C51强大功能及其高效率的重要体现之一在于其丰富的可直接调用的库函数，多使用库函数使程序代码简单，结构清晰，易于调试和维护，下面介绍C51的库函数系统。-C51 powerful features and its high efficiency in one important manifestation of its rich can directly call the library, multi-use library functions to make the code simple, clear and easy to debug and maintenance, and C51 introduced below the library system.