A clarification: You can certainly assume that char is 8 bits if you don't mind losing portability to a small minority of systems.

If I'm being pedantic, I might add something like

    #if CHAR_BIT != 8
    #error "This code assumes 8-bit char"
    #endif

But realistically, if I'm using headers defined by either POSIX or Windows, that's probably enough of a guarantee. (Though I'd still use CHAR_BIT rather than 8 to refer to the number of bits in a byte.)