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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) ; |
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Note |
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Compile and link with |
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.
The pthread_setcancelstate
()
can fail with the following error:
Invalid value for state
.
The pthread_setcanceltype
()
can fail with the following error:
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? 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.
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
().