.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (C) Markus Kuhn, 1996 .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, .\" USA. .\" .\" 1995-11-26 Markus Kuhn .\" First version written .\" .TH UTF-8 7 "1995-11-26" "Linux" "Linux Programmer's Manual" .SH NAME UTF-8 \- an ASCII compatible multibyte Unicode encoding .SH DESCRIPTION The .B Unicode character set occupies a 16-bit code space. The most obvious Unicode encoding (known as .BR UCS-2 ) consists of a sequence of 16-bit words. Such strings can contain as parts of many 16-bit characters 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, .B UCS-2 is not a suitable external encoding of .B Unicode in filenames, text files, environment variables, etc. The .BR "ISO 10646 Universal Character Set (UCS)" , a superset of Unicode, occupies even a 31-bit code space and the obvious .B 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 does not have these problems and is the way to go for using the .B Unicode character set under Unix-style operating systems. .SH PROPERTIES The .B UTF-8 encoding has the following nice properties: .TP 0.2i * .B UCS characters 0x00000000 to 0x0000007f (the classical .B 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 and .BR UTF-8 . .TP * All .B UCS characters > 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 e.g. '\\0' or '/'. .TP * The lexicographic sorting order of .B UCS-4 strings is preserved. .TP * All possible 2^31 UCS codes can be encoded using .BR UTF-8 . .TP * The bytes 0xfe and 0xff are never used in the .B 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 0xc0 to 0xfd and indicates how long this multibyte sequence is. All further bytes in a multibyte sequence are in the range 0x80 to 0xbf. 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 .B Unicode characters can only be up to three bytes long. As Linux uses only the 16-bit .B Unicode subset of .BR UCS , under Linux, .B UTF-8 multibyte sequences can only be one, two or three bytes long. .SH 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: .TP 0.4i 0x00000000 - 0x0000007F: .RI 0 xxxxxxx .TP 0x00000080 - 0x000007FF: .RI 110 xxxxx .RI 10 xxxxxx .TP 0x00000800 - 0x0000FFFF: .RI 1110 xxxx .RI 10 xxxxxx .RI 10 xxxxxx .TP 0x00010000 - 0x001FFFFF: .RI 11110 xxx .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .TP 0x00200000 - 0x03FFFFFF: .RI 111110 xx .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .TP 0x04000000 - 0x7FFFFFFF: .RI 1111110 x .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .RI 10 xxxxxx .PP The .I 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. .SH EXAMPLES The .B Unicode character 0xa9 = 1010 1001 (the copyright sign) is encoded in UTF-8 as .PP .RS 11000010 10101001 = 0xc2 0xa9 .RE .PP and character 0x2260 = 0010 0010 0110 0000 (the "not equal" symbol) is encoded as: .PP .RS 11100010 10001001 10100000 = 0xe2 0x89 0xa0 .RE .SH STANDARDS ISO 10646, Unicode 1.1, XPG4, Plan 9. .SH AUTHOR Markus Kuhn .SH SEE ALSO .B unicode(7)