pthread_cleanup_pop(3p) — Linux manual page

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PTHREAD_CLEANUP_POP(3P) POSIX Programmer's ManualPTHREAD_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.

       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         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.1‐2017 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         top

       None.

SEE ALSO         top

       pthread_cancel(3p), pthread_setcancelstate(3p)

       The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)

COPYRIGHT         top

       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(3P)

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