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PTHREAD_CLEANUP_POP(P)	   POSIX Programmer's Manual	PTHREAD_CLEANUP_POP(P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       pthread_cleanup_pop, pthread_cleanup_push - establish cancellation han-
       dlers

SYNOPSIS
       #include <pthread.h>

       void pthread_cleanup_pop(int execute);
       void pthread_cleanup_push(void (*routine)(void*), void *arg);


DESCRIPTION
       The  pthread_cleanup_pop() function shall remove the routine at the top
       of the calling  thread's	 cancellation  cleanup	stack  and  optionally
       invoke it (if execute is non-zero).

       The  pthread_cleanup_push() function shall push the specified cancella-
       tion cleanup handler routine onto  the  calling	thread's  cancellation
       cleanup	stack.	The  cancellation cleanup handler shall be popped from
       the cancellation cleanup stack and invoked with the argument arg when:

	* The thread exits (that is, calls pthread_exit()).


	* The thread acts upon a cancellation request.


	* The thread calls pthread_cleanup_pop() with a non-zero execute argu-
	  ment.


       These  functions	 may  be  implemented as macros. The application shall
       ensure that they appear as statements, and in  pairs  within  the  same
       lexical scope (that is, the pthread_cleanup_push() macro may be thought
       to  expand  to  a  token	 list  whose   first   token   is   '{'	  with
       pthread_cleanup_pop() expanding to a token list whose last token is the
       corresponding '}' ).

       The effect of calling longjmp() or siglongjmp() is undefined  if	 there
       have  been any calls to pthread_cleanup_push() or pthread_cleanup_pop()
       made without the matching call since the jump buffer  was  filled.  The
       effect  of calling longjmp() or siglongjmp() from inside a cancellation
       cleanup handler is also undefined  unless  the  jump  buffer  was  also
       filled in the cancellation cleanup handler.

RETURN VALUE
       The  pthread_cleanup_push()  and	 pthread_cleanup_pop() functions shall
       not return a value.

ERRORS
       No errors are defined.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES
       The following is an example using thread primitives to implement a can-
       celable, writers-priority read-write lock:


	      typedef struct {
		  pthread_mutex_t lock;
		  pthread_cond_t rcond,
		      wcond;
		  int lock_count; /* < 0 .. Held by writer. */
				  /* > 0 .. Held by lock_count readers. */
				  /* = 0 .. Held by nobody. */
		  int waiting_writers; /* Count of waiting writers. */
	      } rwlock;


	      void
	      waiting_reader_cleanup(void *arg)
	      {
		  rwlock *l;


		  l = (rwlock *) arg;
		  pthread_mutex_unlock(&l->lock);
	      }


	      void
	      lock_for_read(rwlock *l)
	      {
		  pthread_mutex_lock(&l->lock);
		  pthread_cleanup_push(waiting_reader_cleanup, l);
		  while ((l->lock_count < 0) && (l->waiting_writers != 0))
		      pthread_cond_wait(&l->rcond, &l->lock);
		  l->lock_count++;
		 /*
		  * Note the pthread_cleanup_pop executes
		  * waiting_reader_cleanup.
		  */
		  pthread_cleanup_pop(1);
	      }


	      void
	      release_read_lock(rwlock *l)
	      {
		  pthread_mutex_lock(&l->lock);
		  if (--l->lock_count == 0)
		      pthread_cond_signal(&l->wcond);
		  pthread_mutex_unlock(l);
	      }


	      void
	      waiting_writer_cleanup(void *arg)
	      {
		  rwlock *l;


		  l = (rwlock *) arg;
		  if ((--l->waiting_writers == 0) && (l->lock_count >= 0)) {
		     /*
		      * This only happens if we have been canceled.
		      */
		      pthread_cond_broadcast(&l->wcond);
	      }
		  pthread_mutex_unlock(&l->lock);
	      }


	      void
	      lock_for_write(rwlock *l)
	      {
		  pthread_mutex_lock(&l->lock);
		  l->waiting_writers++;
		  pthread_cleanup_push(waiting_writer_cleanup, l);
		  while (l->lock_count != 0)
		      pthread_cond_wait(&l->wcond, &l->lock);
		  l->lock_count = -1;
		 /*
		  * Note the pthread_cleanup_pop executes
		  * waiting_writer_cleanup.
		  */
		  pthread_cleanup_pop(1);
	      }


	      void
	      release_write_lock(rwlock *l)
	      {
		  pthread_mutex_lock(&l->lock);
		  l->lock_count = 0;
		  if (l->waiting_writers == 0)
		      pthread_cond_broadcast(&l->rcond)
		  else
		      pthread_cond_signal(&l->wcond);
		  pthread_mutex_unlock(&l->lock);
	      }


