Why use fopen() mode 'b' (stdio.h) when output can be non-ASCII regardless?

Question

With the C standard library stdio.h, I read that to output ASCII/text data, one should use mode "w" and to output binary data, one should use "wb". But why the difference?

In either case, I'm just outputting a byte (char) array, right? And if I output a non-ASCII byte in ASCII mode, the program still outputs the correct byte.

Answer 1

Some operating systems - mostly named "windows" - don't guarantee that they will read and write ascii to files exactly the way you pass it in. So on windows they actually map \r\n to \n. This is fine and transparent when reading and writing ascii. But it would trash a stream of binary data. Basically just always give windows the 'b' flag if you want it to faithfully read and write data to files exactly the way you passed it in.