6.48. Unnamed struct/union fields within structs/unions.

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. Such constructs are not supported and must be avoided. In the future, such constructs may be detected and treated as compilation errors.