Subroutine

A subroutine is a named sequence of program instructions that can be invoked from multiple points in a program. It accepts inputs, called parameters or arguments, and transfers control back to the caller when finished. Subroutines help modularize code, promote reuse, and reduce duplication by encapsulating a task in a single, reusable unit.

Historically, the term subroutine comes from early programming language design, where it competed with terms such as procedure, function, and method to describe similar concepts. In some languages, the word subroutine implies a unit that does not return a value (a procedure), while in others it is a neutral term encompassing both value-returning and non-returning units. The distinction between subroutine and function is language-dependent; in some environments, a function is a subroutine that returns a value, whereas in others the terms are used more interchangeably.

Implementation and behavior vary by language, but a subroutine generally has an interface (a name and a parameter list) and an implementation (the body). When called, an activation record or stack frame is created to hold parameters, local variables, and the return address. Parameters may be passed by value, by reference, or by other mechanisms. Subroutines can be recursive and may have side effects on global state or I/O. They can be part of a module or library and may be inlined by a compiler under certain conditions. Effective use of subroutines supports separation of concerns, easier maintenance, and reuse across a codebase.