195 lines
6.9 KiB
HTML
195 lines
6.9 KiB
HTML
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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) 1999-2016 Free Software Foundation, Inc.
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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 the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU Fortran Compiler: UNION and MAP</title>
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<meta name="description" content="The GNU Fortran Compiler: UNION and MAP">
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<meta name="keywords" content="The GNU Fortran Compiler: UNION and MAP">
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<meta name="distribution" content="global">
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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="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" rel="up" title="Extensions implemented in GNU Fortran">
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<link href="Extensions-not-implemented-in-GNU-Fortran.html#Extensions-not-implemented-in-GNU-Fortran" rel="next" title="Extensions not implemented in GNU Fortran">
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<link href="STRUCTURE-and-RECORD.html#STRUCTURE-and-RECORD" rel="prev" title="STRUCTURE and RECORD">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="UNION-and-MAP"></a>
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<div class="header">
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<p>
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Previous: <a href="STRUCTURE-and-RECORD.html#STRUCTURE-and-RECORD" accesskey="p" rel="prev">STRUCTURE and RECORD</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="UNION-and-MAP-1"></a>
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<h4 class="subsection">6.1.21 <code>UNION</code> and <code>MAP</code></h4>
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<a name="index-UNION"></a>
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<a name="index-MAP"></a>
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<p>Unions are an old vendor extension which were commonly used with the
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non-standard <a href="STRUCTURE-and-RECORD.html#STRUCTURE-and-RECORD">STRUCTURE and RECORD</a> extensions. Use of <code>UNION</code> and
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<code>MAP</code> is automatically enabled with <samp>-fdec-structure</samp>.
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</p>
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<p>A <code>UNION</code> declaration occurs within a structure; within the definition of
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each union is a number of <code>MAP</code> blocks. Each <code>MAP</code> shares storage
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with its sibling maps (in the same union), and the size of the union is the
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size of the largest map within it, just as with unions in C. The major
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difference is that component references do not indicate which union or map the
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component is in (the compiler gets to figure that out).
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</p>
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<p>Here is a small example:
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</p><div class="smallexample">
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<pre class="smallexample">structure /myunion/
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union
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map
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character(2) w0, w1, w2
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end map
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map
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character(6) long
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end map
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end union
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end structure
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record /myunion/ rec
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! After this assignment...
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rec.long = 'hello!'
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! The following is true:
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! rec.w0 === 'he'
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! rec.w1 === 'll'
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! rec.w2 === 'o!'
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</pre></div>
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<p>The two maps share memory, and the size of the union is ultimately six bytes:
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</p>
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<div class="example">
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<pre class="example">0 1 2 3 4 5 6 Byte offset
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-------------------------------
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-------------------------------
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^ W0 ^ W1 ^ W2 ^
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\-------/ \-------/ \-------/
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^ LONG ^
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\---------------------------/
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</pre></div>
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<p>Following is an example mirroring the layout of an Intel x86_64 register:
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</p>
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<div class="example">
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<pre class="example">structure /reg/
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union ! U0 ! rax
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map
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character(16) rx
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end map
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map
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character(8) rh ! rah
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union ! U1
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map
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character(8) rl ! ral
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end map
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map
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character(8) ex ! eax
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end map
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map
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character(4) eh ! eah
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union ! U2
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map
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character(4) el ! eal
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end map
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map
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character(4) x ! ax
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end map
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map
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character(2) h ! ah
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character(2) l ! al
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end map
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end union
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end map
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end union
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end map
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end union
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end structure
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record /reg/ a
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! After this assignment...
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a.rx = 'AAAAAAAA.BBB.C.D'
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! The following is true:
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a.rx === 'AAAAAAAA.BBB.C.D'
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a.rh === 'AAAAAAAA'
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a.rl === '.BBB.C.D'
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a.ex === '.BBB.C.D'
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a.eh === '.BBB'
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a.el === '.C.D'
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a.x === '.C.D'
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a.h === '.C'
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a.l === '.D'
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="STRUCTURE-and-RECORD.html#STRUCTURE-and-RECORD" accesskey="p" rel="prev">STRUCTURE and RECORD</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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