5.7. Platform-Dependent Character Sets
In addition to the standard character sets discussed previously, many vendors have at one time or another produced proprietary character sets to meet the needs of their specific platform. Often, they contain special characters the vendor saw a need for, such as Apple's trademarked open apple or the box-drawing characters such as and used for cell boundaries in early DOS spreadsheets. Microsoft, IBM, and Apple are the three most prolific inventors of character sets. The single most common such set is probably Microsoft's Cp1252, a variant of Latin-1 that replaces the C1 controls with more graphic characters. Hundreds of such platform-dependent character sets are in use today. Documentation for these ranges from excellent to nonexistent.
Platform-specific character sets like these should be used only within a single system. They should never be placed on the wire or used to transfer data between systems. Doing so can lead to nasty surprises in unexpected places. For example, displaying a file that contains some of the extra Cp1252 characters , , ^, , ", , ..., , , , ·, `, ', ", ", -, --, , , ™, , and ~ on a VT-220 terminal can effectively disable the screen. Nonetheless, these character sets are in common use and often seen on the Web even when they don't belong there. There's no absolute rule that says you can't use them for an XML document, provided that you include the proper encoding declaration and your parser understands it. The one advantage to using these sets is that existing text editors are likely to be much more comfortable with them than with Unicode and its friends. Nonetheless, we strongly recommend that you don't use them and stick to the documented standards that are much more broadly supported across platforms.
The most common platform-dependent character set, and the one you're most likely to encounter on the Internet, is Cp1252, also (and incorrectly) known as Windows ANSI. This is the default character set used by most American and Western European Windows PCs, which explains its ubiquity. Cp1252 is a single-byte character set almost identical to the standard ISO-8859-1 character set--indeed, many Cp1252 documents are often incorrectly labeled as being Latin-1 documents. However, this set replaces the C1 controls between code points 128 and 159 with additional graphics characters, such as , , and . These characters won't cause problems on other Windows systems. However, other platforms will have difficulty viewing them properly and may even crash in extreme cases. Cp1252 (and its siblings used in non-Western Windows systems) should be avoided.
The Mac OS uses a different nonstandard, single-byte character set that's a superset of ASCII. The version used in the Americas and most of Western Europe is called MacRoman. Variants for other countries include MacGreek, MacHebrew, MacIceland, and so forth. Most Java-based XML processors can make sense out of these encodings if they're properly labeled, but most other non-Macintosh tools cannot.
For instance, if the French sentence "Au cours des dernières années, XML a été adapte dans des domaines aussi diverse que l'aéronautique, le multimédia, la gestion de hôpitaux, les télécommunications, la théologie, la vente au détail et la littérature médiévale" is written on a Macintosh and then read on a PC, what the PC user will see is "Au cours des derni?res annžes, XML a žtž adapte dans des domaines aussi diverse que l'ažronautique, le multimždia, la gestion de h™pitaux, les tžlžcommunications, la thžologie, la vente au džtail et la littžrature mždižvale," not the same thing at all. Generally, the result is at least marginally intelligible if most of the text is ASCII, but it certainly doesn't lend itself to high fidelity or quality. Mac-specific character sets should also be avoided.
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