# pthread_cancel

## NAME



           pthread_cancel - send a cancelation request to a thread
    


## LIBRARY



           POSIX threads library (_libpthread_ , _-lpthread_)
    


## SYNOPSIS



           **#include <pthread.h>**
    
           **int pthread_cancel(pthread_t** _thread_**);**
    


## DESCRIPTION



           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 request is acted on, the following steps occur
           for _thread_ (in this order):
    
           (1)  Cancelation 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.)
    


## 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 │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## VERSIONS



           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.
    


## STANDARDS



           POSIX.1-2008.
    


## 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; cancelation disabled
               main(): sending cancelation request
               thread_func(): about to enable cancelation
               main(): thread was canceled
    
       **Program source**
    
           #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);
           }
    


* * *