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<TITLE>PTHREAD_SPECIFIC(3) manual page</TITLE>
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
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<P><A HREF="index.html">Reference Index</A></P>
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<P><A HREF="#toc">Table of Contents</A></P>
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<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
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<P>pthread_key_create, pthread_key_delete, pthread_setspecific,
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pthread_getspecific - management of thread-specific data
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</P>
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<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
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<P><B>#include <pthread.h></B>
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</P>
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<P><B>int pthread_key_create(pthread_key_t *</B><I>key</I><B>, void
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(*</B><I>destr_function</I><B>) (void *));</B>
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</P>
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<P><B>int pthread_key_delete(pthread_key_t </B><I>key</I><B>);</B>
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</P>
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<P><B>int pthread_setspecific(pthread_key_t </B><I>key</I><B>, const
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void *</B><I>pointer</I><B>);</B>
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</P>
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<P><B>void * pthread_getspecific(pthread_key_t </B><I>key</I><B>);</B>
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</P>
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<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
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<P>Programs often need global or static variables that have different
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values in different threads. Since threads share one memory space,
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this cannot be achieved with regular variables. Thread-specific data
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is the POSIX threads answer to this need.
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</P>
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<P>Each thread possesses a private memory block, the thread-specific
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data area, or TSD area for short. This area is indexed by TSD keys.
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The TSD area associates values of type <B>void *</B> to TSD keys. TSD
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keys are common to all threads, but the value associated with a given
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TSD key can be different in each thread.
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</P>
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<P>For concreteness, the TSD areas can be viewed as arrays of <B>void
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*</B> pointers, TSD keys as integer indices into these arrays, and
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the value of a TSD key as the value of the corresponding array
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element in the calling thread.
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</P>
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<P>When a thread is created, its TSD area initially associates <B>NULL</B>
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with all keys.
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</P>
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<P><B>pthread_key_create</B> allocates a new TSD key. The key is
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stored in the location pointed to by <I>key</I>. There is a limit of
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<B>PTHREAD_KEYS_MAX</B> on the number of keys allocated at a given
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time. The value initially associated with the returned key is <B>NULL</B>
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in all currently executing threads.
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</P>
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<P>The <I>destr_function</I> argument, if not <B>NULL</B>, specifies
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a destructor function associated with the key. When a thread
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terminates via <B>pthread_exit</B> or by cancellation, <I>destr_function</I>
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is called with arguments the value associated with the key in that
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thread. The <I>destr_function</I> is not called if that value is <B>NULL</B><SPAN STYLE="font-weight: medium">
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or the key has been deleted</SPAN>. The order in which destructor
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functions are called at thread termination time is unspecified.
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</P>
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<P>Before the destructor function is called, the <B>NULL</B> value is
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associated with the key in the current thread. A destructor function
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might, however, re-associate non- <B>NULL</B> values to that key or
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some other key. To deal with this, if after all the destructors have
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been called for all non- <B>NULL</B> values, there are still some
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non- <B>NULL</B> values with associated destructors, then the process
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is repeated.</P>
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<P><B>pthread_key_delete</B> deallocates a TSD key. It does not check
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whether non- <B>NULL</B> values are associated with that key in the
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currently executing threads, nor call the destructor function
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associated with the key.
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</P>
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<P><B>pthread_setspecific</B> changes the value associated with <I>key</I>
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in the calling thread, storing the given <I>pointer</I> instead.
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</P>
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<P><B>pthread_getspecific</B> returns the value currently associated
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with <I>key</I> in the calling thread.
