Final word is Bruce's quote:
C9x specifies the BSD4.4-Lite behaviour:
[#3] ... Thus, the
null-terminated output has been completely written if and
only if the returned value is less than n.
It means that if we not have any null-terminated output as for n == 0
we can't return value less than n, so we forced to return value
equal to n i.e. 0
The next good thing is glibc compatibility, of course.
2) Do check for too big n in machine-independent way.
3) Minor optimization assuming EOF is < 0
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