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       pthread_cancel - send a cancellation request to a thread


       #include <pthread.h>

       int pthread_cancel(pthread_t thread);

       Compile and link with -pthread.


       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

       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

       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

       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


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


       ESRCH  No thread with the ID thread could be found.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue   │
       │pthread_cancel() │ Thread safety │ MT-Safe │


       POSIX.1-2001, POSIX.1-2008.


       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.


       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");
           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;

           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");
               printf("main(): thread wasn't canceled (shouldn't happen!)\n");


       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)


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