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 repre‐
sents its characters in 21 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 21-bit quantity representing a Unicode charac‐
ter. 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 the Unicode Standard 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(2).
Letting numbers be binary, a rune x is converted to a multibyte UTF
sequence as follows:
01. x in [000000.00000000.0bbbbbbb] → 0bbbbbbb
10. x in [000000.00000bbb.bbbbbbbb] → 110bbbbb, 10bbbbbb
11. x in [000000.bbbbbbbb.bbbbbbbb] → 1110bbbb, 10bbbbbb, 10bbbbbb
100. x in [bbbbbb.bbbbbbbb.bbbbbbbb] → 1110bbbb, 10bbbbbb, 10bbbbbb,
Conversion 01 provides a one-byte sequence that spans the ASCII charac‐
ter set in a compatible way. Conversions 10, 11 and 100 represent
higher-valued characters as sequences of two, three or four bytes with
the high bit set. Plan 9 does not support the 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 hexadecimal FFFD.
table of characters and descriptions, suitable for look(1).
SEE ALSOascii(1), tcs(1), rune(2), keyboard(6), The Unicode Standard.