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diff --git a/share/doc/gcc/Unnamed-Fields.html b/share/doc/gcc/Unnamed-Fields.html new file mode 100644 index 0000000..f01de9f --- /dev/null +++ b/share/doc/gcc/Unnamed-Fields.html @@ -0,0 +1,166 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 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). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Unnamed Fields</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Unnamed Fields"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Unnamed Fields"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> +<link href="Thread_002dLocal.html#Thread_002dLocal" rel="next" title="Thread-Local"> +<link href="Loop_002dSpecific-Pragmas.html#Loop_002dSpecific-Pragmas" rel="previous" title="Loop-Specific Pragmas"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Unnamed-Fields"></a> +<div class="header"> +<p> +Next: <a href="Thread_002dLocal.html#Thread_002dLocal" accesskey="n" rel="next">Thread-Local</a>, Previous: <a href="Pragmas.html#Pragmas" accesskey="p" rel="previous">Pragmas</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Unnamed-Structure-and-Union-Fields"></a> +<h3 class="section">6.63 Unnamed Structure and Union Fields</h3> +<a name="index-struct"></a> +<a name="index-union"></a> + +<p>As permitted by ISO C11 and for compatibility with other compilers, +GCC allows you to define +a structure or union that contains, as fields, structures and unions +without names. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct { + int a; + union { + int b; + float c; + }; + int d; +} foo; +</pre></div> + +<p>In this example, you are able to access members of the unnamed +union with code like ‘<samp>foo.b</samp>’. Note that only unnamed structs and +unions are allowed, you may not have, for example, an unnamed +<code>int</code>. +</p> +<p>You must never create such structures that cause ambiguous field definitions. +For example, in this structure: +</p> +<div class="smallexample"> +<pre class="smallexample">struct { + int a; + struct { + int a; + }; +} foo; +</pre></div> + +<p>it is ambiguous which <code>a</code> is being referred to with ‘<samp>foo.a</samp>’. +The compiler gives errors for such constructs. +</p> +<a name="index-fms_002dextensions-2"></a> +<p>Unless <samp>-fms-extensions</samp> is used, the unnamed field must be a +structure or union definition without a tag (for example, ‘<samp>struct +{ int a; };</samp>’). If <samp>-fms-extensions</samp> is used, the field may +also be a definition with a tag such as ‘<samp>struct foo { int a; +};</samp>’, a reference to a previously defined structure or union such as +‘<samp>struct foo;</samp>’, or a reference to a <code>typedef</code> name for a +previously defined structure or union type. +</p> +<a name="index-fplan9_002dextensions-1"></a> +<p>The option <samp>-fplan9-extensions</samp> enables +<samp>-fms-extensions</samp> as well as two other extensions. First, a +pointer to a structure is automatically converted to a pointer to an +anonymous field for assignments and function calls. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct s1 { int a; }; +struct s2 { struct s1; }; +extern void f1 (struct s1 *); +void f2 (struct s2 *p) { f1 (p); } +</pre></div> + +<p>In the call to <code>f1</code> inside <code>f2</code>, the pointer <code>p</code> is +converted into a pointer to the anonymous field. +</p> +<p>Second, when the type of an anonymous field is a <code>typedef</code> for a +<code>struct</code> or <code>union</code>, code may refer to the field using the +name of the <code>typedef</code>. +</p> +<div class="smallexample"> +<pre class="smallexample">typedef struct { int a; } s1; +struct s2 { s1; }; +s1 f1 (struct s2 *p) { return p->s1; } +</pre></div> + +<p>These usages are only permitted when they are not ambiguous. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Thread_002dLocal.html#Thread_002dLocal" accesskey="n" rel="next">Thread-Local</a>, Previous: <a href="Pragmas.html#Pragmas" accesskey="p" rel="previous">Pragmas</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |