PTHREAD_SETCANCELSTATE(3) Linux Programmer's Manual PTHREAD_SETCANCELSTATE(3)
pthread_setcancelstate, pthread_setcanceltype - set cancelability state and type
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 immedi-
ately upon receiving a cancellation request, but the system doesn't guarantee
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.
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? Further-
more, 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 func-
tions. 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.
The Linux threading implementations permit the oldstate argument of pthread_setcancel-
state() 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().
pthread_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3), pthreads(7)
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Linux 2008-11-24 PTHREAD_SETCANCELSTATE(3)