Using macro in C11 anonymous struct definition

Question

The typical C99 way to extending stuct is something like

struct Base {
    int x;
    /* ... */
};

struct Derived {
    struct Base base_part;
    int y;
    /* ... */
};

Then we may cast instance of struct Derived * to struct Base * and then access x.

I want to access base elements of struct Derived * obj; directly, for example obj->x and obj->y. C11 provide extended structs, but as explained here we can use this feature only with anonymous definitions. Then how about to write

#define BASE_BODY { \
    int x; \
}

struct Base BASE_BODY;

struct Derived {
    struct BASE_BODY;
    int y;
};

Then I may access Base members same as it's part of Derived without any casts or intermediate members. I can cast Derived pointer to Base pointer if need do.

Is this acceptable? Are there any pitfalls?

Answer 1

There are pitfalls.

Consider:

#define BASE_BODY { \
    double a; \
    short b; \
}

struct Base BASE_BODY;

struct Derived {
    struct BASE_BODY;
    short c;
};

On some implementation it could be that sizeof(Base) == sizeof(Derived), but:

struct Base {
    double a;
    // Padding here
    short b;
}

struct Derived {
    double a;
    short b;
    short c;
};

There is no guarantee that at the beginning of the struct memory layout is the same. Therefore you cannot pass this kind of Derived * to function expecting Base *, and expect it to work.

And even if padding would not mess up the layout, there is a still potential problem with trap presenstation:

If again sizeof(Base) == sizeof(Derived), but c ends up to a area which is covered by the padding at the end of Base. Passing pointer of this struct to function which expects Base* and modifies it, might affect padding bits too (padding has unspecified value), thus possibly corrupting c and maybe even creating trap presentation.