This is threads/thread_incr_rwlock.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 33.

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

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download threads/thread_incr_rwlock.c

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/* thread_incr_rwlock.c

   This program employs two POSIX threads that increment the same global
   variable, synchronizing their access using a read/write lock. As a
   consequence, updates are not lost. Compare with thread_incr.c,
   thread_incr_mutex.c, and thread_incr_spinlock.c.
#include <pthread.h>
#include "tlpi_hdr.h"

static volatile int glob = 0;
static pthread_rwlock_t rwlock;
static void *                   /* Loop 'arg' times incrementing 'glob' */
threadFunc(void *arg)
    int loops = *((int *) arg);
    int loc, s;

    for (int j = 0; j < loops; j++) {
        s = pthread_rwlock_wrlock(&rwlock);
        if (s != 0)
            errExitEN(s, "pthread_rwlock_wrlock");

        loc = glob;
        glob = loc;

        s = pthread_rwlock_unlock(&rwlock);
        if (s != 0)
            errExitEN(s, "pthread_rwlock_unlock");

    return NULL;
main(int argc, char *argv[])
    int loops = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-loops") : 10000000;

    int s = pthread_rwlock_init(&rwlock, NULL);
    if (s != 0)
        errExitEN(s, "pthread_rwlock_init");

    pthread_t t1, t2;
    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");

    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);


Download threads/thread_incr_rwlock.c

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