Provided by: sam_4.3-18.2_i386
UTF, Unicode, ASCII, rune - character set and format
The Plan 9 character set and representation are based on Unicode and on
a proposed X-Open multibyte FSS-UCS-TF (File System Safe Universal
Character Set Transformation Format) encoding. Unicode represents its
characters in 16 bits; FSS-UCS-TF, or just UTF, represent such values
in an 8-bit byte stream.
In Plan 9, a rune is a 16-bit quantity representing a Unicode
character. Internally, programs may store characters as runes.
However, any external manifestation of textual information, in files or
at the interface between programs, uses a machine-independent, byte-
stream encoding called UTF.
UTF is designed so the 7-bit ASCII set (values hexadecimal 00 to 7F),
appear only as themselves in the encoding. Runes with values above 7F
appear as sequences of two or more bytes with values only from 80 to
The UTF encoding of Unicode is backward compatible with ASCII: programs
presented only with ASCII work on Plan 9 even if not written to deal
with UTF, as do programs that deal with uninterpreted byte streams.
However, programs that perform semantic processing on ASCII graphic
characters must convert from UTF to runes in order to work properly
with non-ASCII input. See rune(3g).
Letting numbers be binary, a rune x is converted to a multibyte UTF
sequence as follows:
01. x in [00000000.0bbbbbbb] → 0bbbbbbb
10. x in [00000bbb.bbbbbbbb] → 110bbbbb, 10bbbbbb
11. x in [bbbbbbbb.bbbbbbbb] → 1110bbbb, 10bbbbbb, 10bbbbbb
Conversion 01 provides a one-byte sequence that spans the ASCII
character set in a compatible way. Conversions 10 and 11 represent
higher-valued characters as sequences of two or three bytes with the
high bit set. Plan 9 does not support the 4, 5, and 6 byte sequences
proposed by X-Open. When there are multiple ways to encode a value,
for example rune 0, the shortest encoding is used.
In the inverse mapping, any sequence except those described above is
incorrect and is converted to rune 0080.
ascii(7), rune(3g), keyboard(5g), The Unicode Standard.