sig_sync.c

#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define SIG_SYNC SIGUSR1

uint32_t *Cnt = 0;

void sig_sync_handler(int sig)
{
}

int main(int argc, char const* argv[])
{
    int shm_fd;
    pid_t pid;
    struct sigaction sa;
    sigset_t block_mask, empty_mask;


    setbuf(stdout, NULL);

    sa.sa_flags = 0;
    sa.sa_handler = sig_sync_handler;
    sigaction(SIG_SYNC, &sa, NULL);

    sigemptyset(&empty_mask);

    sigemptyset(&block_mask);
    sigaddset(&block_mask, SIG_SYNC);

    sigprocmask(SIG_BLOCK, &block_mask, NULL);

    // Open shm with Cnt
    shm_fd = shm_open("/3_11_shm", O_CREAT | O_RDWR, 0600);
    ftruncate(shm_fd, sizeof(Cnt));
    Cnt = (uint32_t *)mmap(NULL, sizeof(Cnt), PROT_READ | PROT_WRITE,
                           MAP_SHARED, shm_fd, 0);
    *Cnt = 0;

    switch (pid = fork()) {
    // Child prints even numbers
    case 0: {
        pid_t ppid = getppid();

        for (;;) {
            sigsuspend(&empty_mask);
            *Cnt += 1;
            printf("[Child ]: Cnt=%d\n", *Cnt);
            kill(ppid, SIG_SYNC);
        }

        _exit(EXIT_SUCCESS);
    }
    default: {
        // Parent prints odd numbers
        while (*Cnt < 10) {
            *Cnt += 1;
            printf("[Parent]: Cnt=%d\n", *Cnt);
            kill(pid, SIG_SYNC);
            sigsuspend(&empty_mask);
        }

        kill(pid, SIGTERM);

        int status;
        wait(&status);

        exit(EXIT_SUCCESS);
    }
    case -1:
        perror("fork");
        exit(EXIT_FAILURE);
    }
}