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PTHREAD_SETCANCELSTATE(3) Linux Programmer's Manual PTHREAD_SETCANCELSTATE(3)
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 call-
ing 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_can-
cel(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 opera-
tions 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 cancel-
lation 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 mal-
loc(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_can-
cel(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 analo-
gous set of statements applies for the oldtype argument of pthread_set-
canceltype().
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/.
Linux 2014-05-13 PTHREAD_SETCANCELSTATE(3)
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