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)
- 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 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 |
POSIX.1-2001, POSIX.1-2008.
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.
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
#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)