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JMÉNO
UTF-8 - vícebytové Unicode kódování, kompatibilní 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.
Kódování UTF-8 pro Unicode a UCS tyto problémy nemá, a proto je obvyklou cestou pro využívání Unicode v
UNIXových (a podobných) operačních systémech.
Vlastnosti
Kódování UTF-8 má několik pěkných vlastností:
• 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.
• 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 '/'.
• Lexikografické uspořádání znaků z UCS-4 se zachovává.
• Libovolnou z 2^31 kombinací UCS lze zakódovat pomocí UTF-8.
• Kódy 0xc0, 0xc1, 0xfe a 0xff nejsou použity v kódování UTF-8.
• První byte vícebytové sekvence reprezentující jeden ne-ASCII znak UCS je vždy v intervalu 0xc0 až 0xfd
a indikuje, jak dlouhá je sekvence bytů. Všechny následující byty jsou v rozmezí 0x80 až 0xbf. Toto
umožňuje jednoduchou resynchronizaci při výpadku bytu(ů), protože se jedná o bezstavové kódování.
• UTF-8 kóduje znaky UCS až do šestibytových sekvencí, nicméně standard Unicode nespecifikuje znaky nad
0x10ffff, takže Unicode znaky mohou být v UTF-8 dlouhé nejvýše čtyři byty.
Kódování
Následující sekvence jsou použity pro reprezentování znaků. Typ použité sekvence závisí na kódu UCS
daného 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.
PŘÍKLAD
Znak Unicode 0xa9 = 1010 1001 (copyright) je kódován v UTF-8 jako:
11000010 10101001 = 0xc2 0xa9
a znak 0x2260 = 0010 0010 0110 0000 (není rovno) je kódován jako:
11100010 10001001 10100000 = 0xe2 0x89 0xa0
Praktické poznámky
Uživatelé musejí vybrat UTF-8 locale, např. pomocí
export LANG=en_GB.UTF-8
aby aktivovali podporu UTF-8 v aplikacích.
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.
DALŠÍ INFORMACE
locale(1), nl_langinfo(3), setlocale(3), charsets(7), unicode(7)
PŘEKLAD
Překlad této příručky do češtiny vytvořili Pavel Heimlich <tropikhajma@gmail.com>
Tento překlad je bezplatná dokumentace; Přečtěte si GNU General Public License Version 3 nebo novější
ohledně podmínek autorských práv. Neexistuje ŽÁDNÁ ODPOVĚDNOST.
Pokud narazíte na nějaké chyby v překladu této příručky, pošlete e-mail na adresu translation-team-
cs@lists.sourceforge.net.
Linux man-pages 6.9.1 15. června 2024 UTF-8(7)