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Note that the two different ways a character is used as an int (as an unsigned char + EOF, or as a plain char, converted to int) can lead to confusion. For example, isspace('\200') results in undefined behavior when char is signed.

unsigned char

• Used internally for string comparison functions, even though these operate on character data.  Consequently, the result of a string comparison does not depend on whether plain char is signed.
• Used for situations where the object being manipulated might be of any type, and it is necessary to access all bits of that object, as with fwrite().

Unlike other integer types, unsigned char has the unique property that (quoting from Section 6.2.6.1 of C99):

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That is, objects of type unsigned char may have no padding bits and thus no trap representation. Thus, non-bit field objects of any type may be copied into an array of unsigned char (e.g., via memcpy()) and have their representation examined one byte at a time.

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wchar_t

• Wide characters are used for natural-language character data.

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