PTHREAD_CANCEL

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
AUTHOR
SEE ALSO
BUGS

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();