how to declare dependant structs in C?

Question

I need to declare a struct that both use one another. For, example, how do I compile this?

z3 src # cat dependant.h
typedef struct egg_t egg_t;
typedef struct chicken_t chicken_t;

typedef struct egg_t {
        int                             egg_num;
        struct chicken_t                chicken;
} egg_t;

typedef struct chicken_t {
        int                     chicken_num;
        struct egg_t                    egg;
} chicken_t;
z3 src # gcc -c dependant.h
dependant.h:6:20: error: field 'chicken' has incomplete type
dependant.h:7:3: error: redefinition of typedef 'egg_t'
dependant.h:1:22: note: previous declaration of 'egg_t' was here
dependant.h:12:3: error: redefinition of typedef 'chicken_t'
dependant.h:2:26: note: previous declaration of 'chicken_t' was here
z3 src #

Answer 1

You can create pointers to incomplete types (nixing the typedefs since they don't add anything):

struct egg_t;
struct chicken_t;

struct egg_t {
  int egg_num;
  struct chicken_t *chicken;
};

struct chicken_t {
  int chicken_num;
  struct egg_t *egg;
};

Of course, the right answer is to avoid this kind of circular dependency in the first place, but that's not always possible.