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  can-
       cellation  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_setcancel-
       state(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_can-
       cel()  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 4.10 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				  2015-07-23		     PTHREAD_CANCEL(3)