pthread_cancel(3) — Linux manual page


pthread_cancel(3)       Library Functions Manual       pthread_cancel(3)

NAME         top

       pthread_cancel - send a cancelation request to a thread

LIBRARY         top

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS         top

       #include <pthread.h>

       int pthread_cancel(pthread_t thread);

DESCRIPTION         top

       The pthread_cancel() function sends a cancelation request to the
       thread thread.  Whether and when the target thread reacts to the
       cancelation 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 cancelation, then
       a cancelation request remains queued until the thread enables
       cancelation.  If a thread has enabled cancelation, then its
       cancelability type determines when cancelation occurs.

       A thread's cancelation 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 cancelation will be delayed until the thread next
       calls a function that is a cancelation point.  A list of
       functions that are or may be cancelation points is provided in

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

       (1)  Cancelation clean-up handlers are popped (in the reverse of
            the order in which they were pushed) and called.  (See

       (2)  Thread-specific data destructors are called, in an
            unspecified order.  (See pthread_key_create(3).)

       (3)  The thread is terminated.  (See pthread_exit(3).)

       The above steps happen asynchronously with respect to the
       pthread_cancel() call; the return status of pthread_cancel()
       merely informs the caller whether the cancelation 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
       cancelation has completed.)

RETURN VALUE         top

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

ERRORS         top

       ESRCH  No thread with the ID thread could be found.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ pthread_cancel()                    │ Thread safety │ MT-Safe │

VERSIONS         top

       On Linux, cancelation 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.

STANDARDS         top


HISTORY         top

       glibc 2.0 POSIX.1-2001.

EXAMPLES         top

       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:

           $ ./a.out
           thread_func(): started; cancelation disabled
           main(): sending cancelation request
           thread_func(): about to enable cancelation
           main(): thread was canceled

   Program source

       #include <errno.h>
       #include <pthread.h>
       #include <stdio.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 cancelation for a while, so that we don't
              immediately react to a cancelation request. */

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

           printf("%s(): started; cancelation disabled\n", __func__);
           printf("%s(): about to enable cancelation\n", __func__);

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

           /* sleep() is a cancelation point. */

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

           /* Should never get here. */

           printf("%s(): not canceled!\n", __func__);
           return NULL;

           pthread_t thr;
           void *res;
           int s;

           /* Start a thread and then send it a cancelation 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("%s(): sending cancelation request\n", __func__);
           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("%s(): thread was canceled\n", __func__);
               printf("%s(): thread wasn't canceled (shouldn't happen!)\n",

SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)                pthread_cancel(3)

Pages that refer to this page: pthread_cleanup_push(3)pthread_cleanup_push_defer_np(3)pthread_create(3)pthread_detach(3)pthread_join(3)pthread_kill_other_threads_np(3)pthread_setcancelstate(3)pthread_testcancel(3)pthreads(7)