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<h3 class="section">6.56 Thread-Local Storage</h3>

<p><a name="index-Thread_002dLocal-Storage-3153"></a><a name="index-g_t_0040acronym_007bTLS_007d-3154"></a><a name="index-g_t_005f_005fthread-3155"></a>
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 run-time 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 (<samp><span class="command">ld</span></samp>), dynamic linker (<samp><span class="command">ld.so</span></samp>), and
system libraries (<samp><span class="file">libc.so</span></samp> and <samp><span class="file">libpthread.so</span></samp>), so it
is not available everywhere.

 <p>At the user level, the extension is visible with a new storage
class keyword: <code>__thread</code>.  For example:

<pre class="smallexample">     __thread int i;
     extern __thread struct state s;
     static __thread char *p;
</pre>
 <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>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>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>No static initialization may refer to the address of a thread-local variable.

 <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>See <a href="http://people.redhat.com/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 run-time
is expected to function.

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