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119 lines
5.2 KiB
HTML
119 lines
5.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1992-2019 Free Software Foundation, Inc.
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Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
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and David MacKenzie.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled "GNU
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Free Documentation License". -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Stack Variables (STABS)</title>
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<meta name="description" content="Stack Variables (STABS)">
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<meta name="keywords" content="Stack Variables (STABS)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Variables.html#Variables" rel="up" title="Variables">
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<link href="Global-Variables.html#Global-Variables" rel="next" title="Global Variables">
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<link href="Variables.html#Variables" rel="prev" title="Variables">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.format {font-family: inherit}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Stack-Variables"></a>
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<div class="header">
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<p>
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Next: <a href="Global-Variables.html#Global-Variables" accesskey="n" rel="next">Global Variables</a>, Up: <a href="Variables.html#Variables" accesskey="u" rel="up">Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Automatic-Variables-Allocated-on-the-Stack"></a>
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<h3 class="section">4.1 Automatic Variables Allocated on the Stack</h3>
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<p>If a variable’s scope is local to a function and its lifetime is only as
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long as that function executes (C calls such variables
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<em>automatic</em>), it can be allocated in a register (see <a href="Register-Variables.html#Register-Variables">Register Variables</a>) or on the stack.
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</p>
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<a name="index-N_005fLSYM_002c-for-stack-variables"></a>
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<a name="index-C_005fLSYM"></a>
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<p>Each variable allocated on the stack has a stab with the symbol
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descriptor omitted. Since type information should begin with a digit,
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‘<samp>-</samp>’, or ‘<samp>(</samp>’, only those characters precluded from being used
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for symbol descriptors. However, the Acorn RISC machine (ARM) is said
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to get this wrong: it puts out a mere type definition here, without the
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preceding ‘<samp><var>type-number</var>=</samp>’. This is a bad idea; there is no
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guarantee that type descriptors are distinct from symbol descriptors.
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Stabs for stack variables use the <code>N_LSYM</code> stab type, or
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<code>C_LSYM</code> for XCOFF.
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</p>
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<p>The value of the stab is the offset of the variable within the
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local variables. On most machines this is an offset from the frame
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pointer and is negative. The location of the stab specifies which block
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it is defined in; see <a href="Block-Structure.html#Block-Structure">Block Structure</a>.
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</p>
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<p>For example, the following C code:
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</p>
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<div class="example">
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<pre class="example">int
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main ()
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{
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int x;
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}
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</pre></div>
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<p>produces the following stabs:
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</p>
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<div class="example">
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<pre class="example">.stabs "main:F1",36,0,0,_main # <span class="roman">36 is N_FUN</span>
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.stabs "x:1",128,0,0,-12 # <span class="roman">128 is N_LSYM</span>
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.stabn 192,0,0,LBB2 # <span class="roman">192 is N_LBRAC</span>
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.stabn 224,0,0,LBE2 # <span class="roman">224 is N_RBRAC</span>
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</pre></div>
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<p>See <a href="Procedures.html#Procedures">Procedures</a> for more information on the <code>N_FUN</code> stab, and
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<a href="Block-Structure.html#Block-Structure">Block Structure</a> for more information on the <code>N_LBRAC</code> and
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<code>N_RBRAC</code> stabs.
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</p>
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</body>
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</html>
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