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Utf-8 Basic Informations:

History
2> By early 1992 the search was on for a good byte-stream encoding of multi-byte character sets. The draft ISO 10646 standard contained a non-required annex called UTF-1 that provided a byte-stream encoding of its 32-bit code points. This encoding was not satisfactory on performance grounds, but did introduce the notion that bytes in the ASCII range of 0–127 represent themselves in UTF, thereby providing backward compatibility. In July 1992, the X/Open committee XoJIG was looking for a better encoding. Dave Prosser of Unix System Laboratories submitted a proposal for one that had faster implementation characteristics and introduced the improvement that 7-bit ASCII characters would only represent themselves; all multibyte sequences would include only bytes where the high bit was set. In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. Ken Thompson of the Plan 9 operating system group at Bell Labs then made a crucial modification to the encoding to allow it to be self-synchronizing, meaning that it was not necessary to read from the beginning of the string to find code point boundaries. Thompson's design was outlined on September 2, 1992, on a placemat in a New Jersey diner with Rob Pike. The following days, Pike and Thompson implemented it and updated Plan 9 to use it throughout, and then communicated their success back to X/Open.[10] UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25–29, 1993. In November 2003 UTF-8 was restricted by RFC 3629 to four bytes in order to match the constraints of the UTF-16 character encoding. [edit]

Tags:Character,Backward Compatibility,Ascii,Utf-16,Code Points,Bytes,Iso 10646,Annex,Utf-1,X/open,Unix System Laboratories,Ibm,Ken Thompson,Plan 9,Operating System,Bell Labs,Rob Pike,Usenix,
Description
2> The design of UTF‑8 is most easily seen in the table of the scheme as originally proposed by Dave Prosser and subsequently modified by Ken Thompson: Bits Last code point Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6   7 U+007F 0xxxxxxx 11 U+07FF 110xxxxx 10xxxxxx 16 U+FFFF 1110xxxx 10xxxxxx 10xxxxxx 21 U+1FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx 26 U+3FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 31 U+7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx The salient features of the above scheme are as follows: The bytes equal to ASCII characters represent those characters and are used for that purpose only. The first byte indicates the length of a sequence, without having to examine the continuation bytes Single bytes, first bytes, and continuation bytes do not share values. This makes the scheme "self synchronizing", allowing searching to work without having to find character boundaries, and allowing the start of a character to be found by backing up at most five bytes (three bytes in actual UTF‑8 as explained below). The scheme could be extended beyond 6-byte sequences and the lead bytes FE and FF are available for this. The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode. This includes Latin letters with diacritics and characters from the Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac and Tāna alphabets. Three bytes are needed for the rest of the Basic Multilingual Plane (which contains virtually all characters in common use). Four bytes are needed for characters in the other planes of Unicode, which include less common CJK characters and various historic scripts and mathematical symbols. The original specification covered numbers up to 31 bits (the original limit of the Universal Character Set). In November 2003 UTF-8 was restricted by RFC 3629 to end at U+10FFFF, in order to match the constraints of the UTF-16 character encoding. This removed the 5 and 6-byte sequences, and about ½ the 4-byte sequences. [edit]

Tags:Unicode,Latin,Diacritics,Greek,Cyrillic,Coptic,Armenian,Hebrew,Arabic,Syriac,Tāna,Basic Multilingual Plane,
Examples
3> Character Binary code point Binary UTF-8 Hexadecimal UTF-8 $ U+0024 00100100 00100100 24 ¢ U+00A2 00000000 10100010 11000010 10100010 C2 A2 € U+20AC 00100000 10101100 11100010 10000010 10101100 E2 82 AC 𤭢 U+24B62 00000010 01001011 01100010 11110000 10100100 10101101 10100010 F0 A4 AD A2 [edit]

