分类： BSD
2011-03-10 12:45:14
Q: What is a BOM?
A: A byte order mark (BOM) consists of the character code U+FEFF at the beginning of a data stream, where it can be used as a signature defining the byte order and encoding form, primarily of unmarked plaintext files. Under some higher level protocols, use of a BOM may be mandatory (or prohibited) in the Unicode data stream defined in that protocol.
Q: Where is a BOM useful?
A: A BOM is useful at the beginning of files that are typed as text, but for which it is not known whether they are in big or little endian format—it can also serve as a hint indicating that the file is in Unicode, as opposed to in a legacy encoding and furthermore, it act as a signature for the specific encoding form used. [
Q: What does ‘endian’ mean?
A: Data types longer than a byte can be stored in computer memory with the most significant byte (MSB) first or last. The former is called big-endian, the latter little-endian. When data is exchanged, bytes that appear in the "correct" order on the sending system may appear to be out of order on the receiving system. In that situation, a BOM would look like 0xFFFE which is a noncharacter, allowing the receiving system to apply byte reversal before processing the data. UTF-8 is byte oriented and therefore does not have that issue. Nevertheless, an initial BOM might be useful to identify the datastream as UTF-8.
Q: When a BOM is used, is it only in 16-bit Unicode text?
A: No, a BOM can be used as a signature no matter how the Unicode text is transformed: UTF-16, UTF-8, UTF-7, etc. The exact bytes comprising the BOM will be whatever the Unicode character FEFF is converted into by that transformation format. In that form, the BOM serves to indicate both that it is a Unicode file, and which of the formats it is in. Examples:
| Bytes | Encoding Form |
|---|---|
| 00 00 FE FF | UTF-32, big-endian |
| FF FE 00 00 | UTF-32, little-endian |
| FE FF | UTF-16, big-endian |
| FF FE | UTF-16, little-endian |
| EF BB BF | UTF-8 |
Q: Can a UTF-8 data stream contain the BOM character (in UTF-8 form)? If yes, then can I still assume the remaining UTF-8 bytes are in big-endian order?
A: Yes, UTF-8 can contain a BOM. However, it makes no difference as to the endianness of the byte stream. UTF-8 always has the same byte order. An initial BOM is only used as a signature — an indication that an otherwise unmarked text file is in UTF-8. Note that some recipients of UTF-8 encoded data do not expect a BOM. Where UTF-8 is used transparently in 8-bit environments, the use of a BOM will interfere with any protocol or file format that expects specific ASCII characters at the beginning, such as the use of "#!" of at the beginning of Unix shell scripts.
Q: What should I do with U+FEFF in the middle of a file?
A: In the absence of a protocol supporting its use as a BOM and when not at the beginning of a text stream, U+FEFF should normally not occur. For backwards compatibility it should be treated as ZERO WIDTH NON-BREAKING SPACE (ZWNBSP), and is then part of the content of the file or string. The use of U+2060 WORD JOINER is strongly preferred over ZWNBSP for expressing word joining semantics since it cannot be confused with a BOM. When designing a markup language or data protocol, the use of U+FEFF can be restricted to that of Byte Order Mark. In that case, any FEFF occurring in the middle of a file can be treated as an .
Q: I am using a protocol that has BOM at the start of text. How do I represent an initial ZWNBSP?
A: Use U+2060 WORD JOINER instead.
Q: How do I tag data that does not interpret FEFF as a BOM?
A: Use the tag UTF-16BE to indicate big-endian UTF-16 text, and UTF-16LE to indicate little-endian UTF-16 text. If you do use a BOM, tag the text as simply UTF-16.
Q: Why wouldn’t I always use a protocol that requires a BOM?
A: Where the data is typed, such as a field in a database, a BOM is unnecessary. In particular, if a text data stream is marked as UTF-16BE, UTF-16LE, UTF-32BE or UTF-32LE, a BOM is neither necessary nor permitted. Any FEFF would be interpreted as a ZWNBSP.
Do not tag every string in a database or set of fields with a BOM, since it wastes space and complicates string concatenation. Moreover, it also means two data fields may have precisely the same content, but not be binary-equal (where one is prefaced by a BOM).
Q: How I should deal with BOMs?
A: Here are some guidelines to follow:
A particular protocol (e.g. Microsoft conventions for .txt files) may require use of the BOM on certain Unicode data streams, such as files. When you need to conform to such a protocol, use a BOM.
Some protocols allow optional BOMs in the case of untagged text. In those cases,
Where a text data stream is known to be plain text, but of unknown encoding, BOM can be used as a signature. If there is no BOM, the encoding could be anything.
Where a text data stream is known to be plain Unicode text (but not which endian), then BOM can be used as a signature. If there is no BOM, the text should be interpreted as big-endian.
Some byte oriented protocols expect ASCII characters at the beginning of a file. If UTF-8 is used with these protocols, use of the BOM as encoding form signature should be avoided.
Where the precise type of the data stream is known (e.g. Unicode big-endian or Unicode little-endian), the BOM should not be used. In particular, whenever a data stream is declared to be UTF-16BE, UTF-16LE, UTF-32BE or UTF-32LE a BOM must not be used. (See also .)
