Section: Linux Programmer's Manual (7)
The Unicode 3.0 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---as part of many 16-bit characters---bytes such as '\0' or '/', which have a special meaning in filenames and other C library function arguments. In addition, the majority of UNIX tools expect 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, and so on. The ISO 10646 Universal Character Set (UCS), a superset of Unicode, occupies an even larger code space---31 bits---and the obvious UCS-4 encoding for it (a sequence of 32-bit words) has the same problems.
The UTF-8 encoding has the following nice properties:
The following byte sequences are used to represent a character. The sequence to be used depends on the UCS code number of the character:
The xxx bit positions are filled with the bits of the character code number in binary representation. Only the shortest possible multibyte sequence which can represent the code number of the character can be used.
The Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded in UTF-8 as
and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is encoded as:
Users have to select a UTF-8 locale, for example with
in order to activate the UTF-8 support in applications.
Application software that has to be aware of the used character encoding should always set the locale with for example
and programmers can then test the expression
to determine whether a UTF-8 locale has been selected and whether therefore all plaintext standard input and output, terminal communication, plaintext file content, filenames and environment variables are encoded in UTF-8.
Programmers accustomed to single-byte encodings such as US-ASCII or ISO 8859 have to be aware that two assumptions made so far are no longer valid in UTF-8 locales. Firstly, a single byte does not necessarily correspond any more to a single character. Secondly, since modern terminal emulators in UTF-8 mode also support Chinese, Japanese, and Korean double-width characters as well as nonspacing combining characters, outputting a single character does not necessarily advance the cursor by one position as it did in ASCII. Library functions such as mbsrtowcs?(3) and wcswidth?(3) should be used today to count characters and cursor positions.
The official ESC sequence to switch from an ISO 2022 encoding scheme (as used for instance by VT100 terminals) to UTF-8 is ESC % G ("\x1b%G"). The corresponding return sequence from UTF-8 to ISO 2022 is ESC % @ ("\x1b%@"). Other ISO 2022 sequences (such as for switching the G0 and G1 sets) are not applicable in UTF-8 mode.
The Unicode and UCS standards require that producers of UTF-8 shall use the shortest form possible, for example, producing a two-byte sequence with first byte 0xc0 is nonconforming. Unicode 3.1 has added the requirement that conforming programs must not accept non-shortest forms in their input. This is for security reasons: if user input is checked for possible security violations, a program might check only for the ASCII version of "/../" or ";" or NUL and overlook that there are many non-ASCII ways to represent these things in a non-shortest UTF-8 encoding.
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