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pthread_setcancelstate, pthread_setcanceltype - set cancelability state
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:
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.
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:
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.
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
On success, these functions return 0; on error, they return a nonzero
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.
For details of what happens when a thread is canceled, see
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_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3),
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