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 fu