	      /*
	       * This function is called to initialize the read/write lock.
	       */
	      void
	      initialize_rwlock(rwlock *l)
	      {
		  pthread_mutex_init(&l->lock, pthread_mutexattr_default);
		  pthread_cond_init(&l->wcond, pthread_condattr_default);
		  pthread_cond_init(&l->rcond, pthread_condattr_default);
		  l->lock_count = 0;
		  l->waiting_writers = 0;
	      }


	      reader_thread()
	      {
		  lock_for_read(&lock);
		  pthread_cleanup_push(release_read_lock, &lock);
		 /*
		  * Thread has read lock.
		  */
		  pthread_cleanup_pop(1);
	      }


	      writer_thread()
	      {
		  lock_for_write(&lock);
		  pthread_cleanup_push(release_write_lock, &lock);
		 /*
		  * Thread has write lock.
		  */
	      pthread_cleanup_pop(1);
	      }

APPLICATION USAGE
       The  two	 routines  that	 push  and  pop cancellation cleanup handlers,
       pthread_cleanup_push() and pthread_cleanup_pop(), can be thought of  as
       left and right parentheses.  They always need to be matched.

RATIONALE
       The  restriction	 that  the two routines that push and pop cancellation
       cleanup	handlers,  pthread_cleanup_push()  and	pthread_cleanup_pop(),
       have  to appear in the same lexical scope allows for efficient macro or
       compiler implementations and efficient  storage	management.  A	sample
       implementation of these routines as macros might look like this:


	      #define pthread_cleanup_push(rtn,arg) { \
		  struct _pthread_handler_rec __cleanup_handler, **__head; \
		  __cleanup_handler.rtn = rtn; \
		  __cleanup_handler.arg = arg; \
		  (void) pthread_getspecific(_pthread_handler_key, &__head); \
		  __cleanup_handler.next = *__head; \
		  *__head = &__cleanup_handler;


	      #define pthread_cleanup_pop(ex) \
		  *__head = __cleanup_handler.next; \
		  if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \
	      }

       A  more ambitious implementation of these routines might do even better
       by allowing the compiler to note that the cancellation cleanup  handler
       is a constant and can be expanded inline.

       This  volume  of	 IEEE Std 1003.1-2001 currently leaves unspecified the
       effect of calling longjmp() from a signal handler executing in a	 POSIX
       System  Interfaces  function.  If an implementation wants to allow this
       and give the programmer reasonable behavior, the longjmp() function has
       to call all cancellation cleanup handlers that have been pushed but not
       popped since the time setjmp() was called.

       Consider a multi-threaded function called by a thread  that  uses  sig-
       nals.  If a signal were delivered to a signal handler during the opera-
       tion of qsort() and that handler were  to  call	longjmp()  (which,  in
       turn,  did  not	call  the  cancellation	 cleanup  handlers) the helper
       threads	created	 by  the  qsort()  function  would  not	 be  canceled.
       Instead,	 they  would  continue	to execute and write into the argument
       array even though the array might have been popped off the stack.

       Note that the specified cleanup handling mechanism is  especially  tied
       to  the	C  language and, while the requirement for a uniform mechanism
       for expressing cleanup is language-independent, the mechanism  used  in
       other  languages may be quite different. In addition, this mechanism is
       really only necessary due to the lack of a real exception mechanism  in
       the C language, which would be the ideal solution.

       There  is  no  notion  of  a  cancellation cleanup-safe function. If an
       application has no cancellation points in its signal  handlers,	blocks
       any  signal  whose  handler  may have cancellation points while calling
       async-unsafe functions, or disables cancellation while  calling	async-
       unsafe  functions, all functions may be safely called from cancellation
       cleanup routines.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_cancel() , pthread_setcancelstate() , the Base Definitions vol-
       ume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003	by  the	 Institute  of
       Electrical  and	Electronics  Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained	online
       at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group		     2003		PTHREAD_CLEANUP_POP(P)
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