Byte Order Mark (BOM)

A byte order mark (BOM) is a sequence of bytes used to indicate Unicode encoding of a text file. The underlying character code, U+FEFF, takes one of the following forms depending on the character encoding.

BOM use is optional. If used, it must be at the very beginning of the text. The BOM gives the producer of the text a way to describe the encoding such as UTF-8 or UTF-16, and in the case of UTF-16 and UTF-32, its endianness. The BOM is important for text interchange, when files move between systems that use different byte orders or different encodings, rather than in normal text handling in a closed environment.

As UTF-8 has become the most common text encoding, EFBBBF (shown here as three hexadecimal values) is the most commonly occurring BOM form, also known as the UTF-8 signature. HTML5 browsers are required to recognize the UTF-8 BOM and use it to detect the encoding of the page. Software may alternatively recognize UTF-8 encoding by looking for bytes with the high order bit set (values 0x80 through 0xFF) followed by bytes that define valid UTF-8 sequences.

The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but warns that it may be encountered at the start of a file.

一般存成无BOM的UTF-8文件