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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. |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="C99-Thread_002dLocal-Edits.html#C99-Thread_002dLocal-Edits">C99 Thread-Local Edits</a> |
| <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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