Importing a C DLL's functions into a C++ program - c++

Question

I have a 3rd party library that's written in C. It exports all of its functions to a DLL.
I have the .h file, and I'm trying to load the DLL from my C++ program.
The first thing I tried was surrounding the parts where I #include the 3rd party lib in
#ifdef __cplusplus
extern "C" {
#endif
and, at the end
#ifdef __cplusplus
} // extern "C"
#endif
But the problem there was, all of the DLL file function linkage looked like this in their header files:
a_function = (void *)GetProcAddress(dll, "a_function");
While really a_function had type int (*a_function) (int *). Apparently MSVC++ compiler doesn't like this, while MSVC compiler does not seem to mind.
So I went through (brutal torture) and fixed them all to the pattern
typedef int (*_x_a_function) (int *); // using _a_function will not work, C uses it!
_x_a_function a_function ;
Then, to link it to the DLL code, in main():
a_function = (_x_a_function)GetProcAddress(dll, "a_function");
This SEEMS to make the compiler MUCH, MUCH happier, but it STILL complains with this final set of 143 errors, each saying for each of the DLL link attempts:
error LNK2005: _x_a_function already defined in main.obj main.obj
Multiple symbol definition errors.. sounds like a job for extern! SO I went and made ALL the function pointer declarations as follows:
function_pointers.h
typedef int (*_x_a_function) (int *);
extern _x_a_function a_function ;
And in a cpp file:
function_pointers.cpp
#include "function_pointers.h"
_x_a_function a_function ;
ALL fine and dandy.. except for linker errors now of the form:
error LNK2001: unresolved external symbol _a_function main.obj
Main.cpp includes "function_pointers.h", so it should know where to find each of the functions..
I am bamboozled. Does any one have any pointers to get me functional? (Pardon the pun..)

Answer 1

Linker errors like that suggest you've defined all the functions in function_pointers.cpp, but forgotten to add it to the project/makefile.
Either that, or you've forgotten to "extern C" the functions in function_pointers.cpp too.

Answer 2

I believe that if you declared the typedefs and/or the prototype as extern "C", you must remember to extern "C" the definition too.

Answer 3

When you link C functions the prototypes will get a leading _ in front of them by default so
when you do a typedef using the same name
typedef int (*_a_function) (int *);
_a_function a_function
you'll get issues because there is already a function in the dll named _a_function.

Answer 4

Usually you declare a function in yourlibrary.h like extern "C" __declspec(dllexport) int __cdecl factorial(int); then create that function in yourlibrary.c:
extern "C" __declspec(dllexport) int __cdecl factorial(int x) {
if(x == 0)
return 1;
else
return x * factorial(x - 1);
}
After compiling your DLL you get your .dll and .lib files. The latter is used when you want to import your functions to the project. You put #include "yourlibrary.h" and #pragma comment(lib, "yourlibrary.lib") in your project, after this you can use int factorial(int) in you application.