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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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pthread_cancel(3) Library Functions Manual pthread_cancel(3)
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 order):
(1) Cancelation 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 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).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ pthread_cancel() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
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.
POSIX.1-2008.
glibc 2.0 POSIX.1-2001.
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__);
sleep(5);
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;
}
int
main(void)
{
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__);
else
printf("%s(): thread wasn't canceled (shouldn't happen!)\n",
__func__);
exit(EXIT_SUCCESS);
}
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 pthread_cancel(3)
Pages that refer to this page: pthread_cleanup_push(3), pthread_cleanup_push_defer_np(3), pthread_create(3), pthread_detach(3), pthread_join(3), pthread_kill_other_threads_np(3), pthread_mutex_init(3), pthread_setcancelstate(3), pthread_testcancel(3), pthreads(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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