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.

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_setcancelstate(3), pthread_setcanceltype(3), pthread_testcancel(3), pthreads(7)

COLOPHON

       This page is part of release 3.54 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/.