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142 lines
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HTML
142 lines
6.7 KiB
HTML
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<head>
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<title>Thread-Local (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Thread-Local (Using the GNU Compiler Collection (GCC))">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="C99-Thread_002dLocal-Edits.html#C99-Thread_002dLocal-Edits" rel="next" title="C99 Thread-Local Edits">
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<a name="Thread_002dLocal"></a>
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<div class="header">
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<p>
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Next: <a href="Binary-constants.html#Binary-constants" accesskey="n" rel="next">Binary constants</a>, Previous: <a href="Unnamed-Fields.html#Unnamed-Fields" accesskey="p" rel="prev">Unnamed Fields</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Thread_002dLocal-Storage"></a>
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<h3 class="section">6.63 Thread-Local Storage</h3>
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<a name="index-Thread_002dLocal-Storage"></a>
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<a name="index-TLS"></a>
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<a name="index-_005f_005fthread"></a>
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<p>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 (<code>ld</code>), dynamic linker (<code>ld.so</code>), and
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system libraries (<samp>libc.so</samp> and <samp>libpthread.so</samp>), so it
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is not available everywhere.
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</p>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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>
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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>
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<p>No static initialization may refer to the address of a thread-local variable.
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</p>
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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>
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<p>See <a href="https://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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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="C99-Thread_002dLocal-Edits.html#C99-Thread_002dLocal-Edits" accesskey="1">C99 Thread-Local Edits</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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<tr><td align="left" valign="top">• <a href="C_002b_002b98-Thread_002dLocal-Edits.html#C_002b_002b98-Thread_002dLocal-Edits" accesskey="2">C++98 Thread-Local Edits</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Binary-constants.html#Binary-constants" accesskey="n" rel="next">Binary constants</a>, Previous: <a href="Unnamed-Fields.html#Unnamed-Fields" accesskey="p" rel="prev">Unnamed Fields</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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