UTF8(5) File Formats Manual UTF8(5)

utf8UTF-8, a transformation format of ISO 10646


The UTF-8 encoding represents UCS-4 characters as a sequence of octets, using between 1 and 6 for each character. It is backwards compatible with ASCII, so 0x00-0x7f refer to the ASCII character set. The multibyte encoding of non-ASCII characters consist entirely of bytes whose high order bit is set. The actual encoding is represented by the following table:

[0x00000000 - 0x0000007f] [00000000.0bbbbbbb] -> 0bbbbbbb
[0x00000080 - 0x000007ff] [00000bbb.bbbbbbbb] -> 110bbbbb, 10bbbbbb
[0x00000800 - 0x0000ffff] [bbbbbbbb.bbbbbbbb] ->
	1110bbbb, 10bbbbbb, 10bbbbbb
[0x00010000 - 0x001fffff] [00000000.000bbbbb.bbbbbbbb.bbbbbbbb] ->
	11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
[0x00200000 - 0x03ffffff] [000000bb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
	111110bb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb
[0x04000000 - 0x7fffffff] [0bbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb] ->
	1111110b, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb, 10bbbbbb

If more than a single representation of a value exists (for example, 0x00; 0xC0 0x80; 0xE0 0x80 0x80) the shortest representation is always used. Longer ones are detected as an error as they pose a potential security risk, and destroy the 1:1 character:octet sequence mapping.


Rob Pike and Ken Thompson, Hello World, Proceedings of the Winter 1993 USENIX Technical Conference, USENIX Association, January 1993.

F. Yergeau, UTF-8, a transformation format of ISO 10646, January 1998, RFC 2279.

The Unicode Standard, Version 3.0, The Unicode Consortium, 2000, as amended by the Unicode Standard Annex #27: Unicode 3.1 and by the Unicode Standard Annex #28: Unicode 3.2.

The utf8 encoding is compatible with RFC 2279 and Unicode 3.2.

April 7, 2004 macOS 14.4