Section (3) pthread_cancel
Name
pthread_cancel — send a cancellation request to a thread
Synopsis
#include <pthread.h>
int
pthread_cancel( |
pthread_t thread) ; |
![]() |
Note |
---|---|
Compile and link with |
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_zsingle_quotesz_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_zsingle_quotesz_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):
-
Cancellation clean-up handlers are popped (in the reverse of the order in which they were pushed) and called. (See pthread_cleanup_push(3).)
-
Thread-specific data destructors are called, in an unspecified order. (See pthread_key_create(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_zsingle_quotesz_s exit status. (Joining with a thread is the only way
to know that cancellation has completed.)
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
pthread_cancel () |
Thread safety | MT-Safe |
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_zsingle_quotesz_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 ); sleep(5); printf(thread_func(): about to enable cancellation ); 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! ); 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 ); 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 ); else printf(main(): thread wasn_zsingle_quotesz_t canceled (shouldn_zsingle_quotesz_t happen!) ); 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 5.04 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/.
Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |