How is it legal to reference an undefined type inside a structure?

Question

As part of answering another question, I came across a piece of code like this, which gcc compiles without complaint.

typedef struct {
    struct xyz *z;
} x         


        

Answer 1

You do have to name it. In this:

typedef struct {
    struct xyz *z;
} xyz;

will not be able to point to itself as z refers to some complete other type, not to the unnamed struct you just defined. Try this:

int main()
{
    xyz me1;
    xyz me2;
    me1.z = &me2;   // this will not compile
}

You'll get an error about incompatible types.