321 lines
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321 lines
12 KiB
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<title>pthread_cleanup_pop</title>
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<basefont size="3"> <a name="pthread_cleanup_pop"></a> <a name="tag_03_514"></a><!-- pthread_cleanup_pop -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_514_01"></a>NAME</h4>
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<blockquote>pthread_cleanup_pop, pthread_cleanup_push - establish cancelation handlers</blockquote>
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<h4><a name="tag_03_514_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/pthread.h.html">pthread.h</a>><br>
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<br>
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void pthread_cleanup_pop(int</tt> <i>execute</i><tt>);<br>
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void pthread_cleanup_push(void (*</tt><i>routine</i><tt>)(void*), void *</tt><i>arg</i><tt>); <img src="../images/opt-end.gif"
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alt="[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_514_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>pthread_cleanup_pop</i>() function shall remove the routine at the top of the calling thread's cancelation cleanup stack
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and optionally invoke it (if <i>execute</i> is non-zero).</p>
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<p>The <i>pthread_cleanup_push</i>() function shall push the specified cancelation cleanup handler <i>routine</i> onto the calling
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thread's cancelation cleanup stack. The cancelation cleanup handler shall be popped from the cancelation cleanup stack and invoked
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with the argument <i>arg</i> when:</p>
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<ul>
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<li>
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<p>The thread exits (that is, calls <a href="../functions/pthread_exit.html"><i>pthread_exit</i>()</a>).</p>
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</li>
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<li>
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<p>The thread acts upon a cancelation request.</p>
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</li>
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<li>
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<p>The thread calls <i>pthread_cleanup_pop</i>() with a non-zero <i>execute</i> argument.</p>
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</li>
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</ul>
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<p>These functions may be implemented as macros. The application shall ensure that they appear as statements, and in pairs within
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the same lexical scope (that is, the <i>pthread_cleanup_push</i>() macro may be thought to expand to a token list whose first token
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is <tt>'{'</tt> with <i>pthread_cleanup_pop</i>() expanding to a token list whose last token is the corresponding <tt>'}'</tt>
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).</p>
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<p>The effect of calling <a href="../functions/longjmp.html"><i>longjmp</i>()</a> or <a href=
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"../functions/siglongjmp.html"><i>siglongjmp</i>()</a> is undefined if there have been any calls to <i>pthread_cleanup_push</i>()
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or <i>pthread_cleanup_pop</i>() made without the matching call since the jump buffer was filled. The effect of calling <a href=
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"../functions/longjmp.html"><i>longjmp</i>()</a> or <a href="../functions/siglongjmp.html"><i>siglongjmp</i>()</a> from inside a
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cancelation cleanup handler is also undefined unless the jump buffer was also filled in the cancelation cleanup handler.</p>
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</blockquote>
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<h4><a name="tag_03_514_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>The <i>pthread_cleanup_push</i>() and <i>pthread_cleanup_pop</i>() functions shall not return a value.</p>
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</blockquote>
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<h4><a name="tag_03_514_05"></a>ERRORS</h4>
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<blockquote>
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<p>No errors are defined.</p>
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<p>These functions shall not return an error code of [EINTR].</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_514_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>The following is an example using thread primitives to implement a cancelable, writers-priority read-write lock:</p>
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<pre>
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<tt>typedef struct {
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pthread_mutex_t lock;
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pthread_cond_t rcond,
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wcond;
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int lock_count; /* < 0 .. Held by writer. */
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/* > 0 .. Held by lock_count readers. */
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/* = 0 .. Held by nobody. */
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int waiting_writers; /* Count of waiting writers. */
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} rwlock;
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<br>
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void
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waiting_reader_cleanup(void *arg)
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{
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rwlock *l;
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<br>
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l = (rwlock *) arg;
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pthread_mutex_unlock(&l->lock);
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}
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<br>
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void
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lock_for_read(rwlock *l)
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{
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pthread_mutex_lock(&l->lock);
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pthread_cleanup_push(waiting_reader_cleanup, l);
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while ((l->lock_count < 0) && (l->waiting_writers != 0))
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pthread_cond_wait(&l->rcond, &l->lock);
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l->lock_count++;
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/*
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* Note the pthread_cleanup_pop executes
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* waiting_reader_cleanup.
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*/
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pthread_cleanup_pop(1);
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}
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<br>
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void
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release_read_lock(rwlock *l)
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{
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pthread_mutex_lock(&l->lock);
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if (--l->lock_count == 0)
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pthread_cond_signal(&l->wcond);
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pthread_mutex_unlock(l);
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}
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<br>
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void
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waiting_writer_cleanup(void *arg)
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{
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rwlock *l;
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<br>
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l = (rwlock *) arg;
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if ((--l->waiting_writers == 0) && (l->lock_count >= 0)) {
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/*
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* This only happens if we have been canceled.
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*/
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pthread_cond_broadcast(&l->wcond);
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}
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pthread_mutex_unlock(&l->lock);
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}
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<br>
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void
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lock_for_write(rwlock *l)
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{
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pthread_mutex_lock(&l->lock);
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l->waiting_writers++;
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pthread_cleanup_push(waiting_writer_cleanup, l);
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while (l->lock_count != 0)
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pthread_cond_wait(&l->wcond, &l->lock);
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l->lock_count = -1;
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/*
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* Note the pthread_cleanup_pop executes
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* waiting_writer_cleanup.
