A demonstration of pthread conditional variables.

pthreads_cond_demo.c

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

#define check(x) fatal(x, __LINE__)

int var = 0;
pthread_t thread;
pthread_mutex_t lock;
pthread_cond_t cond;

void fatal(int err, int line) {
	if (err) {
		printf("%d: %d\n", line, err);
		exit(1);
	}
}

void setup() {
	check(pthread_mutex_init(&lock, NULL));
	check(pthread_cond_init(&cond, NULL));
}

void teardown() {
	check(pthread_join(thread, NULL));
	pthread_mutex_destroy(&lock);
	pthread_cond_destroy(&cond);
}

/*
 * Wait for the main thread to change var, then also wait for
 * it to release the mutex it was using to change var in the
 * first place. pthread_cond_wait will relinquish it's ownership
 * of the mutex, but it is also also reponsible for getting it
 * back before retuning. That's why main's 10 second sleep after
 * pthread_cond_signal but before releasing the mutex affects the
 * progress of this thread.
*/
void *mySubroutine() {
	check(pthread_mutex_lock(&lock));
	while (var == 0) {
		check(pthread_cond_wait(&cond, &lock));
	}
	printf("Thread conditional waiting complete.\n");
	check(pthread_mutex_unlock(&lock));
}

int main() {
	setup();
	check(pthread_create(&thread, NULL, mySubroutine, NULL));

	check(pthread_mutex_lock(&lock));
	var = 1;
	pthread_cond_signal(&cond);
	// Since we still hold the mutex, despite using the conditional signal,
	// no other thread should be able to wake up till we release the mutex.
	// Under the hood, pthread_cond_wait will need to acquire the mutex
	// first before returning.
	printf("Signal sent. Sleeping before release.\n");
	sleep(10);
	check(pthread_mutex_unlock(&lock));

	teardown();
	return 0;
}