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<h3 class="section">6.61 Unnamed Structure and Union Fields</h3>
<p><a name="index-g_t_0040code_007bstruct_007d-4189"></a><a name="index-g_t_0040code_007bunion_007d-4190"></a>
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:
<pre class="smallexample"> struct {
int a;
union {
int b;
float c;
};
int d;
} foo;
</pre>
<p class="noindent">In this example, you are able to access members of the unnamed
union with code like ‘<samp><span class="samp">foo.b</span></samp>’. Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
<code>int</code>.
<p>You must never create such structures that cause ambiguous field definitions.
For example, in this structure:
<pre class="smallexample"> struct {
int a;
struct {
int a;
};
} foo;
</pre>
<p class="noindent">it is ambiguous which <code>a</code> is being referred to with ‘<samp><span class="samp">foo.a</span></samp>’.
The compiler gives errors for such constructs.
<p><a name="index-fms_002dextensions-4191"></a>Unless <samp><span class="option">-fms-extensions</span></samp> is used, the unnamed field must be a
structure or union definition without a tag (for example, ‘<samp><span class="samp">struct
{ int a; };</span></samp>’). If <samp><span class="option">-fms-extensions</span></samp> is used, the field may
also be a definition with a tag such as ‘<samp><span class="samp">struct foo { int a;
};</span></samp>’, a reference to a previously defined structure or union such as
‘<samp><span class="samp">struct foo;</span></samp>’, or a reference to a <code>typedef</code> name for a
previously defined structure or union type.
<p><a name="index-fplan9_002dextensions-4192"></a>The option <samp><span class="option">-fplan9-extensions</span></samp> enables
<samp><span class="option">-fms-extensions</span></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:
<pre class="smallexample"> struct s1 { int a; };
struct s2 { struct s1; };
extern void f1 (struct s1 *);
void f2 (struct s2 *p) { f1 (p); }
</pre>
<p class="noindent">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>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>.
<pre class="smallexample"> typedef struct { int a; } s1;
struct s2 { s1; };
s1 f1 (struct s2 *p) { return p->s1; }
</pre>
<p>These usages are only permitted when they are not ambiguous.
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