Question

I wanted to cancel a thread right before the process de-initialization is done, like the following.

rc2 = pthread_attr_init(&attr);
ERR_IF( rc2 != 0 );

rc2 = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
ERR_IF( rc2 != 0 );

rc2 = pthread_create(&destroy_thread, &attr, destroy_expired_sessions, NULL);
ERR_IF( rc2 != 0 );
...
rc2 = pthread_cancel(destroy_thread);
ERR_IF( rc2 != 0 );

usleep(10); // without the sleep here, the real cancellation will be postponed.

rc2 = authSessionListDeInit();
ERR_IF( rc2 != 0 );

...

static void *destroy_expired_sessions(void *t)
{
    int rc2 = 0;

    (void)t;

    pthread_cleanup_push(cleanup_handler, NULL);

    rc2 = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
    if (rc2 != 0)
        AUTH_DEBUG5("pthread_setcancelstate(): rc2 == %d\n", rc2);

    rc2 = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);
    if (rc2 != 0)
        AUTH_DEBUG5("pthread_setcanceltype(): rc2 == %d\n", rc2);

    ... // real work of the thread is done here
}

The problem is, although PTHREAD_CANCEL_ASYNCHRONOUS is set here, the real thread cancellation always happens after authSessionListDeInit(), unless I force a usleep() in between.

My understanding is the cancellation should happen right after the cancel request is sent through pthread_cancel(), shouldn’t it?

If my understanding is not correct, how to ensure the thread gets cancelled before authSessionListDeInit() is called?

Answer 1

From the docs:

The cancellation processing in the target thread shall run
asynchronously with respect to the calling thread returning from
pthread_cancel().

So pthread_cancel can return before the cancellation happens, meaning your assumption is wrong.

Instead of using pthread_cancel you may consider the more popular and arguably safer method (risk of failure with pthread_cancel) of using a cancel flag and/or condition variables to signal when the function actually ends.