How to call a function in DLL defined another project - c++

Question

I'd like to call a function in DLL (lets say foo function defined in a X project ). So DLL never knows the exact implementation of this foo function. But later I want to use this DLL in the X project (that contains the foo function).
When I try to compile the DLL, linker gives the "Unresolved external" error. How can I tell the compiler that foo function is defined in another project.
For example;
foo.cpp in DLL project
extern void foo();
void DLLFunction()
{
foo();
}
impl.cpp in X project
void foo()
{
//Do something
}
And if I can compile foo.cpp in the DLL project, I will use this DLL in the X Project.

Answer 1

If I understand the question correctly, you want to have a DLL, which imports a function from the executable it is linked against.
In general it is possible, but tricky, so first thing you should consider is if you really need to do that or if you can change the DLL function to e.g. accept a function pointer (which then does not require to import a symbol from executable into the DLL).
It is still possible to use symbol from executable in the DLL. It is possible to export a symbol (function) from the executable and use in the DLL (normally the same way you export from DLL, e.g. "__declspec(dllexport)" on Windows). However the tricky part is, that by default you need the dependency to link the particular object, i.e. in this case you'd need the DLL to link the executable and the executable to link the DLL, which is a cyclic dependency.
So you need to break the cyclic dependency. There are some possibilities:
Loading the DLL dynamically as a plugin - using dlopen() on Posix and LoadLibrary() under Windows. Then you do not need the DLL to build the executable, and you can link the EXE providing the symbol with the DLL.
The other way around - on Windows, you can dynamically import the exported symbol from the executable into the DLL (GetModuleHandle(), GetProcAddress()).
Allowing the DLL to be build with unresolved symbols. Then you build the DLL with the symbol unresolved, and build the EXE against the DLL. When executed, the executable will load the DLL and will provide the symbol.
To allow building the DLL with unresolved symbols:
In Linux GCC this is the default behaviour (shared objects do not enforce symbols to be resolved), unless you build with "-Wl,--no-undefined".
In Windows MSVC, you can ignore the unresolved symbols when building the DLL by providing the "/FORCE:UNRESOLVED" parameter to the linker. Note that on Windows/MSVC, after you build the EXE .lib file, you then need to rebuild the DLL again against that file to import the symbol.

Answer 2

What you can do is to instead of writing extern, you can load the DLL dynamically
e.g.
auto hd = LoadLibrary("YourDll.DLL");
typedef void(*userfunc)() ;
userfunc p = GetProcAddress(hd, "foo");
p();
Alt. link with the .lib file created when you built the DLL
discalimer: code for cleanup, error checks etc omitted.

Answer 3

If you are using a recent version of the Visual Studio IDE, you can simply reference it - Visual Studio will take care of the rest. To do that, right click on the "References" entry in your Solution Explorer for the project you want to link against your DLL. From there, select the other DLL project in your solution:
This will take care of all the linking required and saves you the trouble of changing your project settings if you rename your projects or change the name of the output-files.
If you don't have the other project in your solution you will have to stick with the regular static linking procedure:
Project Settings -> Linker -> General -> Additional Library Directories
Project Settings -> Linker -> Input -> Additional Dependencies
If you want to dynamically link against the DLL at runtime, you can use the method described by Andreas K.