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You are right, the code should use iswprint instead.

The point is that multiple subsequent bytes can make up a
single printable character. Not every character above 127 is
necessarily printable (e.g. in Latin-1, only characters
above 160 are printable). Likewise, a single byte may not be
printable, but a combination will print fine. So this code
is supposed to catch only those cases where printing will
actually work.