Subroutines

A subroutine is a named sequence of instructions within a program that can be invoked from multiple places. It accepts zero or more input values, may produce a result, and returns control to the caller after completing its task.

In many languages, the terms subroutine, procedure, and function refer to similar concepts, with variation in whether a return value is produced. Object-oriented languages often use the term method for subroutines associated with objects.

Subroutines enable modularity, reuse, and clarity by encapsulating a task. They typically have parameters, which may be passed by value or by reference, and local variables whose scope is limited to the subroutine. A call creates a frame on the call stack.

Control flow transfers when a subroutine is called and returns when finished. Subroutines may be recursive, where a subroutine calls itself. Proper termination and base cases are essential to avoid infinite recursion and stack overflow.

Implementation notes: Subroutine calls carry overhead compared to inlined code. Techniques such as tail call optimization and inlining affect performance. Languages differ in parameter passing conventions and how activation records are structured.

History and usage: Subroutines appeared in early procedural programming and remain fundamental for structuring programs, enabling abstraction and reuse. They are implemented in many forms, including standalone procedures, functions, and methods within libraries and APIs.