/* Compile with cc -Og -o thread_sleeps -pthread thread_sleeps.c */

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

int main(int argc, char *argv[]);
void* sender(void*);
void* receiver(void*);

static pthread_mutex_t  mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t   sent = PTHREAD_COND_INITIALIZER;

int                     n_sends = 0;

#define NPASSES         20

int main(int argc, char *argv[])
{
    pthread_t sender_tid;
    pthread_t receiver_tid;

    if (pthread_create (&sender_tid, NULL, sender, NULL) != 0)
        {
        fprintf(stderr, "Couldn't create sender thread\n");
        return 1;
        }
    if (pthread_create (&receiver_tid, NULL, receiver, NULL) != 0)
        {
        fprintf(stderr, "Couldn't create receiver thread\n");
        return 1;
        }
    if (pthread_join (sender_tid, NULL) != 0)
        {
        fprintf(stderr, "Couldn't join with sender thread\n");
        return 1;
        }
    if (pthread_join (receiver_tid, NULL) != 0)
        {
        fprintf(stderr, "Couldn't join with receiver thread\n");
        return 1;
        }
    return 0;

}

/*
 * The sender sends one signal per second, bumping n_sends each time.
 */
void* sender(void* data)
{
    int i;

    for (i = 0;  i < NPASSES;  i++) {
        sleep(1);
        printf("Sender sent %d time(s)\n", i + 1);

        pthread_mutex_lock(&mutex);
        ++n_sends;
        pthread_cond_signal(&sent);
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}

/*
 * The receiver counts the number of sends it has seen.  After
 * receiving a message, it might sleep up to 3 seconds.
 */
void* receiver(void* data)
{
    int messages_seen = 0;

    while (1) {
        if (messages_seen >= NPASSES) {
            printf("  Receiver saw %d messages\n", messages_seen);
            return NULL;
        }

        pthread_mutex_lock(&mutex);
        while (n_sends == messages_seen) {
            pthread_cond_wait(&sent, &mutex);
        }
        pthread_mutex_unlock(&mutex);

        ++messages_seen;

        printf("  Receiver saw %d messages...", messages_seen);
        if (n_sends < messages_seen) {
            printf("Oops!  Saw more messages than sent!\n");
            exit(1);
        }
        else if (n_sends == messages_seen) {
            int sleep_time = random() % 4;
            if (sleep_time == 0)
                printf("but not sleeping\n");
            else {
                printf("sleeping %d second(s)\n", sleep_time);
                sleep(sleep_time);
            }
        }
        else
            printf("continuing immediately\n");
    }
}