PTHREAD_SETCANCELSTATE
PTHREAD_SETCANCELSTATE
NAMESYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO
NOTES
EXAMPLE
SEE ALSO
COLOPHON
NAME
pthread_setcancelstate, pthread_setcanceltype − set cancelability state and type
SYNOPSIS
#include <pthread.h>
int
pthread_setcancelstate(int state, int
*oldstate);
int pthread_setcanceltype(int type, int
*oldtype);
Compile and link with −pthread.
DESCRIPTION
The
pthread_setcancelstate() sets the cancelability state
of the calling thread to the value given in state.
The previous cancelability state of the thread is returned
in the buffer pointed to by oldstate. The
state argument must have one of the following values:
PTHREAD_CANCEL_ENABLE
The thread is cancelable. This is the default cancelability state in all new threads, including the initial thread. The thread’s cancelability type determines when a cancelable thread will respond to a cancellation request.
PTHREAD_CANCEL_DISABLE
The thread is not cancelable. If a cancellation request is received, it is blocked until cancelability is enabled.
The
pthread_setcanceltype() sets the cancelability type
of the calling thread to the value given in type. The
previous cancelability type of the thread is returned in the
buffer pointed to by oldtype. The type
argument must have one of the following values:
PTHREAD_CANCEL_DEFERRED
A cancellation request is deferred until the thread next calls a function that is a cancellation point (see pthreads(7)). This is the default cancelability type in all new threads, including the initial thread.
PTHREAD_CANCEL_ASYNCHRONOUS
The thread can be canceled at any time. (Typically, it will be canceled immediately upon receiving a cancellation request, but the system doesn’t guarantee this.)
The set-and-get operation performed by each of these functions is atomic with respect to other threads in the process calling the same function.
RETURN VALUE
On success, these functions return 0; on error, they return a nonzero error number.
ERRORS
The pthread_setcancelstate() can fail with the following error:
|
EINVAL |
Invalid value for state. |
The pthread_setcanceltype() can fail with the following error:
|
EINVAL |
Invalid value for type. |
ATTRIBUTES
Multithreading
(see pthreads(7))
The pthread_setcancelstate() and
pthread_setcanceltype() functions are
thread-safe.
CONFORMING TO
POSIX.1-2001.
NOTES
For details of what happens when a thread is canceled, see pthread_cancel(3).
Briefly disabling cancelability is useful if a thread performs some critical action that must not be interrupted by a cancellation request. Beware of disabling cancelability for long periods, or around operations that may block for long periods, since that will render the thread unresponsive to cancellation requests.
Asynchronous
cancelability
Setting the cancelability type to
PTHREAD_CANCEL_ASYNCHRONOUS is rarely useful. Since
the thread could be canceled at any time, it cannot
safely reserve resources (e.g., allocating memory with
malloc(3)), acquire mutexes, semaphores, or locks,
and so on. Reserving resources is unsafe because the
application has no way of knowing what the state of these
resources is when the thread is canceled; that is, did
cancellation occur before the resources were reserved, while
they were reserved, or after they were released?
Furthermore, some internal data structures (e.g., the linked
list of free blocks managed by the malloc(3) family
of functions) may be left in an inconsistent state if
cancellation occurs in the middle of the function call.
Consequently, clean-up handlers cease to be useful.
Functions that can be safely asynchronously canceled are called async-cancel-safe functions. POSIX.1-2001 requires only that pthread_cancel(3), pthread_setcancelstate(), and pthread_setcanceltype() be async-cancel-safe. In general, other library functions can’t be safely called from an asynchronously cancelable thread.
One of the few circumstances in which asynchronous cancelability is useful is for cancellation of a thread that is in a pure compute-bound loop.
Portability
notes
The Linux threading implementations permit the
oldstate argument of pthread_setcancelstate()
to be NULL, in which case the information about the previous
cancelability state is not returned to the caller. Many
other implementations also permit a NULL oldstat
argument, but POSIX.1-2001 does not specify this point, so
portable applications should always specify a non-NULL value
in oldstate. A precisely analogous set of statements
applies for the oldtype argument of
pthread_setcanceltype().
EXAMPLE
See pthread_cancel(3).
SEE ALSO
pthread_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3), pthreads(7)
COLOPHON
This page is part of release 3.69 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man−pages/.
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