calling a function declared in header file

Question

I have xh file and yh file, zc file. zc includes yh and yh includes xh I have a function that need to be declared in xh and defined in zc, because it needs to use some other functions in yh

I tried like this

// x.h
int foo();

// y.h
#include "x.h"

// z.c
#include"y.h"
int foo() {
   blah;
   blah;
   return 1;
}

this is all in one project lets say in x_y.dll and it compiled well and i have x_y.dll, x_y.lib

Now in other project a_b in one of the ac file I am trying to include xh and call foo(); compiles well, but I am having the linker error lnk2019 and lnk1120 unresolved external symbols

I am working on VS2008, I have the path set to the folder where x_y.lib located.

Answer 1

Create a macro like so:

#ifndef DLL_IFACE
#ifdef DLL_IFACE_EXPORT
#define DLL_IFACE _declspec( dllexport )
#else  // !DLL_IFACE_EXPORT
#define DLL_IFACE _declspec( dllimport )
#endif // !DLL_IFACE_EXPORT
#endif // !DLL_IFACE

And put this in some header included by everyone. Then in xh:

DLL_IFACE int foo();

then in zc, start that file off with:

#define DLL_IFACE_EXPORT

This will cause the function to be declared "_declspec( dllexport )" when you build the dll that defines the function. And will cause the function to be declared "_declspec( dllimport )" in other dlls.

The dllexport tells the compiler that the given function should be part of a dll's public interface. It put the function in the dll's dynamic symbol table. dllimport tells the compiler that the function will be imported through dynamic linking later.