pthread_cancel(3) - Linux manual page (original) (raw)
pthreadcancel(3) Library Functions Manual pthreadcancel(3)
NAME top
pthread_cancel - send a cancelation request to a threadLIBRARY top
POSIX threads library (_libpthread_, _-lpthread_)SYNOPSIS top
**#include <pthread.h>**
**int pthread_cancel(pthread_t** _thread_**);**DESCRIPTION top
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)](../man3/pthread%5Fsetcancelstate.3.html), 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)](../man3/pthread%5Fsetcanceltype.3.html), 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)](../man7/pthreads.7.html).
When a cancelation request 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)](../man3/pthread%5Fcleanup%5Fpush.3.html).)
(2) Thread-specific data destructors are called, in an
unspecified order. (See [pthread_key_create(3)](../man3/pthread%5Fkey%5Fcreate.3.html).)
(3) The thread is terminated. (See [pthread_exit(3)](../man3/pthread%5Fexit.3.html).)
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)](../man3/pthread%5Fjoin.3.html) obtains **PTHREAD_CANCELED** as the thread's
exit status. (Joining with a thread is the only way to know that
cancelation has completed.)RETURN VALUE top
On success, **pthread_cancel**() returns 0; on error, it returns a
nonzero error number.ERRORS top
**ESRCH** No thread with the ID _thread_ could be found.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **pthread_cancel**() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘VERSIONS top
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.STANDARDS top
POSIX.1-2008.HISTORY top
glibc 2.0 POSIX.1-2001.EXAMPLES top
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 canceledProgram 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);
}SEE ALSO top
[pthread_cleanup_push(3)](../man3/pthread%5Fcleanup%5Fpush.3.html), [pthread_create(3)](../man3/pthread%5Fcreate.3.html), [pthread_exit(3)](../man3/pthread%5Fexit.3.html),
[pthread_join(3)](../man3/pthread%5Fjoin.3.html), [pthread_key_create(3)](../man3/pthread%5Fkey%5Fcreate.3.html), [pthread_setcancelstate(3)](../man3/pthread%5Fsetcancelstate.3.html),
[pthread_setcanceltype(3)](../man3/pthread%5Fsetcanceltype.3.html), [pthread_testcancel(3)](../man3/pthread%5Ftestcancel.3.html), [pthreads(7)](../man7/pthreads.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-05 pthreadcancel(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)