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PTHREAD_SETCANCELSTATE(3) Linux Programmer's Manual PTHREAD_SETCANCELSTATE(3)
pthread_setcancelstate, pthread_setcanceltype - set cancelability state and
type
#include <pthread.h>
int pthread_setcancelstate(int state, int *oldstate);
int pthread_setcanceltype(int type, int *oldtype);
Compile and link with -pthread.
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.
On success, these functions return 0; on error, they return a nonzero error
number.
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.
POSIX.1-2001.
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.
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 only requires 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.
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().
See pthread_cancel(3).
pthread_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3), pthreads(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-11-24 PTHREAD_SETCANCELSTATE(3)
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