PTHREAD_CANCEL

Section: Linux Programmer's Manual (3)
Updated: 2020-06-09
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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 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.)

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.

EXAMPLES

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); }

COLOPHON

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