DESCRIPTION

The pthread_cancel() function sends a cancellation request to the thread thread. Whether and when the target thread reacts to the cancellation request depends on two attributes that are under the control of that thread: its cancelability state and type.

A thread's cancelability state, determined by pthread_setcancelstate(3), can be enabled (the default for new threads) or disabled. If a thread has disabled cancellation, then a cancellation request remains queued until the thread enables cancellation. If a thread has enabled cancellation, then its cancelability type determines when cancellation occurs.

A thread's cancellation type, determined by pthread_setcanceltype(3), may be either asynchronous or deferred (the default for new threads). Asynchronous cancelability means that the thread can be canceled at any time (usually immediately, but the system does not guarantee this). Deferred cancelability means that cancellation will be delayed until the thread next calls a function that is a cancellation point. A list of functions that are or may be cancellation points is provided in pthreads(7).

When a cancellation requested is acted on, the following steps occur for thread (in this order):

The above steps happen asynchronously with respect to the pthread_cancel() call; the return status of pthread_cancel() merely informs the caller whether the cancellation request was successfully queued.

After a canceled thread has terminated, a join with that thread using pthread_join(3) obtains PTHREAD_CANCELED as the thread's exit status. (Joining with a thread is the only way to know that cancellation has completed.)

RETURN VALUE

On success, pthread_cancel() returns 0; on error, it returns a nonzero error number.

ERRORS

CONFORMING TO

POSIX.1-2001.

NOTES

On Linux, cancellation is implemented using signals. Under the NPTL threading implementation, the first real-time signal (i.e., signal 32) is used for this purpose. On LinuxThreads, the second real-time signal is used, if real-time signals are available, otherwise SIGUSR2 is used.

EXAMPLE

The program below creates a thread and then cancels it. The main thread joins with the canceled thread to check that its exit status was PTHREAD_CANCELED. The following shell session shows what happens when we run the program:

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

#define handle_error_en(en, msg) \
        do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

static void *
thread_func(void *ignored_argument)
{
    int s;

    /* Disable cancellation for a while, so that we don't
       immediately react to a cancellation request */

    s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
    if (s != 0)
        handle_error_en(s, "pthread_setcancelstate");

    printf("thread_func(): started; cancellation disabled\n");
    sleep(5);
    printf("thread_func(): about to enable cancellation\n");

    s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
    if (s != 0)
        handle_error_en(s, "pthread_setcancelstate");

    /* sleep() is a cancellation point */

    sleep(1000);        /* Should get canceled while we sleep */

    /* Should never get here */

    printf("thread_func(): not canceled!\n");
    return NULL;
}

int
main(void)
{
    pthread_t thr;
    void *res;
    int s;

    /* Start a thread and then send it a cancellation request */

    s = pthread_create(&thr, NULL, &thread_func, NULL);
    if (s != 0)
        handle_error_en(s, "pthread_create");

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

    printf("main(): sending cancellation request\n");
    s = pthread_cancel(thr);
    if (s != 0)
        handle_error_en(s, "pthread_cancel");

    /* Join with thread to see what its exit status was */

    s = pthread_join(thr, &res);
    if (s != 0)
        handle_error_en(s, "pthread_join");

    if (res == PTHREAD_CANCELED)
        printf("main(): thread was canceled\n");
    else
        printf("main(): thread wasn't canceled (shouldn't happen!)\n");
    exit(EXIT_SUCCESS);
}

SEE ALSO

pthread_cleanup_push(3), pthread_create(3), pthread_exit(3), pthread_join(3), pthread_key_create(3), pthread_setcancelstate(3), pthread_setcanceltype(3), pthread_testcancel(3), pthreads(7)