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As permitted by ISO C1X 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:
struct { int a; union { int b; float c; }; int d; } foo;
In this example, the user would be able to access members of the unnamed
union with code like `foo.b'. Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
int
.
You must never create such structures that cause ambiguous field definitions. For example, this structure:
struct { int a; struct { int a; }; } foo;
It is ambiguous which a
is being referred to with `foo.a'.
The compiler gives errors for such constructs.
Unless -fms-extensions is used, the unnamed field must be a
structure or union definition without a tag (for example, `struct
{ int a; };'). If -fms-extensions is used, the field may
also be a definition with a tag such as `struct foo { int a;
};', a reference to a previously defined structure or union such as
`struct foo;', or a reference to a typedef
name for a
previously defined structure or union type.
The option -fplan9-extensions enables -fms-extensions 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:
struct s1 { int a; }; struct s2 { struct s1; }; extern void f1 (struct s1 *); void f2 (struct s2 *p) { f1 (p); }
In the call to f1
inside f2
, the pointer p
is
converted into a pointer to the anonymous field.
Second, when the type of an anonymous field is a typedef
for a
struct
or union
, code may refer to the field using the
name of the typedef
.
typedef struct { int a; } s1; struct s2 { s1; }; s1 f1 (struct s2 *p) { return p->s1; }
These usages are only permitted when they are not ambiguous.