Unexpected behaviour of pthread_cond_broadcast - c

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.