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Codepage layout
3> UTF-8 _0 _1 _2 _3 _4 _5 _6 _7 _8 _9 _A _B _C _D _E _F   0_   NUL 0000 0 SOH 0001 1 STX 0002 2 ETX 0003 3 EOT 0004 4 ENQ 0005 5 ACK 0006 6 BEL 0007 7 BS 0008 8 HT 0009 9 LF 000A 10 VT 000B 11 FF 000C 12 CR 000D 13 SO 000E 14 SI 000F 15   1_   DLE 0010 16 DC1 0011 17 DC2 0012 18 DC3 0013 19 DC4 0014 20 NAK 0015 21 SYN 0016 22 ETB 0017 23 CAN 0018 24 EM 0019 25 SUB 001A 26 ESC 001B 27 FS 001C 28 GS 001D 29 RS 001E 30 US 001F 31   2_   SP 0020 32 ! 0021 33 " 0022 34 # 0023 35 $ 0024 36 % 0025 37 & 0026 38 ' 0027 39 ( 0028 40 ) 0029 41 * 002A 42 + 002B 43 , 002C 44 - 002D 45 . 002E 46 / 002F 47   3_   0 0030 48 1 0031 49 2 0032 50 3 0033 51 4 0034 52 5 0035 53 6 0036 54 7 0037 55 8 0038 56 9 0039 57 : 003A 58 ; 003B 59 < 003C 60 = 003D 61 > 003E 62 ? 003F 63   4_   @ 0040 64 A 0041 65 B 0042 66 C 0043 67 D 0044 68 E 0045 69 F 0046 70 G 0047 71 H 0048 72 I 0049 73 J 004A 74 K 004B 75 L 004C 76 M 004D 77 N 004E 78 O 004F 79   5_   P 0050 80 Q 0051 81 R 0052 82 S 0053 83 T 0054 84 U 0055 85 V 0056 86 W 0057 87 X 0058 88 Y 0059 89 Z 005A 90 [ 005B 91 \ 005C 92 ] 005D 93 ^ 005E 94 _ 005F 95   6_   ` 0060 96 a 0061 97 b 0062 98 c 0063 99 d 0064 100 e 0065 101 f 0066 102 g 0067 103 h 0068 104 i 0069 105 j 006A 106 k 006B 107 l 006C 108 m 006D 109 n 006E 110 o 006F 111   7_   p 0070 112 q 0071 113 r 0072 114 s 0073 115 t 0074 116 u 0075 117 v 0076 118 w 0077 119 x 0078 120 y 0079 121 z 007A 122 { 007B 123 | 007C 124 } 007D 125 ~ 007E 126 DEL 007F 127   8_   • +00 128 • +01 129 • +02 130 • +03 131 • +04 132 • +05 133 • +06 134 • +07 135 • +08 136 • +09 137 • +0A 138 • +0B 139 • +0C 140 • +0D 141 • +0E 142 • +0F 143   9_   • +10 144 • +11 145 • +12 146 • +13 147 • +14 148 • +15 149 • +16 150 • +17 151 • +18 152 • +19 153 • +1A 154 • +1B 155 • +1C 156 • +1D 157 • +1E 158 • +1F 159   A_   • +20 160 • +21 161 • +22 162 • +23 163 • +24 164 • +25 165 • +26 166 • +27 167 • +28 168 • +29 169 • +2A 170 • +2B 171 • +2C 172 • +2D 173 • +2E 174 • +2F 175   B_   • +30 176 • +31 177 • +32 178 • +33 179 • +34 180 • +35 181 • +36 182 • +37 183 • +38 184 • +39 185 • +3A 186 • +3B 187 • +3C 188 • +3D 189 • +3E 190 • +3F 191   C_   2 (0000) 192 2 (0040) 193 2 0080 194 2 00C0 195 2 0100 196 2 0140 197 2 0180 198 2 01C0 199 2 0200 200 2 0240 201 2 0280 202 2 02C0 203 2 0300 204 2 0340 205 2 0380 206 2 03C0 207   D_   2 0400 208 2 0440 209 2 0480 210 2 04C0 211 2 0500 212 2 0540 213 2 0580 214 2 05C0 215 2 0600 216 2 0640 217 2 0680 218 2 06C0 219 2 0700 220 2 0740 221 2 0780 222 2 07C0 223   E_   3 0800* 224 3 1000 225 3 2000 226 3 3000 227 3 4000 228 3 5000 229 3 6000 230 3 7000 231 3 8000 232 3 9000 233 3 A000 234 3 B000 235 3 C000 236 3 D000 237 3 E000 238 3 F000 239   F_   4 10000* 240 4 40000 241 4 80000 242 4 C0000 243 4 100000 244 4 140000 245 4 180000 246 4 1C0000 247 5 200000* 248 5 1000000 249 5 2000000 250 5 3000000 251 6 4000000* 252 6 40000000 253 254 255 Legend: Yellow cells are control characters, blue cells are punctuation, purple cells are digits and green cells are ASCII letters. Orange cells with a large dot are continuation bytes. The hexadecimal number shown after a "+" plus sign is the value of the 6 bits they add. White cells containing a large single-digit number are the start bytes for a sequence of that many bytes. The unbolded hexadecimal code point number shown in the cell is the lowest character value encoded using that start byte. When a start byte could form both overlong and valid encodings, the lowest non-overlong-encoded codepoint is shown, marked by an asterisk "*". Red cells must never appear in a valid UTF-8 sequence. The first two (C0 and C1) could only be used for overlong encoding of basic ASCII characters. The remaining red cells indicate start bytes of sequences that could only encode numbers larger than the 0x10FFFF limit of Unicode. The byte 244 (hex 0xF4) could also encode some values greater than 0x10FFFF; such a sequence is also invalid. [edit]

