What is the meaning of the indicator XXX in the Unicode charts
unicode
Solution
According to the explanation in Ch. 17 (About the Code Charts) of the Unicode Standard, p. 573, by the “Dashed Box Convention”, characters that have no visible rendering as such “are represented by a square dashed box. This box surrounds a short mnemonic abbreviation of the character’s name.” The characters referred to in the questions are control characters, in the C1 Controls area.
The Unicode Standard says, in Ch. 16, p. 544, about C0 and C1 Controls: “The Unicode Standard provides for the intact interchange of these code points, neither adding to nor subtracting from their semantics. The semantics of the control codes are gen-erally determined by the application with which they are used. However, in the absence of specific application uses, they may be interpreted according to the control function semantics specified in ISO/IEC 6429:1992.” And the abbreviations in the square dashed boxes reflect the meanings given in ISO/IEC 6429:1992.
Some code points in the C1 Controls area are not defined in ISO/IEC 6429:1992. For them, such as U+0080, the code chart has “XXX” in place of a mnemonic abbreviation. So this indicates that the Unicode standard does not refer to any meaning for those code points, beyond their being control characters with some abstract properties.
Thus, “XXX” does not mean “invalid”, but rather “completely undefined meaning”. The meaning of such code points can be defined by various standards or other conventions, as long as they are consistent with the general definitions—e.g., it would be incompatible to define U+0080 as a graphic character.
Such code points must not be replaced or omitted in any character-level processing; applications that actually change data may do whatever they want, but any general conversion routines, for example, must keep these code points (characters) intact. They must not be treated as malformed or invalid; but an application may treat them as undefined. By Unicode principles, it’s OK to be ignorant of a character, but not completely wrong about it.
This has nothing to do with the meaning of bytes like 0x80 in 8-bit codes like Windows-1252. But if you send e.g. data labeled as ISO-8859-1 encoded (where e.g. 0x80 is in principle U+0080) to a web browser, it will actually treat it as Windows-1252 encoded. The reason is that characters like U+0080 are practically never used in ISO-8859-1 data; occurrence of 0x80 in ISO-8859-1 labeled data is virtually always either windows-1252 mislabeled or messed-up data that cannot be meaningfully processed. So browsers take the practical route and treat ISO-8859-1 as windows-1252; this is being formalized in HTML5 and related specifications.
Problem
Consider the unicode chart for C1 Controls and Latin-1 supplement in Unicode Charts. If a character has a glyph, it is shown, if it does not have a glyph, a special dotted line and symbolic marker or identifier is given. In this case, both 0080 and 0081 seem to have some "invalid marker", which I think is what "XXX" means. Is that what it means? Secondly, what should be the behaviour of a Unicode aware string type that has a value stored into the string of value 0x80 (hex) or 128 (decimal)? Should it be converted to some other point, such as the mapping like this: - Byte Value 128 in many ANSI Codepages is the EURO marker. - Storing a 128 decimal value is equivalent to storing U+20AC ? The magic "non orthogonality" I have encountered in a particular language or operating system API implementation of its MBCS and Unicode types, and Java's interesting handling, leads me to wonder, what is the real intended use of the U+0080 character? This reference link confuses me by showing that Java treats this character as a Euro symbol (ANSI codepage to Unicode one way friendliness) but that it's name is `<control>` which is not anything I know how to deal with. Wikipedia says it's `PAD` here Can anyone help me? Did I skip a foundational concepts day at Unicode School? What am I missing? Update The block from 0080 to 0098 is non printable control characters. This much I know. What I wonder is what does the XXX mean and how am I to think of this character when I am processing unicode data with this value in it?