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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);

       Compile and link with -pthread.


       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:

              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.

              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:

              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.

              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

       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.


       On success, these functions return 0; on error, they return a nonzero error number.


       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.




       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().


       See pthread_cancel(3).


       pthread_cancel(3), pthread_cleanup_push(3), pthread_testcancel(3), pthreads(7)


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