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
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 FF.
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(3).
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 hexadecimal 0080.