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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
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_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.
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 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
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