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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.
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 3.53 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)