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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.

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 cancel-
         lation 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); }