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 <a name="Thread_002dLocal"></a>
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 <p>
 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> &nbsp; [<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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 <hr>
 <a name="Thread_002dLocal-Storage"></a>
 <h3 class="section">6.61 Thread-Local Storage</h3>
 <a name="index-Thread_002dLocal-Storage"></a>
 <a name="index-TLS"></a>
 <a name="index-_005f_005fthread"></a>

 <p>Thread-local storage (<acronym>TLS</acronym>) is a mechanism by which variables
 are allocated such that there is one instance of the variable per extant
 thread.  The runtime model GCC uses to implement this originates
 in the IA-64 processor-specific ABI, but has since been migrated
 to other processors as well.  It requires significant support from
 the linker (<code>ld</code>), dynamic linker (<code>ld.so</code>), and
 system libraries (<samp>libc.so</samp> and <samp>libpthread.so</samp>), so it
 is not available everywhere.
 </p>
 <p>At the user level, the extension is visible with a new storage
 class keyword: <code>__thread</code>.  For example:
 </p>
 <div class="smallexample">
 <pre class="smallexample">__thread int i;
 extern __thread struct state s;
 static __thread char *p;
 </pre></div>

 <p>The <code>__thread</code> specifier may be used alone, with the <code>extern</code>
 or <code>static</code> specifiers, but with no other storage class specifier.
 When used with <code>extern</code> or <code>static</code>, <code>__thread</code> must appear
 immediately after the other storage class specifier.
 </p>
 <p>The <code>__thread</code> specifier may be applied to any global, file-scoped
 static, function-scoped static, or static data member of a class.  It may
 not be applied to block-scoped automatic or non-static data member.
 </p>
 <p>When the address-of operator is applied to a thread-local variable, it is
 evaluated at run time and returns the address of the current thread&rsquo;s
 instance of that variable.  An address so obtained may be used by any
 thread.  When a thread terminates, any pointers to thread-local variables
 in that thread become invalid.
 </p>
 <p>No static initialization may refer to the address of a thread-local variable.
 </p>
 <p>In C++, if an initializer is present for a thread-local variable, it must
 be a <var>constant-expression</var>, as defined in 5.19.2 of the ANSI/ISO C++
 standard.
 </p>
 <p>See <a href="http://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local Storage</a> for a detailed explanation of
 the four thread-local storage addressing models, and how the runtime
 is expected to function.
 </p>
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 <tr><td align="left" valign="top">&bull; <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>&nbsp;&nbsp;</td><td align="left" valign="top">
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	<a name="Thread_002dLocal"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="Thread_002dLocal-Storage"></a>
	<h3 class="section">6.61 Thread-Local Storage</h3>
	<a name="index-Thread_002dLocal-Storage"></a>
	<a name="index-TLS"></a>
	<a name="index-_005f_005fthread"></a>

	<p>Thread-local storage (<acronym>TLS</acronym>) is a mechanism by which variables
	are allocated such that there is one instance of the variable per extant
	thread. The runtime model GCC uses to implement this originates
	in the IA-64 processor-specific ABI, but has since been migrated
	to other processors as well. It requires significant support from
	the linker (<code>ld</code>), dynamic linker (<code>ld.so</code>), and
	system libraries (<samp>libc.so</samp> and <samp>libpthread.so</samp>), so it
	is not available everywhere.
	</p>
	<p>At the user level, the extension is visible with a new storage
	class keyword: <code>__thread</code>. For example:
	</p>
	<div class="smallexample">
	<pre class="smallexample">__thread int i;
	extern __thread struct state s;
	static __thread char *p;
	</pre></div>

	<p>The <code>__thread</code> specifier may be used alone, with the <code>extern</code>
	or <code>static</code> specifiers, but with no other storage class specifier.
	When used with <code>extern</code> or <code>static</code>, <code>__thread</code> must appear
	immediately after the other storage class specifier.
	</p>
	<p>The <code>__thread</code> specifier may be applied to any global, file-scoped
	static, function-scoped static, or static data member of a class. It may
	not be applied to block-scoped automatic or non-static data member.
	</p>
	<p>When the address-of operator is applied to a thread-local variable, it is
	evaluated at run time and returns the address of the current thread’s
	instance of that variable. An address so obtained may be used by any
	thread. When a thread terminates, any pointers to thread-local variables
	in that thread become invalid.
	</p>
	<p>No static initialization may refer to the address of a thread-local variable.
	</p>
	<p>In C++, if an initializer is present for a thread-local variable, it must
	be a <var>constant-expression</var>, as defined in 5.19.2 of the ANSI/ISO C++
	standard.
	</p>
	<p>See <a href="http://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local Storage</a> for a detailed explanation of
	the four thread-local storage addressing models, and how the runtime
	is expected to function.
	</p>
	<table class="menu" border="0" cellspacing="0">
	<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">
	</td></tr>
	<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">
	</td></tr>
	</table>

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