pthread condition variables API, waking up multiple threads

ddrake-condvar.c

/* Like join.c, but I want to see if multiple threads can wait on a single
condition. It looks like `pthread_cond_broadcast` is the API function for this.

What's weird is that this has some kind of race condition in it. I've seen:

* only the parent woken up
* parent and child2 woken up
* child2 woken up twice!

 */

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include "common.h"
#include "common_threads.h"

pthread_cond_t  c = PTHREAD_COND_INITIALIZER;
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;

int done = 0;

void *child(void *arg) {
    printf("child sleeping for a moment\n");
    sleep(1);
    Mutex_lock(&m);
    done = 1;
    printf("child about to signal\n");
    //    Cond_signal(&c);
    pthread_cond_broadcast(&c);
    Mutex_unlock(&m);
    return NULL;
}

void *child2(void *arg) {
    printf("child 2 about to wait on child 1\n");
    Mutex_lock(&m);
    printf("child 2 got the lock\n");
    while (done == 0) {
      printf("child 2 about to wait\n");
      Cond_wait(&c, &m); // releases lock when going to sleep
    }
    Mutex_unlock(&m);
    printf("child 2 woke up\n");
    return NULL;
}


int main(int argc, char *argv[]) {
    pthread_t p;
    printf("parent: begin\n");
    Pthread_create(&p, NULL, child, NULL);

    Pthread_create(&p, NULL, child2, NULL);

    Mutex_lock(&m);
    printf("parent got the lock\n");
    while (done == 0) {
      printf("parent sees done == 0, about to wait\n");
      Cond_wait(&c, &m); // releases lock when going to sleep
    }
    Mutex_unlock(&m);
    printf("parent: woke up\n");
    return 0;
}