1PTHREAD_CANCEL(3)          Linux Programmer's Manual         PTHREAD_CANCEL(3)
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NAME

6       pthread_cancel - send a cancellation request to a thread
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SYNOPSIS

9       #include <pthread.h>
10
11       int pthread_cancel(pthread_t thread);
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13       Compile and link with -pthread.
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DESCRIPTION

16       The  pthread_cancel()  function  sends  a  cancellation  request to the
17       thread thread.  Whether and when the target thread reacts to  the  can‐
18       cellation  request depends on two attributes that are under the control
19       of that thread: its cancelability state and type.
20
21       A  thread's  cancelability  state,  determined  by   pthread_setcancel‐
22       state(3), can be enabled (the default for new threads) or disabled.  If
23       a thread has disabled cancellation, then a cancellation request remains
24       queued  until the thread enables cancellation.  If a thread has enabled
25       cancellation, then its cancelability type determines when  cancellation
26       occurs.
27
28       A  thread's  cancellation type, determined by pthread_setcanceltype(3),
29       may be either asynchronous or deferred (the default for  new  threads).
30       Asynchronous cancelability means that the thread can be canceled at any
31       time (usually immediately, but the system  does  not  guarantee  this).
32       Deferred  cancelability  means  that cancellation will be delayed until
33       the thread next calls a function that is a cancellation point.  A  list
34       of  functions  that  are  or  may be cancellation points is provided in
35       pthreads(7).
36
37       When a cancellation requested is acted on, the  following  steps  occur
38       for thread (in this order):
39
40       1. Cancellation  clean-up  handlers  are  popped (in the reverse of the
41          order   in   which   they   were   pushed)   and    called.     (See
42          pthread_cleanup_push(3).)
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44       2. Thread-specific  data  destructors  are  called,  in  an unspecified
45          order.  (See pthread_key_create(3).)
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47       3. The thread is terminated.  (See pthread_exit(3).)
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49       The above steps happen asynchronously with respect to the  pthread_can‐
50       cel()  call;  the  return status of pthread_cancel() merely informs the
51       caller whether the cancellation request was successfully queued.
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53       After a canceled thread has terminated, a join with that  thread  using
54       pthread_join(3)  obtains  PTHREAD_CANCELED as the thread's exit status.
55       (Joining with a thread is the only way to know  that  cancellation  has
56       completed.)
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RETURN VALUE

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

63       ESRCH  No thread with the ID thread could be found.
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CONFORMING TO

66       POSIX.1-2001.
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NOTES

69       On Linux, cancellation is implemented using signals.   Under  the  NPTL
70       threading  implementation, the first real-time signal (i.e., signal 32)
71       is used for this purpose.  On LinuxThreads, the second real-time signal
72       is used, if real-time signals are available, otherwise SIGUSR2 is used.
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EXAMPLE

75       The  program  below  creates  a  thread  and then cancels it.  The main
76       thread joins with the canceled thread to check that its exit status was
77       PTHREAD_CANCELED.   The following shell session shows what happens when
78       we run the program:
79
80           $ ./a.out
81           thread_func(): started; cancellation disabled
82           main(): sending cancellation request
83           thread_func(): about to enable cancellation
84           main(): thread was canceled
85
86   Program source
87
88       #include <pthread.h>
89       #include <stdio.h>
90       #include <errno.h>
91       #include <stdlib.h>
92       #include <unistd.h>
93
94       #define handle_error_en(en, msg) \
95               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
96
97       static void *
98       thread_func(void *ignored_argument)
99       {
100           int s;
101
102           /* Disable cancellation for a while, so that we don't
103              immediately react to a cancellation request */
104
105           s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
106           if (s != 0)
107               handle_error_en(s, "pthread_setcancelstate");
108
109           printf("thread_func(): started; cancellation disabled\n");
110           sleep(5);
111           printf("thread_func(): about to enable cancellation\n");
112
113           s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
114           if (s != 0)
115               handle_error_en(s, "pthread_setcancelstate");
116
117           /* sleep() is a cancellation point */
118
119           sleep(1000);        /* Should get canceled while we sleep */
120
121           /* Should never get here */
122
123           printf("thread_func(): not canceled!\n");
124           return NULL;
125       }
126
127       int
128       main(void)
129       {
130           pthread_t thr;
131           void *res;
132           int s;
133
134           /* Start a thread and then send it a cancellation request */
135
136           s = pthread_create(&thr, NULL, &thread_func, NULL);
137           if (s != 0)
138               handle_error_en(s, "pthread_create");
139
140           sleep(2);           /* Give thread a chance to get started */
141
142           printf("main(): sending cancellation request\n");
143           s = pthread_cancel(thr);
144           if (s != 0)
145               handle_error_en(s, "pthread_cancel");
146
147           /* Join with thread to see what its exit status was */
148
149           s = pthread_join(thr, &res);
150           if (s != 0)
151               handle_error_en(s, "pthread_join");
152
153           if (res == PTHREAD_CANCELED)
154               printf("main(): thread was canceled\n");
155           else
156               printf("main(): thread wasn't canceled (shouldn't happen!)\n");
157           exit(EXIT_SUCCESS);
158       }
159

SEE ALSO

161       pthread_cleanup_push(3), pthread_create(3), pthread_exit(3),
162       pthread_join(3), pthread_key_create(3), pthread_setcancelstate(3),
163       pthread_setcanceltype(3), pthread_testcancel(3), pthreads(7)
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COLOPHON

166       This page is part of release 3.53 of the Linux man-pages project.  A
167       description of the project, and information about reporting bugs, can
168       be found at http://www.kernel.org/doc/man-pages/.
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172Linux                             2008-11-17                 PTHREAD_CANCEL(3)
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