apps/rumprun_pthreads/components/pthreads/pthreads.c

/**
 * Copyright 2020, Data61, CSIRO (ABN 41 687 119 230)
 *
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Adapted from https://computing.llnl.gov/tutorials/pthreads/
 */

/******************************************************************************
* FILE: condvar.c
* DESCRIPTION:
*   Example code for using Pthreads condition variables.  The main thread
*   creates three threads.  Two of those threads increment a "count" variable,
*   while the third thread watches the value of "count".  When "count"
*   reaches a predefined limit, the waiting thread is signaled by one of the
*   incrementing threads. The waiting thread "awakens" and then modifies
*   count. The program continues until the incrementing threads reach
*   TCOUNT. The main program prints the final value of count.
* SOURCE: Adapted from example code in "Pthreads Programming", B. Nichols
*   et al. O'Reilly and Associates.
* LAST REVISED: 03/07/17  Blaise Barney
******************************************************************************/

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

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int     count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t)
{
    int i;
    long my_id = (long)t;

    for (i = 0; i < TCOUNT; i++) {
        pthread_mutex_lock(&count_mutex);
        count++;

        /*
        Check the value of count and signal waiting thread when condition is
        reached.  Note that this occurs while mutex is locked.
        */
        if (count == COUNT_LIMIT) {
            printf("inc_count(): thread %ld, count = %d  Threshold reached. ",
                   my_id, count);
            pthread_cond_signal(&count_threshold_cv);
            printf("Just sent signal.\n");
        }
        printf("inc_count(): thread %ld, count = %d, unlocking mutex\n",
               my_id, count);
        pthread_mutex_unlock(&count_mutex);

        /* Do some work so threads can alternate on mutex lock */
        sleep(1);
    }
    pthread_exit(NULL);
}

void *watch_count(void *t)
{
    long my_id = (long)t;

    printf("Starting watch_count(): thread %ld\n", my_id);

    /*
    Lock mutex and wait for signal.  Note that the pthread_cond_wait routine
    will automatically and atomically unlock mutex while it waits.
    Also, note that if COUNT_LIMIT is reached before this routine is run by
    the waiting thread, the loop will be skipped to prevent pthread_cond_wait
    from never returning.
    */
    pthread_mutex_lock(&count_mutex);
    while (count < COUNT_LIMIT) {
        printf("watch_count(): thread %ld Count= %d. Going into wait...\n", my_id, count);
        pthread_cond_wait(&count_threshold_cv, &count_mutex);
        printf("watch_count(): thread %ld Condition signal received. Count= %d\n", my_id, count);
    }
    printf("watch_count(): thread %ld Updating the value of count...\n", my_id);
    count += 125;
    printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
    printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
    pthread_mutex_unlock(&count_mutex);
    pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
    int i, rc;
    long t1 = 1, t2 = 2, t3 = 3;
    pthread_t threads[3];
    pthread_attr_t attr;

    /* Initialize mutex and condition variable objects */
    pthread_mutex_init(&count_mutex, NULL);
    pthread_cond_init(&count_threshold_cv, NULL);

    /* For portability, explicitly create threads in a joinable state */
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    pthread_create(&threads[0], &attr, watch_count, (void *)t1);
    pthread_create(&threads[1], &attr, inc_count, (void *)t2);
    pthread_create(&threads[2], &attr, inc_count, (void *)t3);

    /* Wait for all threads to complete */
    for (i = 0; i < NUM_THREADS; i++) {
        pthread_join(threads[i], NULL);
    }
    printf("Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n",
           NUM_THREADS, count);

    /* Clean up and exit */
    pthread_attr_destroy(&attr);
    pthread_mutex_destroy(&count_mutex);
    pthread_cond_destroy(&count_threshold_cv);
    pthread_exit(NULL);

}