What is dllspec(dllimport) and dllspec(dllexport) means

Question

After googling, i came to know that Dllimport makes the function available for other modules,

is it mandatory to declare function with extern "c" identifier?

Also, Dllexport means, Dll itself uses the function while compiling it says. so by default all

functions present in DLL are dllexport?

Solution 1

__declspec(dllexport) exports a symbol. It makes it available from outside a DLL.

__declspec(dllimport) imports a symbol. It practically says "this symbol is not defined in this application, it needs to be imported from a DLL file".

You don't have to declare it with extern "C". If you don't use extern "C", then the symbol will be exported as a C++ symbol, and you will only be able to call it from C++ (and languages that support calling C++ DLLs). If you use extern "C", then the symbol will be exported as a C symbol, and you will be able to call it from languages that support caling C DLLs.

If you want to use your DLL in C#, you will need to use extern "C".

Here is an excellent tutorial that shows you how to use a C++ DLL in C#: How to marshal a C++ class. I have used solution A in many projects at work.

Also, here is a short tutorial on how you can use a C++ DLL in another C++ application: How to create and use DLL in C++.

Solution 2

No -- dllexport means you're exporting it from the DLL (or from an executable) so that other modules (DLLs or executables) can use that function.

dllimport is used to declare a function that's implemented in a DLL (or, again, executable).

So, in a typical case, you'll have something like:

#ifdef BUILDDLL
#define DLL declspec(dllexport)
#else
#define DLL declspec(dllimport)
#endif

Then each public function the DLL will be marked as DLL:

DLL int dosomething(int);

Then, when you're building the DLL, you'll define BUILDDLL, to have all those functions marked as dllexport. Otherwise, you'll include the same header in client code that needs to use the function(s). It won't define BUILDDLL, so they'll all be marked as dllimport instead, so when it comes to link time, it'll create a link to that DLL instead of trying to satisfy those functions from someplace like the standard library.