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UTF-8 From Wikipedia, the free encyclopedia

UTF-8 is encoding capable of encoding all possible characters, or code points, defined by and originally designed by Ken Thompson and Rob Pike.[1]

The encoding is variable-length and uses 8-bit code units. It was designed for backward compatibility with ASCII and avoid the complications of endianness and byte order marks in the alternative UTF-16 and UTF-32 encodings. The name is derived from Unicode (or Universal Coded Character Set) Transformation Format – 8- bit.[2]

UTF-8 is the dominant for the , accounting for 89.1% of all Web pages in May 2017 (the most popular East Asian encodings, Shift JIS and GB 2312, have 0.9% and 0.7% respectively).[4][5][3] The Mail Consortium (IMC) recommended that all -mail programs be able to display and create mail using UTF-8,[6] and the W3C recommends UTF-8 as the default encoding in XML and HTML.[7]

UTF-8 encodes each of the 1,112,064[8] valid code points in Unicode using one to four 8-bit bytes.[9] Code points with lower numerical values, which tend to occur more frequently, are encoded using fewer bytes. The first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single octet with the same binary value as ASCII, that valid ASCII text is valid UTF-8-encoded Unicode as well. Since ASCII bytes do not occur when encoding non-ASCII code points into UTF-8, UTF-8 is safe to use within most programming and document languages that interpret certain ASCII characters in a special way, such as '/' in filenames, '\' in escape sequences, and '%' in printf. Shows the usage of the main encodings on the web from 2001 to 2012 as recorded by ,[3] with UTF-8 overtaking all others in Contents 2008 and nearing 50% of the web in 2012. Note that the ASCII only figure includes web 1 Description pages with any declared header if they are restricted to ASCII characters. 1.1 Examples

1.2 Codepage layout

1.3 Overlong encodings

1.4 Invalid byte sequences

1.5 Invalid code points

2 Official name and variants 3 Derivatives

3.1 CESU-8

3.2 Modified UTF-8

3.3 WTF-8

4

5 History 6 Advantages and disadvantages

6.1 General

6.1.1 Advantages 6.2 Comparison with single-byte encodings

6.2.1 Advantages

6.2.2 Disadvantages 6.3 Comparison with other multi-byte encodings

6.3.1 Advantages

6.3.2 Disadvantages 6.4 Comparison with UTF-16

6.4.1 Advantages

6.4.2 Disadvantages

7 See also

8 Notes

9 References

10 External links

Description

Since the restriction of the Unicode code- to 21-bit values in 2003, UTF-8 is defined to encode codepoints in one to four bytes, depending on the number of significant bits in the numerical value of the codepoint. The following table shows the structure of the encoding. The x characters are replaced by the bits of the . If the number of significant bits is more than 7, the first line applies; if no more than 11 bits, the second line applies, and so on.

Number Bits for First Last Byte 1 Byte 2 Byte 3 Byte 4 of bytes code point code point code point 1 7 +0000 U+007F 0xxxxxxx 2 11 U+0080 U+07FF 110xxxxx 10xxxxxx 3 16 U+0800 U+FFFF 1110xxxx 10xxxxxx 10xxxxxx 4 21 U+10000 U+10FFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode, which covers the remainder of almost all Latin alphabets, and also Greek, , Coptic, Armenian, , , Syriac, and ' alphabets, as well as Combining Diacritical Marks. Three bytes are needed for characters in the rest of the Basic Multilingual , which contains virtually all characters in common use[10] including most Chinese, Japanese and Korean characters. Four bytes are needed for characters in the other planes of Unicode, which include less common CJK characters, various historic scripts, mathematical symbols, and (pictographic symbols).

The salient features of this scheme are as follows:

Backward compatibility: One-byte codes are used for the ASCII values 0 through 127, so ASCII text is valid UTF-8. Bytes in this range are not used anywhere else, so UTF-8 text can be processed by that can handle extended ASCII but only applies special meaning to ASCII characters, as it will not accidentally see those ASCII characters in the middle of a multi-byte character. Clear indication of byte sequence length: The first byte indicates the number of bytes in the sequence. This makes UTF-8 a prefix code: reading from a stream can instantaneously decode each individual fully received sequence, without first having to wait for either the first byte of a next sequence or an end-of-stream indication. The length of multi-byte sequences is easily determined as it is simply the number of high-order 1s in the leading byte. Self-synchronization: The leading bytes and the continuation bytes do not share values (continuation bytes start with 10 while single bytes start with 0 and longer lead bytes start with 11). This means a search will not accidentally find the sequence for one character starting in the middle of another character. It also means the start of a character can be found from a random position by backing up at most 3 bytes to find the leading byte. Sorting order: The chosen values of the leading bytes and the fact that the continuation bytes have the high-order bits first means that sorting a list of UTF-8 strings will produce the same order as sorting the equivalent UTF-32 strings.

