1<html lang="en"> 2<head> 3<title>Asm Labels - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Constraints.html#Constraints" title="Constraints"> 10<link rel="next" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" title="Explicit Reg Vars"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Funding Free Software'', the Front-Cover 19Texts being (a) (see below), and with the Back-Cover Texts being (b) 20(see below). A copy of the license is included in the section entitled 21``GNU Free Documentation License''. 22 23(a) The FSF's Front-Cover Text is: 24 25 A GNU Manual 26 27(b) The FSF's Back-Cover Text is: 28 29 You have freedom to copy and modify this GNU Manual, like GNU 30 software. Copies published by the Free Software Foundation raise 31 funds for GNU development.--> 32<meta http-equiv="Content-Style-Type" content="text/css"> 33<style type="text/css"><!-- 34 pre.display { font-family:inherit } 35 pre.format { font-family:inherit } 36 pre.smalldisplay { font-family:inherit; font-size:smaller } 37 pre.smallformat { font-family:inherit; font-size:smaller } 38 pre.smallexample { font-size:smaller } 39 pre.smalllisp { font-size:smaller } 40 span.sc { font-variant:small-caps } 41 span.roman { font-family:serif; font-weight:normal; } 42 span.sansserif { font-family:sans-serif; font-weight:normal; } 43--></style> 44<link rel="stylesheet" type="text/css" href="../cs.css"> 45</head> 46<body> 47<div class="node"> 48<a name="Asm-Labels"></a> 49<p> 50Next: <a rel="next" accesskey="n" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>, 51Previous: <a rel="previous" accesskey="p" href="Constraints.html#Constraints">Constraints</a>, 52Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 53<hr> 54</div> 55 56<h3 class="section">6.43 Controlling Names Used in Assembler Code</h3> 57 58<p><a name="index-assembler-names-for-identifiers-2880"></a><a name="index-names-used-in-assembler-code-2881"></a><a name="index-identifiers_002c-names-in-assembler-code-2882"></a> 59You can specify the name to be used in the assembler code for a C 60function or variable by writing the <code>asm</code> (or <code>__asm__</code>) 61keyword after the declarator as follows: 62 63<pre class="smallexample"> int foo asm ("myfoo") = 2; 64</pre> 65 <p class="noindent">This specifies that the name to be used for the variable <code>foo</code> in 66the assembler code should be ‘<samp><span class="samp">myfoo</span></samp>’ rather than the usual 67‘<samp><span class="samp">_foo</span></samp>’. 68 69 <p>On systems where an underscore is normally prepended to the name of a C 70function or variable, this feature allows you to define names for the 71linker that do not start with an underscore. 72 73 <p>It does not make sense to use this feature with a non-static local 74variable since such variables do not have assembler names. If you are 75trying 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 76probably be changed to issue an error, rather than a warning, in the 77future. 78 79 <p>You cannot use <code>asm</code> in this way in a function <em>definition</em>; but 80you can get the same effect by writing a declaration for the function 81before its definition and putting <code>asm</code> there, like this: 82 83<pre class="smallexample"> extern func () asm ("FUNC"); 84 85 func (x, y) 86 int x, y; 87 /* <span class="roman">...</span> */ 88</pre> 89 <p>It is up to you to make sure that the assembler names you choose do not 90conflict with any other assembler symbols. Also, you must not use a 91register name; that would produce completely invalid assembler code. GCC 92does not as yet have the ability to store static variables in registers. 93Perhaps that will be added. 94 95 </body></html> 96 97