pthread_barrier_t in looping threads

Question

I work with 3 different threads (t0, t1 and t2), whom I need to synchronize behaviors this way:

My main thread will be creating these 3 threads, and those will be looping through prints and barrier just like I mentionned.

I tried to write the functions I pass them (f0 for t0, f1 for t1, etc.):

pthread_barrier_t b; //in my main I initialize pthread_barrier_init(&b, NULL, 3);

void *f0(void *arg){

    while(1){
        printf("A\n");
        pthread_barrier_wait(&b);
        pthread_barrier_wait(&b);
        printf("D\n");
    }

}

void *f1(void *arg){

    while(1){
        pthread_barrier_wait(&b);
        printf("B\n");
        pthread_barrier_wait(&b);
    }

}

void *f2(void *arg){

    while(1){
        pthread_barrier_wait(&b);
        printf("C\n");
        pthread_barrier_wait(&b);
    }

}

It worked well till the first barrier is met, but it looked like the barrier was not resetting, I tried to re-init once al threads came across it but it had weird effects.

i think that you need 2 different barrier variables (b1, b2). You also need to really initialize them with pthread_barrier_init. — Serge, Oct 27 '19 at 13:47

score 0 · Answer 1 · answered Oct 27 '19 at 14:30

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

pthread_barrier_t b[2];

pthread_t t[3];

void *f0(void *arg){

    while(1){
        printf("A");
        pthread_barrier_wait(&b[0]);
        pthread_barrier_wait(&b[1]);
        printf("B\n");
        sleep(1);
    }

}

void *f1(void *arg){

    while(1){
        pthread_barrier_wait(&b[0]);
        printf("C");
        pthread_barrier_wait(&b[1]);
    }

}

void *f2(void *arg){

    while(1){
        pthread_barrier_wait(&b[0]);
        printf("D");
        pthread_barrier_wait(&b[1]);
    }

}

void *(*f[3])(void*) = {&f0, &f1, &f2};

int main(){

    for(int barrier = 0; barrier < 2; barrier++){
        if(pthread_barrier_init(&b[barrier], NULL, 3) != 0){
            perror("pthread_barrier_init");
            return 1;
        }
    }

    for(int thread = 0; thread < 3; thread++){
        if(pthread_create(&t[thread], NULL, f[thread], NULL) != 0){
            perror("pthread_create");
            return 2;
        }
    }

    for(int thread = 0; thread < 3; thread++){
        if(pthread_join(t[thread], NULL) != 0){
            perror("pthread_join");
            return 3;
        }
    }

    return 0;

}

This produces the wanted output.

pthread_barrier_t in looping threads

1 Answers1