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NAME

       pthread_cleanup_pop,   pthread_cleanup_push  -  establish  cancellation
       handlers

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
       cancellation   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
          argument.

       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
       cancelable, 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
       signals.   If  a  signal  were delivered to a signal handler during the
       operation 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
       volume 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 .