165 lines
6.6 KiB
HTML
165 lines
6.6 KiB
HTML
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<head>
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<title>Using the GNU Compiler Collection (GCC): Unnamed Fields</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Unnamed Fields">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Unnamed Fields">
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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="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Thread_002dLocal.html#Thread_002dLocal" rel="next" title="Thread-Local">
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<link href="Loop_002dSpecific-Pragmas.html#Loop_002dSpecific-Pragmas" rel="prev" title="Loop-Specific Pragmas">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Unnamed-Fields"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Unnamed-Structure-and-Union-Fields"></a>
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<h3 class="section">6.62 Unnamed Structure and Union Fields</h3>
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<a name="index-struct"></a>
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<a name="index-union"></a>
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<p>As permitted by ISO C11 and for compatibility with other compilers,
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GCC allows you to define
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a structure or union that contains, as fields, structures and unions
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without names. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct {
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int a;
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union {
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int b;
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float c;
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};
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int d;
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} foo;
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</pre></div>
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<p>In this example, you are able to access members of the unnamed
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union with code like ‘<samp>foo.b</samp>’. Note that only unnamed structs and
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unions are allowed, you may not have, for example, an unnamed
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<code>int</code>.
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</p>
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<p>You must never create such structures that cause ambiguous field definitions.
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For example, in this structure:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct {
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int a;
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struct {
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int a;
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};
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} foo;
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</pre></div>
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<p>it is ambiguous which <code>a</code> is being referred to with ‘<samp>foo.a</samp>’.
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The compiler gives errors for such constructs.
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</p>
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<a name="index-fms_002dextensions-2"></a>
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<p>Unless <samp>-fms-extensions</samp> is used, the unnamed field must be a
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structure or union definition without a tag (for example, ‘<samp>struct
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{ int a; };</samp>’). If <samp>-fms-extensions</samp> is used, the field may
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also be a definition with a tag such as ‘<samp>struct foo { int a;
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};</samp>’, a reference to a previously defined structure or union such as
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‘<samp>struct foo;</samp>’, or a reference to a <code>typedef</code> name for a
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previously defined structure or union type.
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</p>
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<a name="index-fplan9_002dextensions-1"></a>
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<p>The option <samp>-fplan9-extensions</samp> enables
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<samp>-fms-extensions</samp> as well as two other extensions. First, a
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pointer to a structure is automatically converted to a pointer to an
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anonymous field for assignments and function calls. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct s1 { int a; };
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struct s2 { struct s1; };
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extern void f1 (struct s1 *);
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void f2 (struct s2 *p) { f1 (p); }
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</pre></div>
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<p>In the call to <code>f1</code> inside <code>f2</code>, the pointer <code>p</code> is
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converted into a pointer to the anonymous field.
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</p>
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<p>Second, when the type of an anonymous field is a <code>typedef</code> for a
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<code>struct</code> or <code>union</code>, code may refer to the field using the
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name of the <code>typedef</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef struct { int a; } s1;
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struct s2 { s1; };
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s1 f1 (struct s2 *p) { return p->s1; }
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</pre></div>
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<p>These usages are only permitted when they are not ambiguous.
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</p>
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<hr>
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<div class="header">
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<p>
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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>
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</div>
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</body>
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