<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8"> <TITLE>PTHREAD_CLEANUP(3) manual page</TITLE> <META NAME="GENERATOR" CONTENT="OpenOffice.org 1.1.3 (Linux)"> <META NAME="CREATED" CONTENT="20050504;20152200"> <META NAME="CHANGED" CONTENT="20050505;18252600"> <!-- manual page source format generated by PolyglotMan v3.2, --> <!-- available at http://polyglotman.sourceforge.net/ --> </HEAD> <BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR"> <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> </BODY> </HTML>