A function may or may not return a value. If yes, you need to specify the type, such as char *f (), return string pointer type data, struct ptl f2 (), and return structured data. If it is not, it needs to be represented by void, such as void f2 (), which means that no data of any type is returned. For the convenience of use, some C languages omit the representation of returning plastic data, such as f4 (), which means that the function f4 () returns a plastic data. System compilers believe that returning shaped data is the most common.