Unexpected behaviour of pthread_cond_broadcast

Question

Building on my question from yesterday, here , I wrote a small code sample that starts a number of counting and a number of waiting threads. The waiting threads are stopped pthread_cond_wait until they receive a signal. The signal is sent after the counting threads finish their tasks.

The waiting threads receive their signal and each thread prints out its given, unique id.

I would expect all waiting threads to receive the signal at the same time, so that each of them can proceed with the program. I noticed however, the outputs are not chaotic, in fact they even appear to be fairly ordered, like in FILO!

There are now various places, where I could have gone wrong.

Here is my code:

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

#define counting_threads 100
#define waiting_threads 100

int count = 0;
int counting_thread_ids[counting_threads];
int waiting_thread_ids[waiting_threads];
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;


void init_ids(){
    for(int i = 0; i < counting_threads; i++)
        counting_thread_ids[i] = 2*i;
    for(int j =0; j < waiting_threads; j++)
        waiting_thread_ids[j] = 2*j+1;  
}

void counting(void *t) 
{
    pthread_mutex_lock(&count_mutex);
    count++;

    if (count == counting_threads) {
    sleep(2);
      printf("inc_count(): count = %d  Threshold reached. Signaling waiting threads. \n", count);
           //~ pthread_cond_signal(&count_threshold_cv);           
           pthread_cond_broadcast(&count_threshold_cv); 
      }
    pthread_mutex_unlock(&count_mutex);
    }

void *waiting(void *t) 
{
  long my_id = (long)t;
  //~ printf("Starting watch_count(): thread %ld\n", my_id);

  pthread_mutex_lock(&count_mutex);

    //~ printf("watch_count(): I start waiting now: %ld \n", my_id);
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received.\n", my_id);
    pthread_mutex_unlock(&count_mutex);
    pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
    init_ids(); 

    pthread_t wt[waiting_threads];
    pthread_t ct[counting_threads];  

  /* Initialize mutex and condition variable objects */
  pthread_mutex_init(&count_mutex, NULL);
  pthread_cond_init (&count_threshold_cv, NULL);

    for(int i = 0; i < waiting_threads; i++)
        pthread_create(&wt[i], NULL, waiting, (void*) waiting_thread_ids[i] );
    for(int i = 0; i < counting_threads; i++)
        pthread_create(&ct[i], NULL, counting, (void*) counting_thread_ids[i] );


  /* Wait for all threads to complete */
  for (int i=0; i<waiting_threads; i++) {
    pthread_join(wt[i], NULL);
  }
  for (int i=0; i<counting_threads; i++) {
    pthread_join(ct[i], NULL);
  }


  /* Clean up and exit */
  pthread_mutex_destroy(&count_mutex);
  pthread_cond_destroy(&count_threshold_cv);
  pthread_exit(NULL);

}

Answer 1

The pthread_cond_signal() call unblocks at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond).

The pthread_cond_broadcast() call unblocks all threads currently blocked on the specified condition variable cond.

If more than one thread is blocked on a condition variable, the scheduling policy determines the order in which threads are unblocked.

More information about the scheduling policies can be found here .