UTF-8
<character> (UCS transformation format 8) An
ASCII-compatible multibyte
Unicode and
UCS encoding, used by
Java and
Plan 9.
The Unicode character set occupies a 16-bit code space.
The most obvious Unicode encoding (known as UCS-2) consists of a sequence of 16-bit words.
Such strings can contain bytes like '\0' or '/' which have a special meaning in filenames and other
C library function parameters.
In addition, the majority of
Unix tools expects ASCII files and can't read 16-bit words as characters without major modifications.
For these reasons, UCS-2 is not a suitable external encoding of Unicode in filenames, text files, environment variables, etc.
The
ISO 10646 Universal Character Set (UCS), a superset of Unicode, occupies a 31-bit code space and the obvious UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
The UTF-8 encoding of Unicode and UCS avoids the problems of fixed-length Unicode encodings because an ASCII file encoded in UTF is exactly same as the original ASCII file and all non-ASCII characters are guaranteed to have the most significant bit set (bit 0x80).
This means that normal tools for text searching etc. work as expected.
UTF-8 is defined in
RFC 2279.
["File System Safe UCS Transformation Format (FSS_UTF)", X/Open Preliminary Specification, X/Open Company Ltd., Document Number: P316.
This information also appears in ISO/IEC 10646, Annex P].
Plan 9 UTF manual entry (ftp://ftp.uu.net/doc/obi/Bell.Labs/plan9pm/09utf.ps.Z).
