Header files recursion in C

Question

I have this situation:

A.h and B.h

In A.h:

typedef struct TypeA TypeA_t;
struct TypeA {
    ...
    TypeA_t *a;
    void (*doSomething)(TypeB_t *);
};

In B.h:

typedef struct TypeB TypeB_t;
struct TypeB {
    ...
    TypeB_t *b;
    TypeA_t something;
};

What is the correct way to include header files in each file?

If I include A.h in B.h and B.h in A.h I get:

error: unknown type name 'TypeB_t' in A.h

and

error: unknown type name 'TypeA_t' in B.h

I found a similar question here but it doesn't work in my case.

Answer 1

The way you've defined your code, TypeA can live with a forward reference to TypeB, but TypeB needs the full declaration of TypeA to compile.

In other words, you need to do two things. First forward define TypeB in a.h before your class definition (because pointers can work with partial definitions):

//a.h
typedef struct TypeB TypeB_t;

typedef struct TypeA TypeA_t;
struct TypeA {
...
TypeA_t *a;
void (*doSomething)(TypeB_t*);
};

And then include a.h from b.h to get the declaration for your class (because you use the full TypeA class as a field type):

// b.h
#include "a.h"

typedef struct TypeB TypeB_t;
struct TypeB {
...
TypeB_t *b;
TypeA_t something;
};