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| <title>Using the GNU Compiler Collection (GCC): Asm Labels</title> |
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| Next: <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" accesskey="n" rel="next">Explicit Reg Vars</a>, Previous: <a href="Constraints.html#Constraints" accesskey="p" rel="prev">Constraints</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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| <a name="Controlling-Names-Used-in-Assembler-Code"></a> |
| <h3 class="section">6.43 Controlling Names Used in Assembler Code</h3> |
| <a name="index-assembler-names-for-identifiers"></a> |
| <a name="index-names-used-in-assembler-code"></a> |
| <a name="index-identifiers_002c-names-in-assembler-code"></a> |
| |
| <p>You can specify the name to be used in the assembler code for a C |
| function or variable by writing the <code>asm</code> (or <code>__asm__</code>) |
| keyword after the declarator as follows: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int foo asm ("myfoo") = 2; |
| </pre></div> |
| |
| <p>This specifies that the name to be used for the variable <code>foo</code> in |
| the assembler code should be ‘<samp>myfoo</samp>’ rather than the usual |
| ‘<samp>_foo</samp>’. |
| </p> |
| <p>On systems where an underscore is normally prepended to the name of a C |
| function or variable, this feature allows you to define names for the |
| linker that do not start with an underscore. |
| </p> |
| <p>It does not make sense to use this feature with a non-static local |
| variable since such variables do not have assembler names. If you are |
| trying to put the variable in a particular register, see <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>. GCC presently accepts such code with a warning, but will |
| probably be changed to issue an error, rather than a warning, in the |
| future. |
| </p> |
| <p>You cannot use <code>asm</code> in this way in a function <em>definition</em>; but |
| you can get the same effect by writing a declaration for the function |
| before its definition and putting <code>asm</code> there, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">extern func () asm ("FUNC"); |
| |
| func (x, y) |
| int x, y; |
| /* <span class="roman">…</span> */ |
| </pre></div> |
| |
| <p>It is up to you to make sure that the assembler names you choose do not |
| conflict with any other assembler symbols. Also, you must not use a |
| register name; that would produce completely invalid assembler code. GCC |
| does not as yet have the ability to store static variables in registers. |
| Perhaps that will be added. |
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