How to partially declare a struct that is typedef'ed when in an include file

Question

I'm trying to minimize interdependence of #include files as a general practice.

In xxx.h I have:

struct my_struct;  // partial decl to satisfy use of my_struct*
void funct(struct my_struct* ms);  // uses the partial def

How to do a similar partial decl with a typedef'd struct? I have an actual decl in some third #include that looks like (say in yyy.h):

typedef struct my_data_s {
  int ival;
  ... struct's other components ...
} my_data_t;

I just want a representative decl in xxx.h that reference the typedef:

typedef struct my_data_s  my_data_t;  // actual full decl is elsewhere
void funct2(my_data_t* md);   

This attempt causes 'redefinition of typedef my_data_t' error. (Using gcc 4.4.3 / Ubuntu 10.4) Other random search attempts (e.g., add '{}' to typedef) also give errors.

I know the compiler only needs to know that the function requires a pointer, so it seems like this should be possible. So far, found nothing that compiles w/o errors/warnings.

I've looked at the other questions and answers, could not find this problem addressed. Seems like there should be a well-known way to do this(?!) (I know that I can #include yyy.y whenever I #include xxx.h - trying to avoid such dependencies.) Thanks.

Answer 1

Have you tried the simple approach:?

xxx.h

struct my_data_s;
typedef struct my_data_s my_data_t;

yyy.h

#include "decl.h"
struct my_data_s {
   int foo;
};

Answer 2

C99 doesn't allow to repeat a typedef, C11 does.

Just do the typedef only once and always have it first:

typedef struct my_data  my_data;

There also is no need to chose different names for the struct tag and the typedef identifier.