PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       pthread_cleanup_pop, pthread_cleanup_push — establish cancellation
       handlers

SYNOPSIS         top

       #include <pthread.h>

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

DESCRIPTION         top

       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.

       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         top

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

ERRORS         top

       No errors are defined.

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

       The following sections are informative.

EXAMPLES         top

       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         top

       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         top

       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.1‐2008 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         top

       None.

SEE ALSO         top

       pthread_cancel(3p), pthread_setcancelstate(3p)

       The Base Definitions volume of POSIX.1‐2008, pthread.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013             PTHREAD_CLEANUP_POP(3P)

Pages that refer to this page: pthread.h(0p)