Example: Using Condition Variables

• This simple example code demonstrates the use of several Pthread condition variable routines.
• The main routine creates three threads.
• Two of the threads perform work and update a “count” variable.
• The third thread waits until the count variable reaches a specified value.

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

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t)
{
  int i;
  long my_id = (long)t;

  for (i = 0; i < TCOUNT; i++) {
    pthread_mutex_lock(&count_mutex);
    count++;

    /* Check the value of count and signal waiting thread when condition is
     * reached. Note that this occurs while mutex is locked.
     */
    if (count == COUNT_LIMIT) {
      printf("inc_count(): thread %ld, count = %d -- threshold reached.",
             my_id, count);
      pthread_cond_signal(&count_threshold_cv);
      printf("Just sent signal.\n");
    }
    printf("inc_count(): thread %ld, count = %d -- unlocking mutex\n",
           my_id, count);
    pthread_mutex_unlock(&count_mutex);

    /* Do some work so threads can alternate on mutex lock */
    sleep(1);
  }
  pthread_exit(NULL);
}

void *watch_count(void *t)
{
  long my_id = (long)t;

  printf("Starting watch_count(): thread %ld\n", my_id);

  /* Lock mutex and wait for signal. Note that the pthread_cond_wait routine
   * will automatically and atomically unlock mutex while it waits.
   * Also, note that if COUNT_LIMIT is reached before this routine is run by
   * the waiting thread, the loop will be skipped to prevent pthread_cond_wait
   * from never returning.
   */
  pthread_mutex_lock(&count_mutex);
  while (count < COUNT_LIMIT) {
    printf("watch_count(): thread %ld Count= %d. Going into wait...\n", my_id,count);
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received. Count= %d\n", my_id,count);
  }
  printf("watch_count(): thread %ld Updating the value of count...\n", my_id);
  count += 125;
  printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
  printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
  pthread_mutex_unlock(&count_mutex);
  pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
  int i, rc;
  long t1 = 1,
       t2 = 2,
       t3 = 3;
  pthread_t threads[3];
  pthread_attr_t attr;

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

  /* For portability, explicitly create threads in a joinable state */
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  pthread_create(&threads[0], &attr, watch_count, (void *)t1);
  pthread_create(&threads[1], &attr, inc_count, (void *)t2);
  pthread_create(&threads[2], &attr, inc_count, (void *)t3);

  /* Wait for all threads to complete */
  for (i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
  }
  printf ("Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n",
          NUM_THREADS, count);

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

}

Source

Output