PTHREAD_CANCEL(3) BSD Library Functions Manual PTHREAD_CANCEL(3)NAME
pthread_cancel -- cancel execution of a thread
POSIX Threads Library (libpthread, -lpthread)
The pthread_cancel() function requests that thread be canceled. The target thread's cancelability state and type determines whether and when
the target thread reacts to the cancellation request.
1. The cancelability state of a thread is determined by the pthread_setcancelstate(3) function. The state can be either:
o PTHREAD_CANCEL_ENABLE: the cancelability type determines when the actual cancellation occurs. This is the default.
o PTHREAD_CANCEL_DISABLE: the request from pthread_cancel() remains queued until the cancellation is enabled by the thread.
2. The cancellation type of a thread is determined by the pthread_setcanceltype(3) function. The type can be either:
o PTHREAD_CANCEL_DEFERRED: the cancellation will be delayed until the thread calls a function that is a cancellation point. This
is the default. The available cancellation points are listed in pthread_setcanceltype(3).
o PTHREAD_CANCEL_ASYNCHRONOUS: the thread can be canceled at any time.
When the tread reacts to the cancellation request, the following occur:
1. The cancellation cleanup handlers for the thread are called; see pthread_cleanup_push(3).
2. When the last cancellation cleanup handler returns, the thread-specific data destructor functions will be called for the thread.
3. When the last destructor function returns, the thread will be terminated; see pthread_exit(3).
The cancellation processing in the target thread runs asynchronously with respect to the calling thread returning from pthread_cancel().
A status of PTHREAD_CANCELED is made available to any threads joining with the target. The symbolic constant PTHREAD_CANCELED expands to a
constant expression of type (void *), whose value matches no pointer to an object in memory nor the value NULL.
If successful, the pthread_cancel() functions will return zero. Otherwise an error number will be returned to indicate the error.
The pthread_cancel() function may fail if:
[ESRCH] No thread could be found corresponding to that specified by the given thread ID.
SEE ALSO pthread_cleanup_pop(3), pthread_join(3), pthread_testcancel(3)STANDARDS
The function conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
BSD July 9, 2010 BSD
Check Out this Related Man Page
PTHREAD_CANCEL(3) Library Functions Manual PTHREAD_CANCEL(3)NAME
pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - thread cancellation
int pthread_cancel(pthread_t thread);
int pthread_setcancelstate(int state, int *oldstate);
int pthread_setcanceltype(int type, int *oldtype);
Cancellation is the mechanism by which a thread can terminate the execution of another thread. More precisely, a thread can send a cancel-
lation 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
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:
All other POSIX threads functions are guaranteed not to be cancellation points. That is, they never perform cancellation in deferred can-
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.
pthread_cancel, pthread_setcancelstate and pthread_setcanceltype return 0 on success and a non-zero error code on error.
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
Xavier Leroy <Xavier.Leroy@inria.fr>
SEE ALSO pthread_exit(3), pthread_cleanup_push(3), pthread_cleanup_pop(3).
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:
retcode = read(fd, buffer, length);