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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 exe-
cution of another thread. More precisely, a thread can send a cancella-
tion 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 exe-
cuting with the return value PTHREAD_CANCELED. See pthread_exit(3) for
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_CAN-
CEL_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_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_CAN-
CEL_ASYNCHRONOUS to cancel the calling thread as soon as the cancella-
tion request is received, or PTHREAD_CANCEL_DEFERRED to keep the can-
cellation 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 cancel-
All other POSIX threads functions are guaranteed not to be cancellation
points. That is, they never perform cancellation in deferred cancella-
pthread_testcancel does nothing except testing for pending cancellation
and executing it. Its purpose is to introduce explicit checks for can-
cellation in long sequences of code that do not call cancellation point
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_setcanceltype returns the following error code on error:
EINVAL the type argument is not PTHREAD_CANCEL_DEFERRED nor
Xavier Leroy <Xavier.Leroy@inria.fr>
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 sig-
nal 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);