>Is there really a functional difference between stdout and stderr?
>Isn't buffering handled the same way for stdout and stderr?

No.  Buffering (on HP-UX) for stdout is full unless it is a terminal,
else it is line buffering.

For stderr it isn't buffered.  But since this is expensive, HP-UX
uses line buffering and a buffer size of 8.

Perhaps stdio(3s) also explains it for MPE?