gistfile1.c

#include <stdio.h>

struct B;

struct A
{
    struct B *ptr;
};

struct B
{
    struct A *ptr;
};

extern struct B b;

struct A a = { &b };
struct B b = { &a };

int main(void)
{
    printf("a = %p, b = %p\n", &a, &b);
    printf("a->ptr = %p, b->ptr = %p\n", a.ptr, b.ptr);
    return 0;
}