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.

CONFORMING TO

       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 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().

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 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2008-11-24                          PTHREAD_SETCANCELSTATE(3)