NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON
PTHREAD_CANCEL(3) Linux Programmer's Manual PTHREAD_CANCEL(3)
pthread_cancel - send a cancellation request to a thread
#include <pthread.h>
int pthread_cancel(pthread_t thread);
Compile and link with -pthread.
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.)
On success, pthread_cancel() returns 0; on error, it returns a non-zero error
number.
ESRCH No thread with the ID thread could be found.
POSIX.1-2001.
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.
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
#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);
}
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 release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-11-17 PTHREAD_CANCEL(3)