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107 lines
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107 lines
5.0 KiB
HTML
4 years ago
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<title>Thread-Local - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Unnamed-Fields.html#Unnamed-Fields" title="Unnamed Fields">
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<link rel="next" href="Binary-constants.html#Binary-constants" title="Binary constants">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Thread-Local"></a>
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<a name="Thread_002dLocal"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Binary-constants.html#Binary-constants">Binary constants</a>,
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Previous: <a rel="previous" accesskey="p" href="Unnamed-Fields.html#Unnamed-Fields">Unnamed Fields</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.62 Thread-Local Storage</h3>
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<p><a name="index-Thread_002dLocal-Storage-4193"></a><a name="index-g_t_0040acronym_007bTLS_007d-4194"></a><a name="index-g_t_0040code_007b_005f_005fthread_007d-4195"></a>
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Thread-local storage (<acronym>TLS</acronym>) is a mechanism by which variables
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are allocated such that there is one instance of the variable per extant
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thread. The runtime model GCC uses to implement this originates
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in the IA-64 processor-specific ABI, but has since been migrated
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to other processors as well. It requires significant support from
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the linker (<samp><span class="command">ld</span></samp>), dynamic linker (<samp><span class="command">ld.so</span></samp>), and
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system libraries (<samp><span class="file">libc.so</span></samp> and <samp><span class="file">libpthread.so</span></samp>), so it
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is not available everywhere.
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<p>At the user level, the extension is visible with a new storage
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class keyword: <code>__thread</code>. For example:
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<pre class="smallexample"> __thread int i;
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extern __thread struct state s;
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static __thread char *p;
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</pre>
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<p>The <code>__thread</code> specifier may be used alone, with the <code>extern</code>
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or <code>static</code> specifiers, but with no other storage class specifier.
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When used with <code>extern</code> or <code>static</code>, <code>__thread</code> must appear
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immediately after the other storage class specifier.
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<p>The <code>__thread</code> specifier may be applied to any global, file-scoped
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static, function-scoped static, or static data member of a class. It may
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not be applied to block-scoped automatic or non-static data member.
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<p>When the address-of operator is applied to a thread-local variable, it is
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evaluated at run time and returns the address of the current thread's
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instance of that variable. An address so obtained may be used by any
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thread. When a thread terminates, any pointers to thread-local variables
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in that thread become invalid.
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<p>No static initialization may refer to the address of a thread-local variable.
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<p>In C++, if an initializer is present for a thread-local variable, it must
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be a <var>constant-expression</var>, as defined in 5.19.2 of the ANSI/ISO C++
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standard.
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<p>See <a href="http://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local Storage</a> for a detailed explanation of
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the four thread-local storage addressing models, and how the runtime
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is expected to function.
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<ul class="menu">
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<li><a accesskey="1" href="C99-Thread_002dLocal-Edits.html#C99-Thread_002dLocal-Edits">C99 Thread-Local Edits</a>
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<li><a accesskey="2" href="C_002b_002b98-Thread_002dLocal-Edits.html#C_002b_002b98-Thread_002dLocal-Edits">C++98 Thread-Local Edits</a>
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</ul>
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</body></html>
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