C/C++ How Does Dynamic Linking Work On Different Platforms?

Question

How does dynamic linking work generally?

On Windows (LoadLibrary), you need a .dll to call at runtime, but at link time, you need to provide a corresponding .lib file or the program won't link... What does the .lib file contain? A description of the .dll methods? Isn't that what the headers contain?

Relatedly, on *nix, you don't need a lib file... How how does the compiler know that the methods described in the header will be available at runtime?

As a newbie, when you think about either one of the two schemes, then the other, neither of them make sense...

Answer 1

To answer your questions one by one:

• Dynamic linking defers part of the linking process to runtime. It can be used in two ways: implicitly and explicitly. Implicitly, the static linker will insert information into the executable which will cause the library to load and resolve the necessary symbols. Explicitly, you must call LoadLibrary or dlopen manually, and then GetProcAddress/dlsym for each symbol you need to use. Implicit loading is used for things like the system library, where the implementation will depend on the version of the system, but the interface is guaranteed. Explicit loading is used for things like plug-ins, where the library to be loaded will be determined at runtime.

• The .lib file is only necessary for implicit loading. It contains the information that the library actually provides this symbol, so the linker won't complain that the symbol is undefined, and it tells the linker in what library the symbols are located, so it can insert the necessary information to cause this library to automatically be loaded. All the header files tell the compiler is that the symbols will exist, somewhere; the linker needs the .lib to know where.

• Under Unix, all of the information is extracted from the .so. Why Windows requires two separate files, rather than putting all of the information in one file, I don't know; it's actually duplicating most of the information, since the information needed in the .lib is also needed in the .dll. (Perhaps licensing issues. You can distribute your program with the .dll, but no one can link against the libraries unless they have a .lib.)

The main thing to retain is that if you want implicit loading, you have to provide the linker with the appropriate information, either with a .lib or a .so file, so that it can insert that information into the executable. And that if you want explicit loading, you can't refer to any of the symbols in the library directly; you have to call GetProcAddress/dlsym to get their addresses yourself (and do some funny casting to use them).