pthread_cancel

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



NAME
       pthread_cancel - send a cancellation request to a thread

SYNOPSIS
       #include <pthread.h>

       int pthread_cancel(pthread_t thread);

       Compile and link with -pthread.

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):

       1. Cancellation clean-up handlers are popped (in the reverse of the
          order in which they were pushed) and called.  (See
          pthread_cleanup_push(3).)

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

ERRORS
       ESRCH  No thread with the ID thread could be found.

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       ┌─────────────────┬───────────────┬─────────┐
       │Interface        Attribute     Value   │
       ├─────────────────┼───────────────┼─────────┤
       │pthread_cancel() │ Thread safety │ MT-Safe │
       └─────────────────┴───────────────┴─────────┘

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

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:

           $ ./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");
           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)

COLOPHON
       This page is part of release 5.03 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                 PTHREAD_CANCEL(3)