library: .lib, .dll

A library is a package of code that is meant to be reused in many programs. Typically, a library includes a header file that contains declarations for everything the library wishes to expose (make public) to users, and a precompiled object that contains all of the implementation code compiled into machine language. These libraries typically have a .lib or .dll extension on Windows, and a .a or .so extension on Unix. Why are libraries precompiled? First, since libraries rarely change, they do not need to be recompiled often, if ever. It would be a waste of time to compile them every time you wrote a program that used them. Second, because precompiled objects are in machine language, it prevents people from accessing or changing the source code, which is important to businesses or people who don’t want to make their source code available for intellectual property reasons.