UTF-8(7) Miscellaneous Information Manual UTF-8(7) JMENO UTF-8 - vicebytove Unicode kodovani, kompatibilni s ASCII POPIS 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/IEC 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. Kodovani UTF-8 pro Unicode a UCS tyto problemy nema, a proto je obvyklou cestou pro vyuzivani Unicode v UNIXovych (a podobnych) operacnich systemech. Vlastnosti Kodovani UTF-8 ma nekolik peknych vlastnosti: o UCS characters 0x00000000 to 0x0000007f (the classic US-ASCII characters) are encoded simply as bytes 0x00 to 0x7f (ASCII compatibility). This means that files and strings which contain only 7-bit ASCII characters have the same encoding under both ASCII and UTF-8. o All UCS characters greater than 0x7f are encoded as a multibyte sequence consisting only of bytes in the range 0x80 to 0xfd, so no ASCII byte can appear as part of another character and there are no problems with, for example, '\0' or '/'. o Lexikograficke usporadani znaku z UCS-4 se zachovava. o Libovolnou z 2^31 kombinaci UCS lze zakodovat pomoci UTF-8. o Kody 0xc0, 0xc1, 0xfe a 0xff nejsou pouzity v kodovani UTF-8. o Prvni byte vicebytove sekvence reprezentujici jeden ne-ASCII znak UCS je vzdy v intervalu 0xc0 az 0xfd a indikuje, jak dlouha je sekvence bytu. Vsechny nasledujici byty jsou v rozmezi 0x80 az 0xbf. Toto umoznuje jednoduchou resynchronizaci pri vypadku bytu(u), protoze se jedna o bezstavove kodovani. o UTF-8 koduje znaky UCS az do sestibytovych sekvenci, nicmene standard Unicode nespecifikuje znaky nad 0x10ffff, takze Unicode znaky mohou byt v UTF-8 dlouhe nejvyse ctyri byty. Kodovani Nasledujici sekvence jsou pouzity pro reprezentovani znaku. Typ pouzite sekvence zavisi na kodu UCS daneho znaku: 0x00000000 - 0x0000007F: 0xxxxxxx 0x00000080 - 0x000007FF: 110xxxxx 10xxxxxx 0x00000800 - 0x0000FFFF: 1110xxxx 10xxxxxx 10xxxxxx 0x00010000 - 0x001FFFFF: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx 0x00200000 - 0x03FFFFFF: 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 0x04000000 - 0x7FFFFFFF: 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx The xxx bit positions are filled with the bits of the character code number in binary representation, most significant bit first (big-endian). Only the shortest possible multibyte sequence which can represent the code number of the character can be used. The UCS code values 0xd800-0xdfff (UTF-16 surrogates) as well as 0xfffe and 0xffff (UCS noncharacters) should not appear in conforming UTF-8 streams. According to RFC 3629 no point above U+10FFFF should be used, which limits characters to four bytes. PRIKLAD Znak Unicode 0xa9 = 1010 1001 (copyright) je kodovan v UTF-8 jako: 11000010 10101001 = 0xc2 0xa9 a znak 0x2260 = 0010 0010 0110 0000 (neni rovno) je kodovan jako: 11100010 10001001 10100000 = 0xe2 0x89 0xa0 Prakticke poznamky Uzivatele museji vybrat UTF-8 locale, napr. pomoci export LANG=en_GB.UTF-8 aby aktivovali podporu UTF-8 v aplikacich. Application software that has to be aware of the used character encoding should always set the locale with for example setlocale(LC_CTYPE, "") and programmers can then test the expression strcmp(nl_langinfo(CODESET), "UTF-8") == 0 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/IEC 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/IEC 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/IEC 2022 is ESC % @ ("\x1b%@"). Other ISO/IEC 2022 sequences (such as for switching the G0 and G1 sets) are not applicable in UTF-8 mode. Security 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. STANDARDY ISO/IEC 10646-1:2000, Unicode 3.1, RFC 3629, Plan 9. DALSI INFORMACE locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7) PREKLAD Preklad teto prirucky do cestiny vytvorili Pavel Heimlich Tento preklad je bezplatna dokumentace; Prectete si GNU General Public License Version 3 nebo novejsi ohledne podminek autorskych prav. Neexistuje ZADNA ODPOVEDNOST. Pokud narazite na nejake chyby v prekladu teto prirucky, poslete e-mail na adresu . Linux man-pages 6.9.1 15. cervna 2024 UTF-8(7)