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</P>
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<P>The routines <B>pthread_setspecific</B>, <B>pthread_getspecific</B>,
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and <B>pthread_key_delete</B> can be called from <I>destr_function</I>
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targeting any valid key including the key on which <I>destr_function</I>
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is currently operating. If <B>pthread_getspecific</B> is called on
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the key whose thread specific data is being destroyed, the value NULL
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is returned, unless <B>pthread_setspecific</B> was called previously
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on that key from within <I>destr_function</I> to set the value to
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non-NULL. For some implementations the effect of calling
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<B>pthread_setspecific</B> from within <I>destr_function</I> can be
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either memory leakage or infinite loops if <I>destr_function</I> has
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already been called at least <B>PTHREAD_DESTRUCTOR_ITERATIONS</B>
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times.</P>
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<P STYLE="font-weight: medium"><B>Pthreads-w32</B> stops running key
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<I>destr_function</I> routines after <B>PTHREAD_DESTRUCTOR_ITERATIONS</B>
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iterations, even if some non- <B>NULL</B> values with associated
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descriptors remain. If memory is allocated and associated with a key
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from within <I>destr_function</I>, that memory may not be reclaimed
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because that key's <I>destr_function</I>, may not run again.</P>
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<H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2>
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<P><B>pthread_key_create</B>, <B>pthread_key_delete</B>, and
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<B>pthread_setspecific</B> return 0 on success and a non-zero error
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code on failure. If successful, <B>pthread_key_create</B> stores the
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newly allocated key in the location pointed to by its <I>key</I>
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argument.
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</P>
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<P><B>pthread_getspecific</B> returns the value associated with <I>key</I>
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on success, and <B>NULL</B> on error.
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</P>
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<H2><A HREF="#toc4" NAME="sect4">Errors</A></H2>
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<P><B>pthread_key_create</B> returns the following error code on
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error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EAGAIN</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 5cm">
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<B>PTHREAD_KEYS_MAX</B> keys are already allocated
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</BLOCKQUOTE>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ENOMEM</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 5cm">
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Insufficient memory to allocate the key.
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</BLOCKQUOTE>
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<P><B>pthread_key_delete</B> and <B>pthread_setspecific</B> return
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the following error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EINVAL</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<I>key</I> is not a valid, allocated TSD key
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</DD></DL>
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</DL>
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<P>
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<B>pthread_getspecific</B> returns <B>NULL</B> if <I>key</I> is not a
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valid, allocated TSD key.
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</P>
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<H2><A HREF="#toc5" NAME="sect5">Author</A></H2>
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<P>Xavier Leroy <Xavier.Leroy@inria.fr>
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</P>
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<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
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<H2><A HREF="#toc6" NAME="sect6">See Also</A></H2>
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<P><A HREF="pthread_create.html">pthread_create(3)</A> ,
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<A HREF="pthread_exit.html">pthread_exit(3)</A> ,
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<A HREF="pthread_cancel.html">pthread_testcancel(3)</A> .
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</P>
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<H2><A HREF="#toc7" NAME="sect7">Example</A></H2>
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<P>The following code fragment allocates a thread-specific array of
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100 characters, with automatic reclamation at thread exit:
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</P>
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<BLOCKQUOTE><BR><BR>
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</BLOCKQUOTE>
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<PRE STYLE="margin-left: 1cm; margin-right: 1cm">/* Key for the thread-specific buffer */
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static pthread_key_t buffer_key;
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/* Once-only initialisation of the key */
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static pthread_once_t buffer_key_once = PTHREAD_ONCE_INIT;
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/* Allocate the thread-specific buffer */
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void buffer_alloc(void)
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{
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pthread_once(&buffer_key_once, buffer_key_alloc);
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pthread_setspecific(buffer_key, malloc(100));
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}
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/* Return the thread-specific buffer */
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char * get_buffer(void)
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{
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return (char *) pthread_getspecific(buffer_key);
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}
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/* Allocate the key */
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static void buffer_key_alloc()
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{
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pthread_key_create(&buffer_key, buffer_destroy);
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}
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/* Free the thread-specific buffer */
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static void buffer_destroy(void * buf)
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{
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free(buf);
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}</PRE>
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<HR>
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<BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"><A NAME="toc"></A>
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<B>Table of Contents</B></BLOCKQUOTE>
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<UL>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return
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Value</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Author</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">See
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Also</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE><A HREF="#sect7" NAME="toc7">Example</A>
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</BLOCKQUOTE>
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</UL>
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