psem/thread_incr_psem.c

This is psem/thread_incr_psem.c (Listing 53-6, page 1101), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* thread_incr_psem.c

   Use a POSIX unnamed semaphore to synchronize access by two threads to
   a global variable.

   See also thread_incr.c and thread_incr_mutex.c.
*/
#include <semaphore.h>
#include <pthread.h>
#include "tlpi_hdr.h"

static int glob = 0;
static sem_t sem;
static void *                   /* Loop 'arg' times incrementing 'glob' */
threadFunc(void *arg)
{
    int loops = *((int *) arg);
    int loc, j;

    for (j = 0; j < loops; j++) {
        if (sem_wait(&sem) == -1)
            errExit("sem_wait");

        loc = glob;
        loc++;
        glob = loc;

        if (sem_post(&sem) == -1)
            errExit("sem_post");
    }

    return NULL;
}
int
main(int argc, char *argv[])
{
    pthread_t t1, t2;
    int loops, s;

    loops = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-loops") : 10000000;

    /* Initialize a semaphore with the value 1 */

    if (sem_init(&sem, 0, 1) == -1)
        errExit("sem_init");

    /* Create two threads that increment 'glob' */

    s = pthread_create(&t1, NULL, threadFunc, &loops);
    if (s != 0)
        errExitEN(s, "pthread_create");
    s = pthread_create(&t2, NULL, threadFunc, &loops);
    if (s != 0)
        errExitEN(s, "pthread_create");

    /* Wait for threads to terminate */

    s = pthread_join(t1, NULL);
    if (s != 0)
        errExitEN(s, "pthread_join");
    s = pthread_join(t2, NULL);
    if (s != 0)
        errExitEN(s, "pthread_join");

    printf("glob = %d\n", glob);
    exit(EXIT_SUCCESS);
}

 

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