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| .TH PTHREAD_CANCEL 3 2015-07-23 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| pthread_cancel \- send a cancellation request to a thread |
| .SH SYNOPSIS |
| .nf |
| .B #include <pthread.h> |
| |
| .BI "int pthread_cancel(pthread_t " thread ); |
| .sp |
| Compile and link with \fI\-pthread\fP. |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR pthread_cancel () |
| function sends a cancellation request to the thread |
| .IR 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 |
| .I state |
| and |
| .IR type . |
| |
| A thread's cancelability state, determined by |
| .BR pthread_setcancelstate (3), |
| can be |
| .I enabled |
| (the default for new threads) or |
| .IR 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 |
| .BR pthread_setcanceltype (3), |
| may be either |
| .IR asynchronous |
| or |
| .IR 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 |
| .IR "cancellation point" . |
| A list of functions that are or may be cancellation points is provided in |
| .BR pthreads (7). |
| |
| When a cancellation requested is acted on, the following steps occur for |
| .IR thread |
| (in this order): |
| .IP 1. 3 |
| Cancellation clean-up handlers are popped |
| (in the reverse of the order in which they were pushed) and called. |
| (See |
| .BR pthread_cleanup_push (3).) |
| .IP 2. |
| Thread-specific data destructors are called, |
| in an unspecified order. |
| (See |
| .BR pthread_key_create (3).) |
| .IP 3. |
| The thread is terminated. |
| (See |
| .BR pthread_exit (3).) |
| .PP |
| The above steps happen asynchronously with respect to the |
| .BR pthread_cancel () |
| call; |
| the return status of |
| .BR pthread_cancel () |
| merely informs the caller whether the cancellation request |
| was successfully queued. |
| .PP |
| After a canceled thread has terminated, |
| a join with that thread using |
| .BR pthread_join (3) |
| obtains |
| .B PTHREAD_CANCELED |
| as the thread's exit status. |
| (Joining with a thread is the only way to know that cancellation |
| has completed.) |
| .SH RETURN VALUE |
| On success, |
| .BR pthread_cancel () |
| returns 0; |
| on error, it returns a nonzero error number. |
| .SH ERRORS |
| .TP |
| .B ESRCH |
| No thread with the ID |
| .I thread |
| could be found. |
| .\" .SH VERSIONS |
| .\" Available since glibc 2.0 |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR pthread_cancel () |
| T} Thread safety MT-Safe |
| .TE |
| |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH 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 |
| .B SIGUSR2 |
| is used. |
| .SH 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 |
| .BR PTHREAD_CANCELED . |
| The following shell session shows what happens when we run the program: |
| |
| .in +4n |
| .nf |
| $ ./a.out |
| thread_func(): started; cancellation disabled |
| main(): sending cancellation request |
| thread_func(): about to enable cancellation |
| main(): thread was canceled |
| .fi |
| .in |
| .SS Program source |
| \& |
| .nf |
| #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\(aqt |
| 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\(aqt canceled (shouldn\(aqt happen!)\\n"); |
| exit(EXIT_SUCCESS); |
| } |
| .fi |
| .SH SEE ALSO |
| .ad l |
| .nh |
| .BR pthread_cleanup_push (3), |
| .BR pthread_create (3), |
| .BR pthread_exit (3), |
| .BR pthread_join (3), |
| .BR pthread_key_create (3), |
| .BR pthread_setcancelstate (3), |
| .BR pthread_setcanceltype (3), |
| .BR pthread_testcancel (3), |
| .BR pthreads (7) |