) == SIG_IGN)
    signal(SIGTERM, SIG_IGN);

  int shm_fd = shm_open(shmem_name, O_RDWR | O_CREAT);
  if (shm_fd == -1) {
    perror("shm_open");
    ret = 1;
    goto cleanup;
  }

  if (ftruncate(shm_fd, sizeof(*running)) == -1) {
    perror("ftruncate");
    ret = 1;
    goto cleanup;
  }

  void *shm = mmap(NULL, sizeof(*running), PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
  if (shm == MAP_FAILED) {
    perror("mmap");
    ret = 1;
    goto cleanup;
  }

  running = shm;
  *running = true;

#define N_CHILD (8)
  pid_t pid[N_CHILD];

  for (int i = 0; i < N_CHILD; i++) {
    pid[i] = fork();
    if (pid[i] == -1) {
      perror("fork");
      ret = 1;
      goto cleanup;
    } else if (pid[i] == 0) {
      do_child_work();
      return 0;
    }
    printf("forked child %d\n", pid[i]);
  }

  for (pid_t *p = pid; p < pid + N_CHILD; p++) {
    printf("waiting on %d\n", *p);
    if (waitpid(*p, NULL, 0) == -1) {
      perror("wait");
      ret = 1;
      goto cleanup;
    }
  }

cleanup:
  printf("cleanup\n");
  if (shm_unlink(shmem_name) == -1) {
    perror("shm_unlink");
    ret = 1;
  }

  return ret;
}