Byte order mark

Rare card · No. 42661

The byte order mark (BOM) is a particular usage of the special Unicode character code, U+FEFF ZERO WIDTH NO-BREAK SPACE, whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: the byte order, or endianness, of the text stream in the cases of 16-bit and 32-bit encodings; the fact that the text stream's encoding is Unicode, to a high level of confidence; which Unicode character encoding is used. BOM use is optional. Its presence interferes with the use of UTF-8 by software that does not expect non-ASCII bytes at the start of a file but that could otherwise handle the text stream. Unicode can be encoded in units of 8-bit, 16-bit, or 32-bit integers. For 16- and 32-bit representations, a computer receiving text from arbitrary sources needs to know which byte order the integers are encoded in. The BOM becomes a noncharacter Unicode code point if its bytes are swapped. Hence, the process accessing the text can examine these...

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