89 lines
4.2 KiB
HTML
89 lines
4.2 KiB
HTML
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<html lang="en">
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<head>
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<title>Unions - STABS</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="STABS">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Types.html#Types" title="Types">
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<link rel="prev" href="Typedefs.html#Typedefs" title="Typedefs">
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<link rel="next" href="Function-Types.html#Function-Types" title="Function Types">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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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.smallexample { font-size:smaller }
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pre.smalllisp { font-size:smaller }
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span.sc { font-variant:small-caps }
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span.roman { font-family:serif; font-weight:normal; }
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Unions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Function-Types.html#Function-Types">Function Types</a>,
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Previous: <a rel="previous" accesskey="p" href="Typedefs.html#Typedefs">Typedefs</a>,
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Up: <a rel="up" accesskey="u" href="Types.html#Types">Types</a>
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<hr>
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</div>
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<h3 class="section">5.10 Unions</h3>
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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>
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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>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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<!-- FIXME: phony line break. Can probably be fixed by using an example -->
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<!-- with fewer fields. -->
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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>
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<p>The symbol descriptor ‘<samp><span class="samp">T</span></samp>’, following the ‘<samp><span class="samp">name:</span></samp>’ means that
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the stab describes an enumeration, structure, or union tag. The type
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descriptor ‘<samp><span class="samp">u</span></samp>’, following the ‘<samp><span class="samp">23=</span></samp>’ of the type definition,
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narrows it down to a union type definition. Following the ‘<samp><span class="samp">u</span></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><span class="samp">:</span><var>type</var><span class="samp">, </span><var>bit
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offset into the union</var><span class="samp">, </span><var>number of bytes for the element</var><span class="samp">;</span></samp>’.
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<p>The stab for the union variable is:
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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>
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<p>‘<samp><span class="samp">-20</span></samp>’ specifies where the variable is stored (see <a href="Stack-Variables.html#Stack-Variables">Stack Variables</a>).
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</body></html>
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