PTHREAD_CANCEL(3)

NAME
pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - thread cancellation

SYNOPSIS
#include <pthread.h> int pthread_cancel(pthread_t thread); int pthread_setcancelstate(int state, int *oldstate); int pthread_setcanceltype(int type, int *oldtype); void pthread_testcancel(void);

DESCRIPTION
Cancellation is the mechanism by which a thread can terminate the execution of another thread. More precisely, a thread can send a cancellation request to another thread. Depending on its settings, the target thread can then either ignore the request, honor it immediately, or defer it till it reaches a cancellation point. When a thread eventually honors a cancellation request, it performs as if !pthread_exit(PTHREAD_CANCELED)! has been called at that point: all cleanup handlers are executed in reverse order, finalization functions for thread-specific data are called, and finally the thread stops executing with the return value !PTHREAD_CANCELED!. See !pthread_exit!(3) for more information. !pthread_cancel! sends a cancellation request to the thread denoted by the |thread| argument. !pthread_setcancelstate! changes the cancellation state for the calling thread -- that is, whether cancellation requests are ignored or not. The |state| argument is the new cancellation state: either !PTHREAD_CANCEL_ENABLE! to enable cancellation, or !PTHREAD_CANCEL_DISABLE! to disable cancellation (cancellation requests are ignored). If |oldstate| is not !NULL!, the previous cancellation state is stored in the location pointed to by |oldstate|, and can thus be restored later by another call to !pthread_setcancelstate!. !pthread_setcanceltype! changes the type of responses to cancellation requests for the calling thread: asynchronous (immediate) or deferred. The |type| argument is the new cancellation type: either !PTHREAD_CANCEL_ASYNCHRONOUS! to cancel the calling thread as soon as the cancellation request is received, or !PTHREAD_CANCEL_DEFERRED! to keep the cancellation request pending until the next cancellation point. If |oldtype| is not !NULL!, the previous cancellation state is stored in the location pointed to by |oldtype|, and can thus be restored later by another call to !pthread_setcanceltype!. Threads are always created by !pthread_create!(3) with cancellation enabled and deferred. That is, the initial cancellation state is !PTHREAD_CANCEL_ENABLE! and the initial type is !PTHREAD_CANCEL_DEFERRED!. Cancellation points are those points in the program execution where a test for pending cancellation requests is performed and cancellation is executed if positive. The following POSIX threads functions are cancellation points: !pthread_join!(3)
!pthread_cond_wait!(3)
!pthread_cond_timedwait!(3)
!pthread_testcancel!(3)
!sem_wait!(3)
!sigwait!(3) All other POSIX threads functions are guaranteed not to be cancellation points. That is, they never perform cancellation in deferred cancellation mode. !pthread_testcancel! does nothing except testing for pending cancellation and executing it. Its purpose is to introduce explicit checks for cancellation in long sequences of code that do not call cancellation point functions otherwise.

RETURN VALUE
!pthread_cancel!, !pthread_setcancelstate! and !pthread_setcanceltype! return 0 on success and a non-zero error code on error.

ERRORS
!pthread_cancel! returns the following error code on error:

    !ESRCH!
        no thread could be found corresponding to that specified by the |thread| ID.
!pthread_setcancelstate! returns the following error code on error:

    !EINVAL!
        the |state| argument is not !PTHREAD_CANCEL_ENABLE! nor !PTHREAD_CANCEL_DISABLE!
!pthread_setcanceltype! returns the following error code on error:

    !EINVAL!
        the |type| argument is not !PTHREAD_CANCEL_DEFERRED! nor !PTHREAD_CANCEL_ASYNCHRONOUS!

AUTHOR
Xavier Leroy <Xavier.Leroy@inria.fr>

SEE ALSO
!pthread_exit!(3), !pthread_cleanup_push!(3), !pthread_cleanup_pop!(3).

BUGS
POSIX specifies that a number of system calls (basically, all system calls that may block, such as !read!(2), !write!(2), !wait!(2), etc.) and library functions that may call these system calls (e.g. !fprintf!(3)) are cancellation points. LinuxThreads is not yet integrated enough with the C library to implement this, and thus none of the C library functions is a cancellation point. For system calls at least, there is a workaround. Cancellation requests are transmitted to the target thread by sending it a signal. That signal will interrupt all blocking system calls, causing them to return immediately with the !EINTR! error. So, checking for cancellation during a !read! system call, for instance, can be achieved as follows:

    pthread_testcancel();
    retcode = read(fd, buffer, length);
    pthread_testcancel();