See ctype(3) and search for CAVEATS:
CAVEATS
The argument of these functions is of type int, but only a very
restricted subset of values are actually valid. The argument must either
be the value of the macro EOF (which has a negative value), or must be a
non-negative value within the range representable as unsigned char.
Passing invalid values leads to undefined behavior.
Clearly casting EOF to unsigned char will give an invalid input value to
any ctype function.
The whole interface is ... suboptimal, but there is nothing we can do about
it.
You need to first test for EOF but then cast any non-EOF characters to
unsigned char to be portable:
for (;;) {
int c = fgetc(c);
if (c == EOF || !isprint((unsigned char)c))
break;
}
This, unfortunately, voids the reasoning (or "benefits") why EOF as int
was allowed to be passed to ctype functions in the first place.
Luckily the main usage nowadays is with in-memory strings and there things
are simpler (as explained in that CAVEAT section).
Martin
