<!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_SPECIFIC(3) manual page</TITLE> <META NAME="GENERATOR" CONTENT="OpenOffice.org 1.1.3 (Linux)"> <META NAME="CREATED" CONTENT="20050504;18425400"> <META NAME="CHANGED" CONTENT="20050509;18220200"> <!-- 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_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> </BODY> </HTML>