C problems with nested structs (looks like 1 instance is defined without explicit definitios)

Question

I've created struct:

typedef struct
{
    short s;
    int i;

    struct Ss
    {
        short s;
    };
} S;

and I have an error (VS2012):

 error C2020: 's' : 'struct' member redefinition

I see that the name "s" is used, but it is in nested named struct, so it shouldn't be a problem. I know that in C11 was intruduced anonymouds structs and unions but in my case it is not anonymous and I have VS2012 which does not support C11.

Another interesting example is:

typedef struct
{
    short s;
    union U
    {
        int uu;
    };

    struct S
    {
        short ss;
    };

    union
    {
        int i;
        double d;
    };

} A;

and interesting think is that: in VS2008 sizeof(A) = 24, but I have only 1 field defined. And when I use code:

A a = {1, 2, 3, 4};
printf("A.s=%d A.uu=%d A.ss=%d A.i=%d \n", a.s, a.uu, a.ss, a.i);

the output is:

A.s=1 A.uu=2 A.ss=3 A.i=4 

So my question is: why is like that, is it correct behaviour?

Answer 1

You first example is not C because the part struct Ss{ short s }; doesn't declare a name or is not an anonymous member. If VS allows that, then it must be an extension. It should be either:

typedef struct
{
    short s;
    int i;

    struct Ss
    {
        short s;
    } m ;
} S;

or if you have C11:

typedef struct
{
    short s;
    int i;

    struct  //anonymous structure
    {
        short s2 ;
    } ;
} S;

In which case the name of the last member must be different.

Just a remainder what is an anonymous structure from C11:

6.7.2.1 p13

An unnamed member whose type specifier is a structure specifier with no tag is called an anonymous structure; an unnamed member whose type specifier is a union specifier with no tag is called an anonymous union. The members of an anonymous structure or union are considered to be members of the containing structure or union. This applies recursively if the containing structure or union is also anonymous.

Which makes this an anonymous structure:

struct  //note the missing tag
{
    int name ;
} ;