C Functions not being exported

Question

After I had finished an assigment, in which one had to implement a Win32 window, I wanted to export my functions to a dll.

Surprisingly the linker complains about unreferenced links, altough

• __declspec(dllexport) / __declspec(dllimport) is defined and used correctly (or as I am used to from C++).
• the library is correctly specified to the linker.
• the functions are visible within dependency walker.

The linker always complains (in german):

error LNK2019: Verweis auf nicht aufgel÷stes externes Symbol ""__declspec(dllimport) struct cw_Window * __cdecl createWindow(char *,unsigned short,unsigned short)" (__imp_?createWindow@@YAPEAUcw_Window@@PEADGG@Z)" in Funktion "main". C:\\Users\\jkirs\\Desktop\\Workspace\\MSVC2015_x86_64-release\\Unit.MSVC2015-x86-64.88a6cdd3\\intermediate.Unit.exe : fatal error LNK1120: 1 nicht aufgel÷ste Externe

My functions prototypes are defined as following:

typedef struct cw_Window    cw_Window_t;
typedef struct cw_Event     cw_Event_t;

API cw_Window_t*    createWindow(char* pTitle, uint16_t nWidth, uint16_t nHeight);
API void            destroyWindow(cw_Window_t* pWindow);
API void            pollEvent(cw_Window_t* pWindow, cw_Event_t* pEvent);

where as "API" is defined as following:

#ifdef _MSC_VER
    #ifdef CW_EXPORT
        #define API __declspec(dllexport)
    #else
        #define API __declspec(dllimport)
    #endif
#else
    #define API // TODO
#endif

I tried to add 'extern "C" to the 'API' definition as well, without a result, but complaining about the string literal 'C'.

Has anyone come across this issue himself yet and can point me towards the right direction?

If it matters: I am uinsg Visual Studio C++ 2015 (MSVC_x86_64); my header files end with '.h' and source files end with '.c'.

EDIT: The lib is supposed to be used in C code again.

Answer 1

So it seems you knew the basic problem and approximate solution with extern "C" . But your mistake is: don't put extern "C" in the API define. Wrap the definitions and declarations with it:

typedef struct cw_Window    cw_Window_t;
typedef struct cw_Event     cw_Event_t;

extern "C" {
API cw_Window_t*    createWindow(char* pTitle, uint16_t nWidth, uint16_t nHeight);
API void            destroyWindow(cw_Window_t* pWindow);
API void            pollEvent(cw_Window_t* pWindow, cw_Event_t* pEvent);
}

I know it looks like extern and what you were trying seems like a sensible place to put extern, but extern "C" is a different beast with different rules; you can read more than you want to about it here In C++ source, what is the effect of extern "C"?

Answer 2

After all, everything I made was just fine but the build system. Turns out the buildsystem treaded the file as an C++ Source File instead of plain C.