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Linux 2.6 - man page for pthread_setcanceltype (linux section 3)


       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  immedi-
	      ately  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?  Further-
       more, 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 func-
       tions.  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_setcancel-
       state()	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)

       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2008-11-24			PTHREAD_SETCANCELSTATE(3)

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