C程序由函数构成，这些函数既可以放在一个文件中，也可以放在几个文件中。在若干函数中有且只能有一个main函数，程序的执行总是从main函数开始，执行完main函数返回系统。-C program constituted by the function, these functions either on a file, can also be placed in several files. And there can be only one main function in certain functions, program execution always start from the main function, performing the main function returns the system.