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*/
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pthread_cleanup_pop(1);
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}
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<br>
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void
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release_write_lock(rwlock *l)
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{
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pthread_mutex_lock(&l->lock);
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l->lock_count = 0;
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if (l->waiting_writers == 0)
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pthread_cond_broadcast(&l->rcond)
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else
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pthread_cond_signal(&l->wcond);
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pthread_mutex_unlock(&l->lock);
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}
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<br>
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/*
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* This function is called to initialize the read/write lock.
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*/
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void
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initialize_rwlock(rwlock *l)
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{
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pthread_mutex_init(&l->lock, pthread_mutexattr_default);
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pthread_cond_init(&l->wcond, pthread_condattr_default);
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pthread_cond_init(&l->rcond, pthread_condattr_default);
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l->lock_count = 0;
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l->waiting_writers = 0;
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}
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<br>
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reader_thread()
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{
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lock_for_read(&lock);
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pthread_cleanup_push(release_read_lock, &lock);
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/*
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* Thread has read lock.
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*/
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pthread_cleanup_pop(1);
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}
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<br>
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writer_thread()
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{
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lock_for_write(&lock);
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pthread_cleanup_push(release_write_lock, &lock);
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/*
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* Thread has write lock.
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*/
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pthread_cleanup_pop(1);
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}
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_514_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The two routines that push and pop cancelation cleanup handlers, <i>pthread_cleanup_push</i>() and <i>pthread_cleanup_pop</i>(),
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can be thought of as left and right parentheses. They always need to be matched.</p>
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</blockquote>
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<h4><a name="tag_03_514_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The restriction that the two routines that push and pop cancelation cleanup handlers, <i>pthread_cleanup_push</i>() and
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<i>pthread_cleanup_pop</i>(), have to appear in the same lexical scope allows for efficient macro or compiler implementations and
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efficient storage management. A sample implementation of these routines as macros might look like this:</p>
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<pre>
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<tt>#define pthread_cleanup_push(rtn,arg) { \
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struct _pthread_handler_rec __cleanup_handler, **__head; \
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__cleanup_handler.rtn = rtn; \
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__cleanup_handler.arg = arg; \
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(void) pthread_getspecific(_pthread_handler_key, &__head); \
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__cleanup_handler.next = *__head; \
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*__head = &__cleanup_handler;
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<br>
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#define pthread_cleanup_pop(ex) \
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*__head = __cleanup_handler.next; \
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if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \
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}
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</tt>
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</pre>
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<p>A more ambitious implementation of these routines might do even better by allowing the compiler to note that the cancelation
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cleanup handler is a constant and can be expanded inline.</p>
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<p>This volume of IEEE Std 1003.1-2001 currently leaves unspecified the effect of calling <a href=
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"../functions/longjmp.html"><i>longjmp</i>()</a> from a signal handler executing in a POSIX System Interfaces function. If an
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implementation wants to allow this and give the programmer reasonable behavior, the <a href=
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"../functions/longjmp.html"><i>longjmp</i>()</a> function has to call all cancelation cleanup handlers that have been pushed but
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not popped since the time <a href="../functions/setjmp.html"><i>setjmp</i>()</a> was called.</p>
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<p>Consider a multi-threaded function called by a thread that uses signals. If a signal were delivered to a signal handler during
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the operation of <a href="../functions/qsort.html"><i>qsort</i>()</a> and that handler were to call <a href=
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"../functions/longjmp.html"><i>longjmp</i>()</a> (which, in turn, did <i>not</i> call the cancelation cleanup handlers) the helper
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threads created by the <a href="../functions/qsort.html"><i>qsort</i>()</a> function would not be canceled. Instead, they would
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continue to execute and write into the argument array even though the array might have been popped off the stack.</p>
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<p>Note that the specified cleanup handling mechanism is especially tied to the C language and, while the requirement for a uniform
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mechanism for expressing cleanup is language-independent, the mechanism used in other languages may be quite different. In
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addition, this mechanism is really only necessary due to the lack of a real exception mechanism in the C language, which would be
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the ideal solution.</p>
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<p>There is no notion of a cancelation cleanup-safe function. If an application has no cancelation points in its signal handlers,
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blocks any signal whose handler may have cancelation points while calling async-unsafe functions, or disables cancelation while
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calling async-unsafe functions, all functions may be safely called from cancelation cleanup routines.</p>
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</blockquote>
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<h4><a name="tag_03_514_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_514_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="pthread_cancel.html"><i>pthread_cancel</i>()</a> , <a href=
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"pthread_setcancelstate.html"><i>pthread_setcancelstate</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001,
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<a href="../basedefs/pthread.h.html"><i><pthread.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_514_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p>
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</blockquote>
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<h4><a name="tag_03_514_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>pthread_cleanup_pop</i>() and <i>pthread_cleanup_push</i>() functions are marked as part of the Threads option.</p>
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<p>The APPLICATION USAGE section is added.</p>
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<p>The DESCRIPTION is updated to avoid use of the term "must" for application requirements.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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