 What distinguishes the call as being to a library function, versus being to another function in the same program, is the way that the code is organized in the system.Library code is organized in such a way that it can be used by multiple programs that have no connection to each other, while code that is part of a program is organized to be used only within that one program.This distinction can gain a hierarchical notion when a program grows large, such as a multi-million-line program.