From 57e792311ae17dfd6975672cb3178ab72a6d0bce Mon Sep 17 00:00:00 2001 From: Michael Kerrisk Date: Tue, 24 Jun 2014 12:08:09 +0200 Subject: [PATCH] utf-8.7: Minor formatting fixes There's no need really to boldface names of standards and character sets. Reported-by: Marko Myllynen Signed-off-by: Michael Kerrisk --- man7/utf-8.7 | 139 +++++++++++++++------------------------------------ 1 file changed, 39 insertions(+), 100 deletions(-) diff --git a/man7/utf-8.7 b/man7/utf-8.7 index 2433a8926..597fad465 100644 --- a/man7/utf-8.7 +++ b/man7/utf-8.7 @@ -30,12 +30,9 @@ .SH NAME UTF-8 \- an ASCII compatible multibyte Unicode encoding .SH DESCRIPTION -The -.B Unicode 3.0 -character set occupies a 16-bit code space. +The Unicode 3.0 character set occupies a 16-bit code space. The most obvious -Unicode encoding (known as -.BR UCS-2 ) +Unicode encoding (known as UCS-2) consists of a sequence of 16-bit words. Such strings can contain\(emas part of many 16-bit characters\(embytes such as \(aq\\0\(aq or \(aq/\(aq, which have a @@ -43,69 +40,48 @@ 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, -.B UCS-2 -is not a suitable external encoding of -.B Unicode +UCS-2 is not a suitable external encoding of Unicode in filenames, text files, environment variables, and so on. -The -.BR "ISO 10646 Universal Character Set (UCS)" , +The ISO 10646 Universal Character Set (UCS), a superset of Unicode, occupies an even larger code space\(em31\ bits\(emand the obvious -.B UCS-4 -encoding for it (a sequence of 32-bit words) has the same problems. +UCS-4 encoding for it (a sequence of 32-bit words) has the same problems. -The -.B UTF-8 -encoding of -.B Unicode -and -.B UCS +The UTF-8 encoding of Unicode and UCS does not have these problems and is the common way in which -.B Unicode -is used on UNIX-style operating systems. +Unicode is used on UNIX-style operating systems. .SS Properties -The -.B UTF-8 -encoding has the following nice properties: +The UTF-8 encoding has the following nice properties: .TP 0.2i * -.B UCS -characters 0x00000000 to 0x0000007f (the classic -.B US-ASCII +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 -.B ASCII +ASCII and -.BR UTF-8 . +UTF-8 . .TP * -All -.B UCS -characters greater than 0x7f are encoded as a multibyte sequence +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, \(aq\\0\(aq or \(aq/\(aq. .TP * -The lexicographic sorting order of -.B UCS-4 -strings is preserved. +The lexicographic sorting order of UCS-4 strings is preserved. .TP * -All possible 2^31 UCS codes can be encoded using -.BR UTF-8 . +All possible 2^31 UCS codes can be encoded using UTF-8. .TP * -The bytes 0xc0, 0xc1, 0xfe, and 0xff are never used in the -.B UTF-8 -encoding. +The bytes 0xc0, 0xc1, 0xfe, and 0xff are never used in the UTF-8 encoding. .TP * The first byte of a multibyte sequence which represents a single non-ASCII -.B UCS -character is always in the range 0xc2 to 0xfd and indicates how long +UCS character is always in the range 0xc2 to 0xfd and indicates how long this multibyte sequence is. All further bytes in a multibyte sequence are in the range 0x80 to 0xbf. @@ -113,14 +89,10 @@ This allows easy resynchronization and makes the encoding stateless and robust against missing bytes. .TP * -.B UTF-8 -encoded -.B UCS -characters may be up to six bytes long, however the -.B Unicode -standard specifies no characters above 0x10ffff, so Unicode characters +UTF-8 encoded UCS characters may be up to six bytes long, however the +Unicode standard specifies no characters above 0x10ffff, so Unicode characters can be only up to four bytes long in -.BR UTF-8 . +UTF-8. .SS Encoding The following byte sequences are used to represent a character. The sequence to be used depends on the UCS code number of the character: @@ -165,16 +137,10 @@ binary representation. Only the shortest possible multibyte sequence which can represent the code number of the character can be used. .PP -The -.B UCS -code values 0xd800\(en0xdfff (UTF-16 surrogates) as well as 0xfffe and -0xffff (UCS noncharacters) should not appear in conforming -.B UTF-8 -streams. +The UCS code values 0xd800\(en0xdfff (UTF-16 surrogates) as well as 0xfffe and +0xffff (UCS noncharacters) should not appear in conforming UTF-8 streams. .SS Example -The -.B Unicode -character 0xa9 = 1010 1001 (the copyright sign) is encoded +The Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded in UTF-8 as .PP .RS @@ -188,17 +154,13 @@ encoded as: 11100010 10001001 10100000 = 0xe2 0x89 0xa0 .RE .SS Application notes -Users have to select a -.B UTF-8 -locale, for example with +Users have to select a UTF-8 locale, for example with .PP .RS export LANG=en_GB.UTF-8 .RE .PP -in order to activate the -.B UTF-8 -support in applications. +in order to activate the UTF-8 support in applications. .PP Application software that has to be aware of the used character encoding should always set the locale with for example @@ -213,69 +175,46 @@ and programmers can then test the expression strcmp(nl_langinfo(CODESET), "UTF-8") == 0 .RE .PP -to determine whether a -.B UTF-8 -locale has been selected and whether +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 -.BR UTF-8 . +variables are encoded in UTF-8. .PP -Programmers accustomed to single-byte encodings such as -.B US-ASCII -or -.B ISO 8859 +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 -.B UTF-8 -locales. +in UTF-8 locales. Firstly, a single byte does not necessarily correspond any more to a single character. -Secondly, since modern terminal emulators -in -.B UTF-8 +Secondly, since modern terminal emulators in UTF-8 mode also support Chinese, Japanese, and Korean -.B double-width characters -as well as nonspacing -.BR "combining characters" , +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 -.BR ASCII . +by one position as it did in ASCII. Library functions such as .BR mbsrtowcs (3) and .BR wcswidth (3) should be used today to count characters and cursor positions. .PP -The official ESC sequence to switch from an -.B ISO 2022 +The official ESC sequence to switch from an ISO 2022 encoding scheme (as used for instance by VT100 terminals) to -.B UTF-8 -is ESC % G +UTF-8 is ESC % G ("\\x1b%G"). The corresponding return sequence from -.B UTF-8 -to ISO 2022 is ESC % @ ("\\x1b%@"). +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. .SS Security -The -.BR Unicode " and " UCS -standards require that producers of -.B UTF-8 +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. -.B Unicode 3.1 -has added the requirement that conforming programs must not accept +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 -.B ASCII +might check only for the ASCII version of "/../" or ";" or NUL and overlook that there are many -.RB non- ASCII -ways to represent these things in a non-shortest -.B UTF-8 +non-ASCII ways to represent these things in a non-shortest UTF-8 encoding. .SS Standards ISO/IEC 10646-1:2000, Unicode 3.1, RFC\ 3629, Plan 9.