UTF(6) UTF(6)
NAME
UTF, Unicode, ASCII, rune - character set and format
DESCRIPTION
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 Uni-
code 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 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(2).
Letting numbers be binary, a rune x is converted to a multi-
byte 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.
FILES
/lib/unicode table of characters and descriptions, suitable
Page 1 Plan 9 (printed 4/25/24)
UTF(6) UTF(6)
for look(1).
SEE ALSO
ascii(1), tcs(1), rune(2), keyboard(6), The Unicode
Standard.
Page 2 Plan 9 (printed 4/25/24)