freqcs.c

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

pthread_mutex_t lock;
pthread_cond_t cond;
static unsigned counter;

static void dec_counter(void) {
    pthread_mutex_lock(&lock);
    while (counter == 0) {
        pthread_cond_wait(&cond, &lock);
    }
    counter -= 1;
    pthread_mutex_unlock(&lock);
}

static void inc_counter(void) {
    pthread_mutex_lock(&lock);
    if (counter == 0) {
        sleep(3);
        pthread_cond_signal(&cond);
    }
    counter += 1;
    pthread_mutex_unlock(&lock);
}

void *thread_main(void *arg) {
    int i = (int)arg;
    if (i == 0) {
        while (1) {
            dec_counter();
        }
    } else {
        while (1) {
            inc_counter();
        }
    }
    return NULL;
}

int main(void) {
    pthread_t t[2];
    int i;
    for (i = 0; i < 2; i++) {
        pthread_create(&t[i], NULL, thread_main, (void*)i);
    }
    for (i = 0; i < 2; i++) {
        pthread_join(t[i], NULL);
    }
    pthread_cond_destroy(&cond);
}