This is threads/thread_cleanup.c (Listing 32-2, page 678), 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_cleanup.c

   An example of thread cancellation using the POSIX threads API:
   demonstrate the use of pthread_cancel() and cleanup handlers.
#include <pthread.h>
#include "tlpi_hdr.h"

static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static int glob = 0;                    /* Predicate variable */
static void     /* Free memory pointed to by 'arg' and unlock mutex */
cleanupHandler(void *arg)
    int s;

    printf("cleanup: freeing block at %p\n", arg);

    printf("cleanup: unlocking mutex\n");
    s = pthread_mutex_unlock(&mtx);
    if (s != 0)
        errExitEN(s, "pthread_mutex_unlock");
static void *
threadFunc(void *arg)
    int s;
    void *buf = NULL;                   /* Buffer allocated by thread */

    buf = malloc(0x10000);              /* Not a cancellation point */
    printf("thread:  allocated memory at %p\n", buf);

    s = pthread_mutex_lock(&mtx);       /* Not a cancellation point */
    if (s != 0)
        errExitEN(s, "pthread_mutex_lock");

    pthread_cleanup_push(cleanupHandler, buf);

    while (glob == 0) {
        s = pthread_cond_wait(&cond, &mtx);     /* A cancellation point */
        if (s != 0)
            errExitEN(s, "pthread_cond_wait");

    printf("thread:  condition wait loop completed\n");
    pthread_cleanup_pop(1);             /* Executes cleanup handler */
    return NULL;
main(int argc, char *argv[])
    pthread_t thr;
    void *res;
    int s;

    s = pthread_create(&thr, NULL, threadFunc, NULL);
    if (s != 0)
        errExitEN(s, "pthread_create");

    sleep(2);                   /* Give thread a chance to get started */

    if (argc == 1) {            /* Cancel thread */
        printf("main:    about to cancel thread\n");
        s = pthread_cancel(thr);
        if (s != 0)
            errExitEN(s, "pthread_cancel");

    } else {                    /* Signal condition variable */
        printf("main:    about to signal condition variable\n");

        s = pthread_mutex_lock(&mtx);   /* See the TLPI page 679 erratum */
        if (s != 0)
            errExitEN(s, "pthread_mutex_lock");

        glob = 1;

        s = pthread_mutex_unlock(&mtx); /* See the TLPI page 679 erratum */
        if (s != 0)
            errExitEN(s, "pthread_mutex_unlock");

        s = pthread_cond_signal(&cond);
        if (s != 0)
            errExitEN(s, "pthread_cond_signal");

    s = pthread_join(thr, &res);
    if (s != 0)
        errExitEN(s, "pthread_join");
    if (res == PTHREAD_CANCELED)
        printf("main:    thread was canceled\n");
        printf("main:    thread terminated normally\n");



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