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  pro-
       vided 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.

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:

           $ ./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_setcancel-
       state(3), pthread_setcanceltype(3), pthread_testcancel(3), pthreads(7)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-11-17                                          PTHREAD_CANCEL(3)