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