PTHREAD_CLEANUP_POP(3)     POSIX Programmer's Manual    PTHREAD_CLEANUP_POP(3)



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

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 .



IEEE/The Open Group                  2017               PTHREAD_CLEANUP_POP(3)
