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

The 1st and 2nd cases are well-defined, because the size and alignment of a pointer is known. The C compiler only needs the size and alignment info to define a struct.

The 3rd case is invalid because the size of that actual struct is unknown.

But beware that for the 1st case to be logical, you need to give a name to the struct:

//             vvv
typedef struct xyz {
    struct xyz *z;
} xyz;

otherwise the outer struct and the *z will be considered two different structs.

---

The 2nd case has a popular use case known as "opaque pointer" (pimpl). For example, you could define a wrapper struct as

 typedef struct {
    struct X_impl* impl;
 } X;
 // usually just: typedef struct X_impl* X;
 int baz(X x);

in the header, and then in one of the .c,

 #include "header.h"
 struct X_impl {
    int foo;
    int bar[123];
    ...
 };
 int baz(X x) {
    return x.impl->foo;
 }

the advantage is out of that .c, you cannot mess with the internals of the object. It is a kind of encapsulation.