pthread_setcancelstate(3) Library Functions Manual pthread_setcancelstate(3)
NAME
pthread_setcancelstate, pthread_setcanceltype - set cancelability state
and type
LIBRARY
POSIX threads library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
int pthread_setcancelstate(int state, int *oldstate);
int pthread_setcanceltype(int type, int *oldtype);
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 cancelation request.
PTHREAD_CANCEL_DISABLE
The thread is not cancelable. If a cancelation request is re-
ceived, 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 cancelation request is deferred until the thread next calls a
function that is a cancelation point (see pthreads(7)). This is
the default cancelability type in all new threads, including the
initial thread.
Even with deferred cancelation, a cancelation point in an asyn-
chronous signal handler may still be acted upon and the effect is
as if it was an asynchronous cancelation.
PTHREAD_CANCEL_ASYNCHRONOUS
The thread can be canceled at any time. (Typically, it will be
canceled immediately upon receiving a cancelation 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(), │ Thread safety │ MT-Safe │
│ pthread_setcanceltype() │ │ │
├──────────────────────────────────────┼─────────────────────┼─────────┤
│ pthread_setcancelstate(), │ Async-cancel safety │ AC-Safe │
│ pthread_setcanceltype() │ │ │
└──────────────────────────────────────┴─────────────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
glibc 2.0 POSIX.1-2001.
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 cancelation request.
Beware of disabling cancelability for long periods, or around operations
that may block for long periods, since that will render the thread unre-
sponsive to cancelation 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)), ac-
quire 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 cancelation
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 cancelation 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 use-
ful is for cancelation 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().
EXAMPLES
See pthread_cancel(3).
SEE ALSO
pthread_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3),
pthreads(7)
Linux man-pages 6.9.1 2024-05-02 pthread_setcancelstate(3)
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