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Invalid byte sequences
3> Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for: the red invalid bytes in the above table an unexpected continuation byte a start byte not followed by enough continuation bytes a sequence that decodes to a value that should use a shorter sequence (an "overlong form"). A 4-byte sequence (starting with F4) that decodes to a value greater than U+10FFFF Many earlier decoders would happily try to decode these. Carefully crafted invalid UTF-8 could make them either skip or create ASCII characters such as NUL, slash, or quotes. Invalid UTF-8 has been used to bypass security validations in high profile products including Microsoft's IIS web server[11] and Apache's Tomcat servlet container.[12] RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[13] The Unicode Standard requires decoders to "...treat any ill-formed code unit sequence as an error condition. This guarantees that it will neither interpret nor emit an ill-formed code unit sequence." Many UTF-8 decoders throw exceptions on encountering errors,[14] since such errors suggest the input is not a UTF-8 string at all. This can turn what would otherwise be harmless errors (producing a message such as "no such file") into a denial of service bug. For instance Python 3.0 would exit immediately if the command line contained invalid UTF-8,[15] so it was impossible to write a Python program that could handle such input. An increasingly popular option is to detect errors with a separate API, and for converters to translate the first byte to a replacement and continue parsing with the next byte. Popular replacements are: The replacement character "�" (U+FFFD) The invalid Unicode code points U+DC80..U+DCFF where the low 8 bits are the byte's value. Interpret the bytes according to ISO-8859-1 or CP1252. Replacing errors is "lossy": more than one UTF-8 string converts to the same Unicode result. Therefore the original UTF-8 should be stored, and translation should only be used when displaying the text to the user. [edit]

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Invalid code points
3> According to the Unicode standard[which?] the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) are not legal Unicode values, and the UTF-8 encoding of them is an invalid byte sequence and thus should be treated as described above.[citation needed] Whether an actual application should do this with surrogate halves is debatable.[who?] Allowing them allows lossless storage of invalid UTF-16, and allows CESU encoding (described below) to be decoded. There are other code points that are far more important to detect and reject, such as the reversed-BOM U+FFFE, or the C1 controls, caused by improper conversion of CP1252 text or double-encoding of UTF-8. These are invalid in HTML. [edit]

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Official name and variants
2> The official name is "UTF-8". All letters are upper-case, and the name is hyphenated. This spelling is used in all the documents relating to the encoding. Alternatively, the name "utf-8" may be used by all standards conforming to the Internet Assigned Numbers Authority (IANA) list (which include CSS, HTML, XML, and HTTP headers),[16] as the declaration is case insensitive.[17] Other descriptions that omit the hyphen or replace it with a space, such as "utf8" or "UTF 8", are not accepted as correct by the governing standards.[18] Despite this, most agents such as browsers can understand them, and so standards intended to describe existing practice (such as HTML5) may effectively require their recognition. MySQL omits the hyphen in the following query: SET NAMES 'utf8' [edit]

Tags:Internet,
Derivatives
2> The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification. [edit]

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CESU-8
3> Main article: CESU-8 Many pieces of software added UTF-8 conversions for UCS-2 data and did not alter their UTF-8 conversion when UCS-2 was replaced with the surrogate-pair supporting UTF-16. The result is that each half of a UTF-16 surrogate pair is encoded as its own 3-byte UTF-8 encoding, resulting in 6-byte sequences rather than 4 for characters outside the Basic Multilingual Plane. Oracle databases use this, as well as Java and Tcl as described below, and probably a great deal of other Windows software where the programmers were unaware of the complexities of UTF-16. Although most usage is by accident, a supposed benefit is that this preserves UTF-16 binary sorting order when CESU-8 is binary sorted. [edit]

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Modified UTF-8
3> In Modified UTF-8,[19] the null character (U+0000) is encoded as 0xC0,0x80; this is not valid UTF-8[20] because it is not the shortest possible representation. Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000,[21] which allows such strings (with a null byte appended) to be processed by traditional null-terminated string functions. All known Modified UTF-8 implementations also treat the surrogate pairs as in CESU-8. In normal usage, the Java programming language supports standard UTF-8 when reading and writing strings through InputStreamReader and OutputStreamWriter. However it uses Modified UTF-8 for object serialization,[22] for the Java Native Interface,[23] and for embedding constant strings in class files.[24] Tcl also uses the same modified UTF-8[25] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data. [edit]

