如何使用信号量使子进程相互同步？

Question

I have N number of childs that needs to do some work in a loop while being synchronized with each other at the same time. Namely, if a child process is at its i'th iteration, all the other childs should be at i'th iteration. I need to synchronize them with semaphores but I can't find how to do it. This is the code I wrote:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/sem.h>

void sem_signal(int semid, int val) {
    struct sembuf semaphore;
    semaphore.sem_num = 0;
    semaphore.sem_op = val;
    semaphore.sem_flg = 0;
    semop(semid, &semaphore, 1);
}

void sem_wait(int semid, int val) {
    struct sembuf semaphore;
    semaphore.sem_num = 0;
    semaphore.sem_op = (-1 * val);
    semaphore.sem_flg = 0;
    semop(semid, &semaphore, 1);
}

int main() {
    int sem_worker = semget(1, 1, 0700 | IPC_CREAT);
    semctl(sem_worker, 0, SETVAL, 0);
    int process_index = 0;
    int N = 4, pid;

    for (process_index = 0; process_index < N; process_index++) {
        pid = fork();
        if (pid == -1) {
            printf("ERROR: cannot fork!\n");
            return EXIT_FAILURE;
        }
        if (pid == 0)
            break;
    }
    if (pid!=0) // parent
        pause();

    else {
        int i = 0;
        while (i < 3) {
            printf("process %d: i: %d\n", process_index, i);
            sem_signal(sem_worker, 1); // increase the semaphore by one
            sem_wait(sem_worker, N);   // wait for all the other childs
            i += 1;
        }
    }
}

But when I run it, it can't continue after the first iteration.

process 0: i: 0
process 1: i: 0
process 3: i: 0
process 2: i: 0
process 0: i: 1

I understand why this happens. It's because one of the processes makes the semaphore 0 and continue to next iteration but all the other ones still waits. So how should I write my code to solve this problem?

PS: I have taken sem_signal and sem_wait functions from somewhere else so I'm not sure how it works but I'm sure that they are working correctly. For example, if I write sem_wait(my_sem, num_of_children) in parent to wait all the child processes and increase my_sem by 1 in childs when they finish, it works.

Answer 1

As it is mentioned in the comments, you can create a barrier using semaphores and use it to synchronize your processes. You need to create your barrier in a shared memory and set a non-zero value for your semaphores' pshared parameter to share it among processes:

#include <semaphore.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/shm.h>
#include <signal.h>
#include <unistd.h>


typedef struct {
    int n;
    int count;
    sem_t mutex;
    sem_t turnstile;
    sem_t turnstile2;
} barrier_t;

void init_barrier(barrier_t *barrier, int n)
{
    barrier->n = n;
    barrier->count = 0;
    sem_init(&barrier->mutex, 1, 1); // second parameter is pshared
    sem_init(&barrier->turnstile, 1, 0);
    sem_init(&barrier->turnstile2, 1, 0);
}

void phase1(barrier_t *barrier)
{
    sem_wait(&barrier->mutex);
    if (++barrier->count == barrier->n) {
        int i;
        for (i = 0; i < barrier->n; i++) {
            sem_post(&barrier->turnstile);
        }
    }
    sem_post(&barrier->mutex);
    sem_wait(&barrier->turnstile);
}

void phase2(barrier_t *barrier)
{
    sem_wait(&barrier->mutex);
    if (--barrier->count== 0) {
        int i;
        for (i = 0; i < barrier->n; i++) {
            sem_post(&barrier->turnstile2);
        }
    }
    sem_post(&barrier->mutex);
    sem_wait(&barrier->turnstile2);
}

void wait_barrier(barrier_t *barrier)
{
    phase1(barrier);
    phase2(barrier);
}

int shmid, KEYSHM=123456;

int main(int argc, char const* argv[]) {
    barrier_t* barrier;
    shmid = shmget(KEYSHM, sizeof(barrier_t), 0700 | IPC_CREAT);
    barrier = (barrier_t*) shmat(shmid, 0, 0);
    int N = 4;
    init_barrier(barrier, N);
    shmdt(barrier);


    int process_index, pid;

    for (process_index = 0; process_index < N; process_index++) {
        pid = fork();
        if (pid == -1) {
            printf("ERROR: cannot fork!\n");
            return EXIT_FAILURE;
        }
        if (pid == 0)
            break;
    }
    if (pid != 0) // parent
        pause();
    else {
        int i = 0;
        while (i < 3) {
            barrier = (barrier_t*) shmat(shmid, 0, 0);
            printf("process %d: i: %d\n", process_index, i);
            i += 1;
            wait_barrier(barrier);
            shmdt(barrier);
        }

        if (process_index == 3){
            kill(getppid(), SIGKILL);
        }
    }
}

process 0: i: 0
process 1: i: 0
process 2: i: 0
process 3: i: 0
process 2: i: 1
process 3: i: 1
process 0: i: 1
process 1: i: 1
process 3: i: 2
process 2: i: 2
process 0: i: 2
process 1: i: 2