Description:
******Use the typedef keyword to define a new data type.
***For example: typedef unsigned short U16, when defining variables, the definition of unsigned short a and U16 a are equivalent.
****** It is not recommended to use #define to define new data types, because #define cannot handle pointer types correctly
***如：#define ptr_to_char char *
ptr_to_char a,b; ------>It is actually char *a,b; #define is just a simple replacement.