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| <H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4> | |
| <P><A HREF="index.html">Reference Index</A></P> | |
| <P><A HREF="#toc">Table of Contents</A></P> | |
| <H2><A HREF="#toc0" NAME="sect0">Name</A></H2> | |
| <P>pthread_cleanup_push, pthread_cleanup_pop - install and remove | |
| cleanup handlers | |
| </P> | |
| <H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2> | |
| <P><B>#include <pthread.h></B> | |
| </P> | |
| <P><B>void pthread_cleanup_push(void (*</B><I>routine</I><B>) (void | |
| *), void *</B><I>arg</I><B>);</B> | |
| </P> | |
| <P><B>void pthread_cleanup_pop(int </B><I>execute</I><B>);</B> | |
| </P> | |
| <H2><A HREF="#toc2" NAME="sect2">Description</A></H2> | |
| <P>Cleanup handlers are functions that get called when a thread | |
| terminates, either by calling <A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> | |
| or because of cancellation. Cleanup handlers are installed and | |
| removed following a stack-like discipline. | |
| </P> | |
| <P>The purpose of cleanup handlers is to free the resources that a | |
| thread may hold at the time it terminates. In particular, if a thread | |
| exits or is cancelled while it owns a locked mutex, the mutex will | |
| remain locked forever and prevent other threads from executing | |
| normally. The best way to avoid this is, just before locking the | |
| mutex, to install a cleanup handler whose effect is to unlock the | |
| mutex. Cleanup handlers can be used similarly to free blocks | |
| allocated with <A HREF="malloc.html"><B>malloc</B>(3)</A> or close | |
| file descriptors on thread termination. | |
| </P> | |
| <P><B>pthread_cleanup_push</B> installs the <I>routine</I> function | |
| with argument <I>arg</I> as a cleanup handler. From this point on to | |
| the matching <B>pthread_cleanup_pop</B>, the function <I>routine</I> | |
| will be called with arguments <I>arg</I> when the thread terminates, | |
| either through <A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> | |
| or by cancellation. If several cleanup handlers are active at that | |
| point, they are called in LIFO order: the most recently installed | |
| handler is called first. | |
| </P> | |
| <P><B>pthread_cleanup_pop</B> removes the most recently installed | |
| cleanup handler. If the <I>execute</I> argument is not 0, it also | |
| executes the handler, by calling the <I>routine</I> function with | |
| arguments <I>arg</I>. If the <I>execute</I> argument is 0, the | |
| handler is only removed but not executed. | |
| </P> | |
| <P>Matching pairs of <B>pthread_cleanup_push</B> and | |
| <B>pthread_cleanup_pop</B> must occur in the same function, at the | |
| same level of block nesting. Actually, <B>pthread_cleanup_push</B> | |
| and <B>pthread_cleanup_pop</B> are macros, and the expansion of | |
| <B>pthread_cleanup_push</B> introduces an open brace <B>{</B> with | |
| the matching closing brace <B>}</B> being introduced by the expansion | |
| of the matching <B>pthread_cleanup_pop</B>. | |
| </P> | |
| <H2 STYLE="margin-top: 0cm"><A HREF="#toc3" NAME="sect3">Return Value</A></H2> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">None. | |
| </BLOCKQUOTE> | |
| <H2 STYLE="margin-top: 0cm"><A HREF="#toc4" NAME="sect4">Errors</A></H2> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">None. | |
| </BLOCKQUOTE> | |
| <H2 STYLE="margin-top: 0cm"><A HREF="#toc5" NAME="sect5">Author</A></H2> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">Xavier Leroy | |
| <Xavier.Leroy@inria.fr> | |
| </BLOCKQUOTE> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">Modified by | |
| Ross Johnson for use with Pthreads-w32.</BLOCKQUOTE> | |
| <H2 STYLE="margin-top: 0cm"><A HREF="#toc6" NAME="sect6">See Also</A></H2> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"><A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> | |
| , <A HREF="pthread_cancel.html"><B>pthread_cancel</B>(3)</A> , | |
| <A HREF="pthread_cancel.html"><B>pthread_setcanceltype</B>(3)</A> . | |
| </BLOCKQUOTE> | |
| <H2 STYLE="margin-top: 0cm"><A HREF="#toc7" NAME="sect7">Example</A></H2> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">Here is how | |
| to lock a mutex <I>mut</I> in such a way that it will be unlocked if | |
| the thread is canceled while <I>mut</I> is locked: | |
| </BLOCKQUOTE> | |
| <PRE>pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut); | |
| pthread_mutex_lock(&mut); | |
| /* do some work */ | |
| pthread_mutex_unlock(&mut); | |
| pthread_cleanup_pop(0);</PRE><BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"> | |
| Equivalently, the last two lines can be replaced by | |
| </BLOCKQUOTE> | |
| <PRE STYLE="margin-bottom: 0.5cm">pthread_cleanup_pop(1);</PRE><BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"> | |
| Notice that the code above is safe only in deferred cancellation mode | |
| (see <A HREF="pthread_cancel.html"><B>pthread_setcanceltype</B>(3)</A> | |
| ). In asynchronous cancellation mode, a cancellation can occur | |
| between <B>pthread_cleanup_push</B> and <B>pthread_mutex_lock</B>, or | |
| between <B>pthread_mutex_unlock</B> and <B>pthread_cleanup_pop</B>, | |
| resulting in both cases in the thread trying to unlock a mutex not | |
| locked by the current thread. This is the main reason why | |
| asynchronous cancellation is difficult to use. | |
| </BLOCKQUOTE> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm">If the code | |
| above must also work in asynchronous cancellation mode, then it must | |
| switch to deferred mode for locking and unlocking the mutex: | |
| </BLOCKQUOTE> | |
| <PRE>pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype); | |
| pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut); | |
| pthread_mutex_lock(&mut); | |
| /* do some work */ | |
| pthread_cleanup_pop(1); | |
| pthread_setcanceltype(oldtype, NULL);</PRE> | |
| <HR> | |
| <BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"><A NAME="toc"></A> | |
| <B>Table of Contents</B></BLOCKQUOTE> | |
| <UL> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return | |
| Value</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Author</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">See | |
| Also</A> | |
| </BLOCKQUOTE> | |
| <LI><BLOCKQUOTE STYLE="margin-right: 0cm"><A HREF="#sect7" NAME="toc7">Example</A> | |
| </BLOCKQUOTE> | |
| </UL> | |
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