pthread_cancel(3) — Linux manual page


PTHREAD_CANCEL(3)       Linux Programmer's Manual      PTHREAD_CANCEL(3)

NAME         top

       pthread_cancel - send a cancellation request to a thread

SYNOPSIS         top

       #include <pthread.h>

       int pthread_cancel(pthread_t thread);

       Compile and link with -pthread.

DESCRIPTION         top

       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

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

       1. Cancellation 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 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         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 │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       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.

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; cancellation disabled
           main(): sending cancellation request
           thread_func(): about to enable cancellation
           main(): thread was canceled

   Program source

       #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");
           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;

           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");
               printf("main(): 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)

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

Linux                          2021-03-22              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)