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

#define LOOP 20000

int toggle = 0;          /* either 0 or 1 */
pthread_mutex_t mutex;   /* protects 'toggle' */
pthread_cond_t cond;     /* for the threads to wait on */

void *ping(void *arg)
{
    int i;
    int rv;

    rv = pthread_mutex_lock(&mutex);
    if (rv) {
        fprintf(stderr, "pthread_mutex_lock failed: %d\n", rv);
        exit(1);
    }
    for (i = 0; i < LOOP; i++) {
        while (toggle == 0) {
            rv = pthread_cond_wait(&cond, &mutex);
            if (rv) {
                fprintf(stderr, "pthread_cond_wait failed: %d\n", rv);
                exit(1);
            }
        }
        toggle--;
        rv = pthread_cond_signal(&cond);
        if (rv) {
            fprintf(stderr, "pthread_cond_signal failed: %d\n", rv);
            exit(1);
        }
    }
    rv = pthread_mutex_unlock(&mutex);
    if (rv) {
        fprintf(stderr, "pthread_mutex_unlock failed: %d\n", rv);
        exit(1);
    }
    return NULL;
}

void *pong(void *arg)
{
    int i;
    int rv;

    rv = pthread_mutex_lock(&mutex);
    if (rv) {
        fprintf(stderr, "pthread_mutex_lock failed: %d\n", rv);
        exit(1);
    }
    for (i = 0; i < LOOP; i++) {
        while (toggle == 1) {
            rv = pthread_cond_wait(&cond, &mutex);
            if (rv) {
                fprintf(stderr, "pthread_cond_wait failed: %d\n", rv);
                exit(1);
            }
        }
        toggle++;
        rv = pthread_cond_signal(&cond);
        if (rv) {
            fprintf(stderr, "pthread_cond_signal failed: %d\n", rv);
            exit(1);
        }
    }
    rv = pthread_mutex_unlock(&mutex);
    if (rv) {
        fprintf(stderr, "pthread_mutex_unlock failed: %d\n", rv);
        exit(1);
    }
    return NULL;
}

int main()
{
    pthread_attr_t attr;
    pthread_t t1, t2;
    int rv;

    rv = pthread_mutex_init(&mutex, NULL);
    if (rv) {
        fprintf(stderr, "pthread_mutex_init failed: %d\n", rv);
        exit(1);
    }
    rv = pthread_cond_init(&cond, NULL);
    if (rv) {
        fprintf(stderr, "pthread_cond_init failed: %d\n", rv);
        exit(1);
    }

    rv = pthread_attr_init(&attr);
    if (rv) {
        fprintf(stderr, "pthread_attr_init failed: %d\n", rv);
        exit(1);
    }
    rv = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    if (rv) {
        fprintf(stderr, "pthread_attr_setdetachstate failed: %d\n", rv);
        exit(1);
    }
    rv = pthread_create(&t1, &attr, ping, NULL);
    if (rv) {
        fprintf(stderr, "pthread_create failed: %d\n", rv);
        exit(1);
    }
    rv = pthread_create(&t2, &attr, pong, NULL);
    if (rv) {
        fprintf(stderr, "pthread_create failed: %d\n", rv);
        exit(1);
    }

    rv = pthread_join(t1, NULL);
    if (rv) {
        fprintf(stderr, "pthread_join failed: %d\n", rv);
        exit(1);
    }
    rv = pthread_join(t2, NULL);
    if (rv) {
        fprintf(stderr, "pthread_join failed: %d\n", rv);
        exit(1);
    }
    
    return 0;
}