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Byte order mark
2> Many Windows programs (including Windows Notepad) add the bytes 0xEF, 0xBB, 0xBF at the start of any document saved as UTF-8. This is the UTF-8 encoding of the Unicode byte order mark (BOM), and is commonly referred to as a UTF-8 BOM, even though it is not relevant to byte order. The BOM can also appear if another encoding with a BOM is translated to UTF-8 without stripping it. Older text editors may display the BOM as "" at the start of the document. The Unicode standard recommends against the BOM for UTF-8.[26] The presence of the UTF-8 BOM may cause interoperability problems with existing software that could otherwise handle UTF-8; for example: Programming language parsers not explicitly designed for UTF-8 can often handle UTF-8 in string constants and comments, but cannot parse the BOM at the start of the file. Programs that identify file types by leading characters may fail to identify the file if a BOM is present even if the user of the file could skip the BOM. An example is the Unix shebang syntax. Another example is Internet Explorer which will render pages in standards mode only when it starts with a document type declaration. If compatibility with existing programs is not important, the BOM could be used to identify UTF-8 encoding. Because checking if text is valid UTF-8 is very reliable (the majority of random byte sequences are not valid UTF-8) such use should not be necessary. Programs that insert information at the start of a file will break this identification (one example is offline browsers that add the originating URL to the start of the file). In Japan especially, "UTF-8 encoding without BOM" is sometimes called "UTF-8N".[citation needed] [edit]

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Advantages and disadvantages
2> This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2009) [edit]

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Advantages
4> The ASCII characters are represented by themselves as single bytes that do not appear anywhere else, which makes UTF-8 work with the majority of existing APIs that take bytes strings but only treat a small number of ASCII codes specially. This removes the need to write a new Unicode version of every API, and makes it much easier to convert existing systems to UTF-8 than any other Unicode encoding. UTF-8 is the only encoding for XML entities that does not require a BOM or an indication of the encoding.[27] UTF-8 and UTF-16 are the standard encodings for Unicode text in HTML documents, with UTF-8 as the preferred and most used encoding. UTF-8 strings can be fairly reliably recognized as such by a simple heuristic algorithm.[28] The chance of a random string of bytes being valid UTF-8 and not pure ASCII is 3.9% for a two-byte sequence, 0.41% for a three-byte sequence and 0.026% for a four-byte sequence.[29] ISO/IEC 8859-1 is even less likely to be mis-recognized as UTF-8: the only non-ASCII characters in it would have to be in sequences starting with either an accented letter or the multiplication symbol and ending with a symbol. This is an advantage that most other encodings do not have, causing errors (mojibake) if the receiving application isn't told and can't guess the correct encoding. Even word-based UTF-16 can be mistaken for byte encodings (like in the "bush hid the facts" bug). Sorting of UTF-8 strings as arrays of unsigned bytes will produce the same results as sorting them based on Unicode code points. Other byte-based encodings can pass through the same API. This means, however, that the encoding must be identified. Because the other encodings are unlikely to be valid UTF-8, a reliable way to implement this is to assume UTF-8 and switch to a legacy encoding only if several invalid UTF-8 byte sequences are encountered. [edit]

Tags:Apis,
Disadvantages
4> A UTF-8 parser that is not compliant with current versions of the standard might accept a number of different pseudo-UTF-8 representations and convert them to the same Unicode output. This provides a way for information to leak past validation routines designed to process data in its eight-bit representation.[30] [edit]

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Advantages
4> UTF-8 can encode any Unicode character, avoiding the need to figure out and set a "code page" or otherwise indicate what character set is in use, and allowing output in multiple languages at the same time. For many languages there has been more than one single-byte encoding in usage, so even knowing the language was insufficient information to display it correctly. The bytes 0xfe and 0xff do not appear, so a valid UTF-8 stream never matches the UTF-16 byte order mark and thus cannot be confused with it. The absence of 0xFF (\377) also eliminates the need to escape this byte in Telnet (and FTP control connection). [edit]

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Disadvantages
4> UTF-8 encoded text is larger than the appropriate single-byte encoding except for plain ASCII characters. In the case of languages which used 8-bit character sets with non-Latin alphabets encoded in the upper half (such as most Cyrillic and Greek alphabet code pages), letters in UTF-8 will be double the size. For some languages such as Thai and Hindi's Devanagari, letters will be triple the size (this has caused objections in India and other countries). This can easily cause trouble when converting systems to Unicode and starting to use them for other languages than English. It is possible in UTF-8 (or any other multi-byte encoding) to split or truncate a string in the middle of a character, which may result in an invalid string. This will not happen in correct handling of UTF-8.

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