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| author | marha <marha@users.sourceforge.net> | 2009-06-28 22:07:26 +0000 |
|---|---|---|
| committer | marha <marha@users.sourceforge.net> | 2009-06-28 22:07:26 +0000 |
| commit | 3562e78743202e43aec8727005182a2558117eca (patch) | |
| tree | 8f9113a77d12470c5c851a2a8e4cb02e89df7d43 /pthreads/manual/pthread_key_create.html | |
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diff --git a/pthreads/manual/pthread_key_create.html b/pthreads/manual/pthread_key_create.html new file mode 100644 index 000000000..5ecca6995 --- /dev/null +++ b/pthreads/manual/pthread_key_create.html @@ -0,0 +1,211 @@ +<!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>
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