__declspec(dllimport) how to load library

问题:

http://msdn.microsoft.com/en-us/library/9h658af8.aspx

MSDN says I can export function from the library with __declspec(dllexport) but how can I load this library in my executable?

I've got an exported function in DLL:

 __declspec(dllexport) void myfunc(){}

And now I would like to use it in my executable:

 __declspec(dllimport) void myfunc(void);

But how my program will know where to find this function?

回答1:

This is the compiler/linker job, it's done automatically as long as you

1. include the .lib in the Linker options
2. provide the DLL at runtime so that it's found by the exe

The .lib file is generated when you compile the DLL, or is shipped with it if it's not your code. In this case the code is compiled with __declspec(dllexport).

When compiling your exe, the compiler sees that the included function is to be found in DLL. In this case the code is compiled with __declspec(dllimpport).

The linker is provided with the .lib file, and generates appropriate instructions in the exe.

These instructions will make the Exe find the DLL and load the exported function at runtime. The DLL just has to be next to the Exe (there are other possible places, however).

Switching between __declspec(dllimpport) and __declspec(dllexport) is done by a macro, provided by Visual C++ when creating a DLL project.

回答2:

If you are using a DLL, you can to use the LoadLibrary and GetProcAddress combination.

//Load the DLL
HMODULE lib = LoadLibrary("testing.dll");

//Create the function
typedef void (*FNPTR)();
FNPTR myfunc = (FNPTR)GetProcAddress(lib, "myfunc");

//EDIT: For additional safety, check to see if it loaded
if (!myfunc) {
    //ERROR.  Handle it.
}

//Call it!
myfunc();

回答3:

Your operating system will be able to find it based on the linking process. If your library is linked properly to your program, it will recognize that there is an external function being used, and look for it in the dll paths. If it can't find it, your linker will throw an error.

I recommend doing some reading into the linking process; it can be confusing at times but understanding it may help you grasp some key concepts in C/C++.