Question

The question seems to be a classic one, but I didn’t find an answer:

I have two struct type written in C —— struct type A and struct type B, and B use A while A use B at the same time.

In a.h:

#include "b.h"
struct B;
typedef struct A {
    void (*func)(struct B* b);
}A;

In b.h:

#include "a.h"
typedef struct B {
    A a;
}B;

though this will work, it has a consequence — when using the function fun “func”, if I pass a variable declared in the form:

B* someb;

not:

struct B* someb;

there will be a warning when compiling, saying incompatible pointer type. Is this normal? could I avoid this warning?

Answer 1

From your a.h header file, simply remove the line #include "b.h". The forward declaration struct B; is all you need.

That change will fix the circular include dependency, and will make any code using these headers more sane.

Then, wherever you want to make use of the B struct, simply include b.h, and use it with or without the struct keyword.

Some code to illustrate : the a.h header file :

/* a.h */
struct B;

typedef struct A {
    void (*func)(struct B* b);
} A;

The b.h header file :

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

typedef struct B {
    A a;
} B;

A file that uses these header files :

/* some_file.c */
#include "b.h"

void fun() {
    B someb;
    someb.a.func(&someb);
}

void fun2() {
    struct B someb;
    someb.a.func(&someb);
}

Both fun and fun2 are just fine (from the compiler point of view – func hasn’t been initialized, so calling it will cause an issue at run time).