1<html lang="en"> 2<head> 3<title>Thread-Local - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Unnamed-Fields.html#Unnamed-Fields" title="Unnamed Fields"> 10<link rel="next" href="Binary-constants.html#Binary-constants" title="Binary constants"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). 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The run-time model GCC uses to implement this originates 65in the IA-64 processor-specific ABI, but has since been migrated 66to other processors as well. It requires significant support from 67the linker (<samp><span class="command">ld</span></samp>), dynamic linker (<samp><span class="command">ld.so</span></samp>), and 68system libraries (<samp><span class="file">libc.so</span></samp> and <samp><span class="file">libpthread.so</span></samp>), so it 69is not available everywhere. 70 71 <p>At the user level, the extension is visible with a new storage 72class keyword: <code>__thread</code>. For example: 73 74<pre class="smallexample"> __thread int i; 75 extern __thread struct state s; 76 static __thread char *p; 77</pre> 78 <p>The <code>__thread</code> specifier may be used alone, with the <code>extern</code> 79or <code>static</code> specifiers, but with no other storage class specifier. 80When used with <code>extern</code> or <code>static</code>, <code>__thread</code> must appear 81immediately after the other storage class specifier. 82 83 <p>The <code>__thread</code> specifier may be applied to any global, file-scoped 84static, function-scoped static, or static data member of a class. It may 85not be applied to block-scoped automatic or non-static data member. 86 87 <p>When the address-of operator is applied to a thread-local variable, it is 88evaluated at run-time and returns the address of the current thread's 89instance of that variable. An address so obtained may be used by any 90thread. When a thread terminates, any pointers to thread-local variables 91in that thread become invalid. 92 93 <p>No static initialization may refer to the address of a thread-local variable. 94 95 <p>In C++, if an initializer is present for a thread-local variable, it must 96be a <var>constant-expression</var>, as defined in 5.19.2 of the ANSI/ISO C++ 97standard. 98 99 <p>See <a href="http://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local Storage</a> for a detailed explanation of 100the four thread-local storage addressing models, and how the run-time 101is expected to function. 102 103<ul class="menu"> 104<li><a accesskey="1" href="C99-Thread_002dLocal-Edits.html#C99-Thread_002dLocal-Edits">C99 Thread-Local Edits</a> 105<li><a accesskey="2" href="C_002b_002b98-Thread_002dLocal-Edits.html#C_002b_002b98-Thread_002dLocal-Edits">C++98 Thread-Local Edits</a> 106</ul> 107 108 </body></html> 109 110