Forward declaration of typedef struct used in another typedef struct

Question

I would like to forward declare a typedef struct, to use it in another struct, and then to implement the original struct.

I have tried the following code but does not compile.

struct _s1;
struct _s2;

typedef struct _s1 s1;
typedef struct _s2 s2;

typedef struct _big_struct {
    s1 my_s1;
    s2 my_s2;
} big_struct;

struct _s1 {
    int i1;
};

struct _s2{
    int i2;
};

Any idea?

Answer 1

You can only forward declare the existence of a type and then use a pointer to it. This is because the size of a pointer is always known, while the size of a forward declared compund type is not, yet.

struct s1;
struct s2;

struct big_struct {
    struct s1* pmy_s1;
    struct s2* pmy_s2;
};

struct s1 {
    int i1;
};

struct s2{
    int i2;
};

Note, because of my background, I am used to writing extremely backward-compatible code.
Jonathan Leffler has provided information on need/not-need in more modern versions of the C standard. See the comments below.

Answer 2

If you are really forced with that order (I don't care why), one thing that comes in my mind to have it compile is by making the struct _big_struct entries pointers:

typedef struct s1 s1;
typedef struct s2 s2;

typedef struct _big_struct {
    s1 *my_s1;
    s2 *my_s2;
} big_struct;