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, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Asm-Labels"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>, 53Previous: <a rel="previous" accesskey="p" href="Constraints.html#Constraints">Constraints</a>, 54Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">6.43 Controlling Names Used in Assembler Code</h3> 59 60<p><a name="index-assembler-names-for-identifiers-2677"></a><a name="index-names-used-in-assembler-code-2678"></a><a name="index-identifiers_002c-names-in-assembler-code-2679"></a> 61You can specify the name to be used in the assembler code for a C 62function or variable by writing the <code>asm</code> (or <code>__asm__</code>) 63keyword after the declarator as follows: 64 65<pre class="smallexample"> int foo asm ("myfoo") = 2; 66</pre> 67 <p class="noindent">This specifies that the name to be used for the variable <code>foo</code> in 68the assembler code should be ‘<samp><span class="samp">myfoo</span></samp>’ rather than the usual 69‘<samp><span class="samp">_foo</span></samp>’. 70 71 <p>On systems where an underscore is normally prepended to the name of a C 72function or variable, this feature allows you to define names for the 73linker that do not start with an underscore. 74 75 <p>It does not make sense to use this feature with a non-static local 76variable since such variables do not have assembler names. If you are 77trying 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 78probably be changed to issue an error, rather than a warning, in the 79future. 80 81 <p>You cannot use <code>asm</code> in this way in a function <em>definition</em>; but 82you can get the same effect by writing a declaration for the function 83before its definition and putting <code>asm</code> there, like this: 84 85<pre class="smallexample"> extern func () asm ("FUNC"); 86 87 func (x, y) 88 int x, y; 89 /* <span class="roman">...</span> */ 90</pre> 91 <p>It is up to you to make sure that the assembler names you choose do not 92conflict with any other assembler symbols. Also, you must not use a 93register name; that would produce completely invalid assembler code. GCC 94does not as yet have the ability to store static variables in registers. 95Perhaps that will be added. 96 97 </body></html> 98 99