# pthread_cleanup_pop

## 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.
    


* * *