Printf sets errno to 25?

[Doug Gwyn <gwyn>]

isatty() is not the only culprit; other "section 3"
C library routines also can set errno as a side-effect
of successful operation.  (This can legitimately be
considered a misfeature, but that's the way it is.)
You should only test errno when a system call (section
2 routine) returns an error indication.  Nothing in
the C library should ever set errno to 0, although
if the misfeature were to be corrected some routines
would have to save and restore errno.