| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 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 5.2, 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="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <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="prev" 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" 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="prev">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="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Unnamed-struct_002funion-fields-within-structs_002funions"></a> |
| <h3 class="section">6.60 Unnamed struct/union fields within structs/unions</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"></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="prev">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="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
