When embedding a structure or union within another structure or union, you do not have to name the internal structure. You can access the contents of the unnamed structure without using .name to reference it.

Unnamed fields are the same as anonymous unions and structures.

struct
{
    int a;
    union
    {
        int b;
        float c;
    };
    int d;
} Unnamed_Fields_0;

int Unnamed_Fields_1()
{
    return Unnamed_Fields_0.b;   
}