Putting opaque struct definition into a separate header file

Question

I'm designing a library with a public interface containing opaque struct declaration:

lib_public.h:

typedef struct lib_struct lib_struct;

void foo(lib_struct *ptr);

void bar(lib_struct *ptr);

The lib_struct opaque struct hides OS-specific implementation details so it seems to be a bad design to put it into the lib_struct.h directly. But I still want to write unit tests for it which use its members. Currently I decided to create a separate private header file containing just the struct definition:

lib_struct_linux.h:

struct lib_struct{
   int epoll;
   int acceptor_socket;
}

So the implementation lib_struct.c as well as unit test lib_struct_test.c will include this header as follows:

lib_struct.c:

#include "lib_struct_linux.h"

//function definition

lib_struct_test.c:

#include "lib_struct_linux.h"

//unit tests

The thing is such a design seems messy in the sense that the struct is defined in one private header file (lib_struct_linux.h), the functions to work with the struct are declared in another public header file (lib_public.h). And the definition of the functions in yet another implementation file (lib_struct.c).

Is it common approach? If no, how would it be possible to design it in a better way.

Answer 1

Yes, this is perfectly fine.

The thing is such a design seems messy in the sense that the struct is defined in one private header file (lib_struct_linux.h), the functions to work with the struct are declared in another public header file (lib_public.h). And the definition of the functions in yet another implementation file (lib_struct.c).

Allow me to reword this: "The public interface is in a public header, the implementation-only declarations are in a private header, and implementation is in a source file." Doesn't sound messy at all, in fact, it sounds like perfectly good design to me.