可以被其它程序调用的具有特定功能的一段相对独立的程序模块，在C语言中称为函数。引入函数的目的主要是解决两个问题一是为了解决代码的重复。如果一个程序段需要在程序中出现多次，每次都写出来既麻烦又使得程序变长此时可把该程序段定义成一个函数。在需用该程序段的地方直接调用。二是结构化模块化编程的需要。(A relatively independent program module that has a specific function that can be invoked by another program, called function in the C language. The main purpose of introducing a function is to solve two problems, one is to solve the duplication of code. If a program section needs to appear multiple times in the program, every time to write out the troublesome and makes the program at this time the share variable length segment is defined as a function. Call directly where it is required. Two is the need of structured modular programming.)