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What is a function and built-in function?

A large program is subdivided into a number of smaller programs or
 subprograms. Each subprogram specifies one or more actions to be
 performed for a large program. such subprograms are functions.
 The function supports only static and extern storage classes. By
 default, function assumes extern storage class. functions have global
 scope. Only register or auto storage class is allowed in the function

parameters. Built-in functions that predefined and supplied along with
 the compiler are known as built-in functions. They are also known as
 library functions.