When I want to call some Windows function, like MessageBox I can import it from user32.dll and call (with LoadLibrary and GetProcAddress). But there is also a static library that Visual C++ uses, so I don't need to manually load DLLs and functions. How do they work? Do they contain wrappers that call LoadLibrary/GetProcAddress every time I call a function?
2 Answers
6
votes
The "static library" that you're referring to is actually an import library. This type of library contains records that tell the linker which library each function actually exists in, and doesn't contain any code itself. The linker creates import records in the executable, which the loader resolves at load time. This fixes up the addresses used at runtime so your code doesn't need to explicitly call LoadLibrary and GetProcAddress.
1
votes
No, they just allow you to use static linking with DLLs. The executable file contain a list of functions that it needs from other DLL files, so when the executable file is loaded the loader parses this list and resolves each function with LoadLibrary and GetProcAddress, saving result to a static table (IAT, Imported Address Table). This is done only once. There is also the notion of "delayed load" of DLL, that will resolve the address only when the function is call for the first time, but it's rarely used.
In this way the lib file for a DLL contains just information needed to build that list (names of exported functions).
