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       pthread_cancel - send a cancelation request to a thread


       POSIX threads library (libpthread, -lpthread)


       #include <pthread.h>

       int pthread_cancel(pthread_t thread);


       The  pthread_cancel()  function sends a cancelation request to the thread thread.  Whether
       and when the target thread reacts to the cancelation 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 cancelation,  then  a
       cancelation  request remains queued until the thread enables cancelation.  If a thread has
       enabled cancelation, then its cancelability type determines when cancelation occurs.

       A thread's  cancelation  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 cancelation will be delayed until the
       thread next calls a function that is a cancelation point.  A list of functions that are or
       may be cancelation points is provided in pthreads(7).

       When  a  cancelation  requested is acted on, the following steps occur for thread (in this

       (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

       (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 cancelation
       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 cancelation has completed.)


       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 attributes(7).

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


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


       On  Linux,  cancelation  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; cancelation disabled
           main(): sending cancelation request
           thread_func(): about to enable cancelation
           main(): thread was canceled

   Program source

       #include <errno.h>
       #include <pthread.h>
       #include <stdio.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 cancelation for a while, so that we don't
              immediately react to a cancelation request. */

           s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
           if (s != 0)
               handle_error_en(s, "pthread_setcancelstate");

           printf("%s(): started; cancelation disabled\n", __func__);
           printf("%s(): about to enable cancelation\n", __func__);

           s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
           if (s != 0)
               handle_error_en(s, "pthread_setcancelstate");

           /* sleep() is a cancelation point. */

           sleep(1000);        /* Should get canceled while we sleep */

           /* Should never get here. */

           printf("%s(): not canceled!\n", __func__);
           return NULL;

           pthread_t thr;
           void *res;
           int s;

           /* Start a thread and then send it a cancelation 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("%s(): sending cancelation request\n", __func__);
           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("%s(): thread was canceled\n", __func__);
               printf("%s(): 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)