UTF, Unicode, ASCII, rune - character set and format
The Plan 9 character set and representation are based on the Unicode Standard and on the
ISO multibyte UTF-8 encoding (Universal Character Set Transformation Format, 8 bits wide).
The Unicode Standard represents its characters in 16 bits; UTF-8 represents such values in
an 8-bit byte stream. Throughout this manual, UTF-8 is shortened to UTF.
In Plan 9, a rune is a 16-bit quantity representing a Unicode character. Internally, pro-
grams 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 them-
selves in the encoding. Runes with values above 7F appear as sequences of two or more
bytes with values only from 80 to FF.
The UTF encoding of the Unicode Standard is backward compatible with ASCII: programs pre-
sented 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 pro-
cessing on ASCII graphic characters must convert from UTF to runes in order to work prop-
erly with non-ASCII input. See rune(2).
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 compat-
ible 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 con-
verted to rune hexadecimal 0080.
table of characters and descriptions, suitable for look(1).
ascii(1), tcs(1), rune(2), keyboard(6), The Unicode Standard.