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<title>The GNU Fortran Compiler: UNION and MAP</title>
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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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<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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-------------------------------
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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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<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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