threads/prod_condvar.c

/* prod_condvar.c

   A simple POSIX threads producer-consumer example using a condition variable.
*/
#include <time.h>
#include <pthread.h>
#include <stdbool.h>
#include "tlpi_hdr.h"

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int avail = 0;

static void *
producer(void *arg)
{
    int cnt = atoi((char *) arg);

    for (int j = 0; j < cnt; j++) {
        sleep(1);

        /* Code to produce a unit omitted */

        int s = pthread_mutex_lock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_lock");

        avail++;        /* Let consumer know another unit is available */

        s = pthread_mutex_unlock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_unlock");

        s = pthread_cond_signal(&cond);         /* Wake sleeping consumer */
        if (s != 0)
            errExitEN(s, "pthread_cond_signal");
    }

    return NULL;
}

int
main(int argc, char *argv[])
{
    time_t t = time(NULL);

    int totRequired = 0;        /* Total number of units that all threads
                                   will produce */
    /* Create all threads */

    for (int j = 1; j < argc; j++) {
        totRequired += atoi(argv[j]);

        pthread_t tid;
        int s = pthread_create(&tid, NULL, producer, argv[j]);
        if (s != 0)
            errExitEN(s, "pthread_create");
    }

    /* Loop to consume available units */

    int numConsumed = 0;        /* Total units so far consumed */
    bool done = false;

    for (;;) {
        int s = pthread_mutex_lock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_lock");

        while (avail == 0) {            /* Wait for something to consume */
            s = pthread_cond_wait(&cond, &mtx);
            if (s != 0)
                errExitEN(s, "pthread_cond_wait");
        }

        /* At this point, 'mtx' is locked... */

        while (avail > 0) {             /* Consume all available units */

            /* Do something with produced unit */

            numConsumed ++;
            avail--;
            printf("T=%ld: numConsumed=%d\n", (long) (time(NULL) - t),
                    numConsumed);

            done = numConsumed >= totRequired;
        }

        s = pthread_mutex_unlock(&mtx);
        if (s != 0)
            errExitEN(s, "pthread_mutex_unlock");

        if (done)
            break;

        /* Perhaps do other work here that does not require mutex lock */

    }

    exit(EXIT_SUCCESS);
}