Extending an API in C in a flexible and unobtrusive way

I'm trying to add some additional functionality to an API. However, I'd like these additional function开发者_如何转开发s to be in an external library of mine and not mixed with the original library's code.

The problem comes when I need to access static functions of the mentioned API from my functions. Of course I can't, so the only solution I see is either to copy these functions' code into my API or to make them non-static in the original API. Both are not too good options for me for obvious reasons.

More precisely:

original_api.c
  void some_function() -> uses some_helper_function()  
  static some_helper_function()

my_api_extension.c
  void some_extended_function() -> needs to use some_helper_function from original_api.c, but can't

Could you suggest which would be the most flexible way to handle this?

I'd like to point out that it's related to C only, not C++.

1. Make the functions static.
2. Create an extern struct with pointers to these functions. Declare it in a separate #include file, available to the extension, but not to the entire world.
3. Use the struct in your extension.

Something like this:

// in the private header
typedef struct
{
  void (*p_myfunc1)(int);
  int (*p_myfunc2)(void);
} privateAPI_t;
extern privateAPI_t privateAPI;

// in the library
static void myfunc1(int);
static int myfunc2(void);

privateAPI_t privateAPI = { myfunc1, myfunc2 };

// in the extension
#include <privateAPI.h>
...
privateAPI.p_myfunc1(privateAPI.p_myfunc2());