Anonymous struct with ANSI C

Question

I want to know if it is possible to declare anonymous structs in ANSI C. The code I have is:

struct A
{
    int x;
};

struct B
{
    struct A;
    int y;
};

When I compile it I get: warning: declaration does not declare anything

I have read that the flag -fms-extensions does the trick, it however only works on windows systems as it produces: warning: anonymous structs are a Microsoft extension [-Wmicrosoft]

Is there any ANSI equivalent extension that I can use?

Answer 1

A trick to get almost this feature in ANSI C is to use an appropriate macro:

struct A {
    int x;
};

struct B {
    struct A A_;
    int y;
};

#define bx A_.x

Then you can simply do

struct B foo, *bar;

foo.bx;
bar->bx;

In C11 though, anonymous structures are supported and you can simply do

struct B {
    struct {
        int x;
    };

    int y;
}

but sadly not

struct A {
    int x;
};

struct B
{
    struct A;
    int y;
};

As the anonymous structure has to be declared inside the structure it is anonymous to.

See this answer for more details on anonymous members in C11.