Example Code - 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>
#define NUM_THREADS 3
#define TCOUNT 10
#define COUNT_LIMIT 12
int count = 0;
int thread_ids[3] = {0,1,2};
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;
void *inc_count(void *idp)
{
int j,i;
double result=0.0;
int *my_id = idp;
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) {
pthread_cond_signal(&count_threshold_cv);
printf("inc_count(): thread %d, count = %d Threshold reached.\n",
*my_id, count);
}
printf("inc_count(): thread %d, count = %d, unlocking mutex\n",
*my_id, count);
pthread_mutex_unlock(&count_mutex);
/* Do some work so threads can alternate on mutex lock */
for (j=0; j < 1000; j++)
result = result + (double)random();
}
pthread_exit(NULL);
}
void *watch_count(void *idp)
{
int *my_id = idp;
printf("Starting watch_count(): thread %d\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) {
pthread_cond_wait(&count_threshold_cv, &count_mutex);
printf("watch_count(): thread %d Condition signal
received.\n", *my_id);
}
pthread_mutex_unlock(&count_mutex);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
int i, rc;
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
so that they can be joined later.
*/
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_create(&threads[0], &attr, inc_count, (void *)&thread_ids[0]);
pthread_create(&threads[1], &attr, inc_count, (void *)&thread_ids[1]);
pthread_create(&threads[2], &attr, watch_count, (void *)&thread_ids[2]);
/* Wait for all threads to complete */
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS);
/* Clean up and exit */
pthread_attr_destroy(&attr);
pthread_mutex_destroy(&count_mutex);
pthread_cond_destroy(&count_threshold_cv);
pthread_exit(NULL);
}
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