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); |
![[Note]](../stylesheet/note.png) |
Note |
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Compile and link with |
DESCRIPTION
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.
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
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
pthread_setcancelstate(),
pthread_setcanceltype() |
Thread safety | MT-Safe |
pthread_setcancelstate(),
pthread_setcanceltype() |
Async-cancel-safety | AC-Safe |
NOTES
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.
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 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 and POSIX.1-2008
require 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.
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 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().
COLOPHON
This page is part of release 4.07 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
https://www.kernel.org/doc/man−pages/.
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