1. Macro definition define takes effect on the entire file, of course, it is defined in the header, it will replace the predefined content in the whole file accordingly. To put it simply, the mutation does not check whether U8 and uint8_t are defined, because they are not variables in nature. So, it doesn't contradict typedef unsigned char unit8_t .
2. can you combine two sentences into one? the answer is yes, but this definition is not intuitive, overly concise, and sometimes not very good.

in fact, there is no direct relationship between the two statements.

-sharpdefine is a macro definition and belongs to the preprocessing phase. That is, in the preprocessing phase, the preprocessor replaces all U8 in the program with uint8_t .
typedef is a type definition, which is equivalent to defining a new type uint8_t , which you must define if you use uint8_t in your code.

1. generally speaking, you only need to have typedef unsigned char unit8_t before using U8 and uint8_t .
2. of course you can write that