1、Binary source data must be a single-dimensioned array of unsigned char.
2、If you need more, read the data in as a stream of unsigned char objects and do the math.
3、Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).
4、For example, if a function returns an unsigned char value but calculated a result of 256 or more (or -1 or less), errno would be set to ERANGE and the function would return some irrelevant value.
