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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 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.

       It  is  unspecified whether pthread_cleanup_push() and pthread_cleanup_pop() are macros or
       functions. If a macro definition is suppressed in order to access an actual function, or a
       program defines an external identifier with any of these names, the behavior is undefined.
       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.

       The  effect  of  the  use of return, break, continue, and goto to prematurely leave a code
       block described by a pair of pthread_cleanup_push()  and  pthread_cleanup_pop()  functions
       calls is undefined.

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->lock);
           }

           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. If the
                   * lock is not held by a writer, there may be readers who
                   * were blocked because waiting_writers was positive; they
                   * can now be unblocked.
                   */
                   pthread_cond_broadcast(&l->rcond);
               }
               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 POSIX.1‐2017 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 POSIX.1‐2017, <pthread.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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 .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .