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<TITLE>PTHREAD_CLEANUP(3) manual page</TITLE>
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<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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