PTHREAD_SETCANCELSTATE
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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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);
Compile and link with -pthread.
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.
RETURN VALUE
On success, these functions return 0;
on error, they return a nonzero error number.
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
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
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().
EXAMPLE
See
pthread_cancel(3).
SEE ALSO
pthread_cancel(3),
pthread_cleanup_push(3),
pthread_testcancel(3),
pthreads(7)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- Asynchronous cancelability
-
- Portability notes
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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