One quirk of these functions, which the authors mention briefly, is that although they accept arguments of type int, it is not legal to pass just any int value to them. If you were to attempt to call isupper(12345), it might do something bizarre. You should only call these functions with arguments which represent valid character values. (Also, they are guaranteed to accept the value EOF gracefully.)
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This page by Steve Summit // Copyright 1995, 1996 // mail feedback