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118 lines
5.0 KiB
HTML
118 lines
5.0 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>Unions (STABS)</title>
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<meta name="description" content="Unions (STABS)">
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<meta name="keywords" content="Unions (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="Types.html#Types" rel="up" title="Types">
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<link href="Function-Types.html#Function-Types" rel="next" title="Function Types">
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<link href="Typedefs.html#Typedefs" rel="prev" title="Typedefs">
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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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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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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="Unions"></a>
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<div class="header">
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<p>
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Next: <a href="Function-Types.html#Function-Types" accesskey="n" rel="next">Function Types</a>, Previous: <a href="Typedefs.html#Typedefs" accesskey="p" rel="prev">Typedefs</a>, Up: <a href="Types.html#Types" accesskey="u" rel="up">Types</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="Unions-1"></a>
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<h3 class="section">5.10 Unions</h3>
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<div class="example">
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<pre class="example">union u_tag {
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int u_int;
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float u_float;
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char* u_char;
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} an_u;
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</pre></div>
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<p>This code generates a stab for a union tag and a stab for a union
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variable. Both use the <code>N_LSYM</code> stab type. If a union variable is
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scoped locally to the procedure in which it is defined, its stab is
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located immediately preceding the <code>N_LBRAC</code> for the procedure’s block
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start.
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</p>
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<p>The stab for the union tag, however, is located preceding the code for
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the procedure in which it is defined. The stab type is <code>N_LSYM</code>. This
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would seem to imply that the union type is file scope, like the struct
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type <code>s_tag</code>. This is not true. The contents and position of the stab
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for <code>u_type</code> do not convey any information about its procedure local
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scope.
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</p>
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<div class="smallexample">
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<pre class="smallexample"># <span class="roman">128 is N_LSYM</span>
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.stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
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128,0,0,0
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</pre></div>
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<p>The symbol descriptor ‘<samp>T</samp>’, following the ‘<samp>name:</samp>’ means that
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the stab describes an enumeration, structure, or union tag. The type
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descriptor ‘<samp>u</samp>’, following the ‘<samp>23=</samp>’ of the type definition,
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narrows it down to a union type definition. Following the ‘<samp>u</samp>’ is
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the number of bytes in the union. After that is a list of union element
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descriptions. Their format is ‘<samp><var>name</var>:<var>type</var>, <var>bit
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offset into the union</var>, <var>number of bytes for the element</var>;</samp>’.
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</p>
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<p>The stab for the union variable is:
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</p>
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<div class="example">
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<pre class="example">.stabs "an_u:23",128,0,0,-20 # <span class="roman">128 is N_LSYM</span>
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</pre></div>
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<p>‘<samp>-20</samp>’ specifies where the variable is stored (see <a href="Stack-Variables.html#Stack-Variables">Stack Variables</a>).
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</p>
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</body>
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</html>
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