Monitor source code in C that synchronizes threads

Question

I want to make a monitor that synchronizes n threads in C according to the following way: Each thread calls barrier_synch() method. First n-1 threads that call this method sleep. When the n-th thread calls this method, all sleeping threads wake up and then all threads continue the execution, while the barrier returns to the initial condition. This is a solution I found:

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

typedef struct barrier{
    int n, count;
    pthread_mutex_t mutex;
    pthread_cond_t cond;
    int blocked;
}barrier;

void barrier_init(barrier *bar, int n){
    bar->n = n;
    bar->count = 0;
    pthread_mutex_init(&bar->mutex, NULL);
    pthread_cond_init(&bar->cond, NULL);
    bar->blocked = 1;
}

void barrier_synch(barrier *bar){
    while(1){
        pthread_mutex_lock(&bar->mutex);
        if (bar->blocked == 1) break;
        pthread_mutex_unlock(&bar->mutex);
    }
    bar->count++;
    if(bar->count == bar->n){
        bar->blocked = 0;
        pthread_cond_broadcast(&bar->cond);
    }

    while(bar->blocked == 1){
        pthread_cond_wait(&bar->cond, &bar->mutex);
    }
    bar->count--;
    if(bar->count == 0){
        bar->blocked = 1;
    }
pthread_mutex_unlock(&bar->mutex);
}

Is this code correct? Can somebody explain me in simple words how this mechanism works?

Answer 1

Check out The Little Book of Semaphores for all sorts of synchronization primitives and uses. It isn't in C, but it should help you in thinking about your code in a structured, correct way.