Avoiding Circular Dependency in C header files

Question

 header1.h   
        -----------------
        struct A{
        int a;
        B *b;
        };

    header2.h  
    -------------------
    #include"header1.h"

    typedef struct b{
    int aa;
    char bb;
    int cc;
    }B;

    main.c  
    --------------------
    #include<header2.h>

    main(){

    struct A *ace;
    ace = malloc(sizeof(struct A));
    ace->b = malloc(sizeof(B));
    }

The problem: Basically, header1.h needs to use a struct defined in header2.h.

The Dilemma: Since header2.h includes header1.h, If i include header2.h in header1.h I will be introducing a circular dependency.

Some solutions: One way to solve the problem would be use a void pointer, but is there any other way? I tried forward declaring it, but it says "redifining a typedef".

Answer 1

So the problem you have is the pointer

B *b;

in struct A. Since this is a pointer an incomplete type is ok as long as you tell it that's what this is.

typedef struct a {
    int a;
    struct B *b;
} A;

or you could use the prototype form:

struct B;
typedef struct a {
    int a;
    B *b;
};

The compiler just wants to know what type of thing the pointer points-to just here, it doesn't actually care what it looks like in more detail until you first start trying to dereference it.

Answer 2

You can easily prevent your header files from being included multiple times by wrapping them like this

#ifndef FOO_H_ /* include guard */
#define FOO_H_

/* * * * * insert foo.h here * * * * */

#endif /* FOO_H_ */