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 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  asyn‐
       chronous 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 guar‐
       antee  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 implementa‐
       tion,  the  first  real-time signal (i.e., signal 32) is used for this purpose.  On Linux‐
       Threads, the second real-time signal is used, if real-time signals are  available,  other‐
       wise 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 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2008-11-17                          PTHREAD_CANCEL(3)