redefinition of typedef

Question

I am possibly doing this incorrectly and this is much a question about why it works in one compiler and not the other.

I have a large C application, and I am

Answer 1

One piece of the idiom is missing. The forward declarations are independent from the definitions, so they should be in a separate header file.

// a_fwd.h

#ifndef A_FWD_H
#define A_FWD_H

typedef struct A_ A;

#endif

// a.h

#ifndef A_H
#define A_H

#include "a_fwd.h"

struct A_ {
};

#endif

Now it's always safe to include any headers in any order.

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It is illegal to have two definitions of anything. A typedef is a definition, not just a declaration, so the one compiler was being quite lax to allow the redundancy.