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diff --git a/share/doc/gfortran/UNION-and-MAP.html b/share/doc/gfortran/UNION-and-MAP.html new file mode 100644 index 0000000..c7e9e8d --- /dev/null +++ b/share/doc/gfortran/UNION-and-MAP.html @@ -0,0 +1,193 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1999-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). 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Use of <code>UNION</code> and +<code>MAP</code> is automatically enabled with <samp>-fdec-structure</samp>. +</p> +<p>A <code>UNION</code> declaration occurs within a structure; within the definition of +each union is a number of <code>MAP</code> blocks. Each <code>MAP</code> shares storage +with its sibling maps (in the same union), and the size of the union is the +size of the largest map within it, just as with unions in C. The major +difference is that component references do not indicate which union or map the +component is in (the compiler gets to figure that out). +</p> +<p>Here is a small example: +</p><div class="smallexample"> +<pre class="smallexample">structure /myunion/ +union + map + character(2) w0, w1, w2 + end map + map + character(6) long + end map +end union +end structure + +record /myunion/ rec +! After this assignment... +rec.long = 'hello!' + +! The following is true: +! rec.w0 === 'he' +! rec.w1 === 'll' +! rec.w2 === 'o!' +</pre></div> + +<p>The two maps share memory, and the size of the union is ultimately six bytes: +</p> +<div class="example"> +<pre class="example">0 1 2 3 4 5 6 Byte offset +------------------------------- +| | | | | | | +------------------------------- + +^ W0 ^ W1 ^ W2 ^ + \-------/ \-------/ \-------/ + +^ LONG ^ + \---------------------------/ +</pre></div> + +<p>Following is an example mirroring the layout of an Intel x86_64 register: +</p> +<div class="example"> +<pre class="example">structure /reg/ + union ! U0 ! rax + map + character(16) rx + end map + map + character(8) rh ! rah + union ! U1 + map + character(8) rl ! ral + end map + map + character(8) ex ! eax + end map + map + character(4) eh ! eah + union ! U2 + map + character(4) el ! eal + end map + map + character(4) x ! ax + end map + map + character(2) h ! ah + character(2) l ! al + end map + end union + end map + end union + end map + end union +end structure +record /reg/ a + +! After this assignment... +a.rx = 'AAAAAAAA.BBB.C.D' + +! The following is true: +a.rx === 'AAAAAAAA.BBB.C.D' +a.rh === 'AAAAAAAA' +a.rl === '.BBB.C.D' +a.ex === '.BBB.C.D' +a.eh === '.BBB' +a.el === '.C.D' +a.x === '.C.D' +a.h === '.C' +a.l === '.D' +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Type-variants-for-integer-intrinsics.html#Type-variants-for-integer-intrinsics" accesskey="n" rel="next">Type variants for integer intrinsics</a>, Previous: <a href="STRUCTURE-and-RECORD.html#STRUCTURE-and-RECORD" accesskey="p" rel="previous">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> +</div> + + + +</body> +</html> |