PTHREAD_CANCEL(3) Linux Programmer's Manual PTHREAD_CANCEL(3)
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 can-
cellation 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_setcancel-
state(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_can-
cel() 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
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 PTHREAD_CANCEL(3)
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