Examples

Consider the encoding of the , €.

1. The Unicode code point for "€" is U+20AC. 2. According to the scheme table above, this will take three bytes to encode, since it is between U+0800 and U+FFFF. 3. 20AC is binary 0010 0000 1010 1100. The two leading zeros are added because, as the scheme table shows, a three-byte encoding needs exactly sixteen bits from the code point. 4. Because the encoding will be three bytes long, its leading byte starts with three 1s, then a 0 (1110...) 5. The first four bits of the code point are stored in the remaining low order four bits of this byte (1110 0010), leaving 12 bits of the code point yet to be encoded (...0000 1010 1100). 6. All continuation bytes contain exactly six bits from the code point. So the next six bits of the code point are stored in the low order six bits of the next byte, and 10 is stored in the high order two bits to mark it as a continuation byte (so 1000 0010). 7. Finally the last six bits of the code point are stored in the low order six bits of the final byte, and again 10 is stored in the high order two bits (1010 1100).

The three bytes 1110 0010 1000 0010 1010 1100 can be more concisely written in hexadecimal, as E2 82 AC.

Since UTF-8 uses groups of six bits, it is sometimes useful to use octal notation which uses 3-bit groups. With a calculator which can convert between hexadecimal and octal it can be easier to manually create or interpret UTF-8 compared with using binary.

Octal 0200–3777 (hex 80-7FF) shall be coded with two bytes. xxyy will be 3xx 2yy. Octal 4000–177777 (hex 800-FFFF) shall be coded with three bytes. xxyyzz will be (340+xx) 2yy 2zz. Octal 200000-4177777 (hex 10000-10FFFF) shall be coded with four bytes. wxxyyzz will be 36w 2xx 2yy 2zz.

The following table summarises this conversion, as well as others with different lengths in UTF-8. The colors indicate how bits from the code point are distributed among the UTF-8 bytes. Additional bits added by the UTF-8 encoding process are shown in black.

Character Octal code point Binary code point Binary UTF-8 Octal UTF-8 Hexadecimal UTF-8 $ U+0024 044 010 0100 00100100 044 24 ¢ U+00A2 0242 000 1010 0010 11000010 10100010 302 242 C2 A2 € U+20AC 020254 0010 0000 1010 1100 11100010 10000010 10101100 342 202 254 E2 82 AC U+10348 0201510 0 0001 0000 0011 0100 1000 11110000 10010000 10001101 10001000 360 220 215 210 F0 90 8D 88

Codepage layout

The following table summarizes usage of UTF-8 code units (individual bytes or octets) in a format. The upper half (0_ to 7_) is for bytes only used in single- byte codes, so it looks like a normal code page; the lower half is for continuation bytes (8_ to B_) and (possible) leading bytes (C_ to F_), and is explained further in the legend below.

UTF-8 _0 _1 _2 _3 _4 _5 _6 _7 _8 _9 _A _B _C _D _E _F NUL SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI 0_ 0000 0001 0002 0003 0004 0005 0006 0007 0008 0009 000A 000B 000C 000D 000E 000F 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US 1_ 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 001A 001B 001C 001D 001E 001F 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 SP ! " # $ % & ' ( ) * + , - . / 2_ 0020 0021 0022 0023 0024 0025 0026 0027 0028 0029 002A 002B 002C 002D 002E 002F 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 0 1 2 3 4 5 6 7 8 9 : ; < = > ? 3_ 0030 0031 0032 0033 0034 0035 0036 0037 0038 0039 003A 003B 003C 003D 003E 003F 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 @ A B C D E F G H K M N 4_ 0040 0041 0042 0043 0044 0045 0046 0047 0048 0049 004A 004B 004C 004D 004E 004F 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 P Q R S T U V W X Y Z [ \ ] ^ _ 5_ 0050 0051 0052 0053 0054 0055 0056 0057 0058 0059 005A 005B 005C 005D 005E 005F 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 ` a b c d e f g h i j k l m n o 6_ 0060 0061 0062 0063 0064 0065 0066 0067 0068 0069 006A 006B 006C 006D 006E 006F 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 p q r s t u v w x y z { | } ~ DEL 7_ 0070 0071 0072 0073 0074 0075 0076 0077 0078 0079 007A 007B 007C 007D 007E 007F 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 • • • • • • • • • • • • • • • • 8_ +00 +01 +02 +03 +04 +05 +06 +07 +08 +09 +0A +0B +0C +0D +0E +0F 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 • • • • • • • • • • • • • • • • 9_ +10 +11 +12 +13 +14 +15 +16 +17 +18 +19 +1A +1B +1C +1D +1E +1F 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 • • • • • • • • • • • • • • • • A_ +20 +21 +22 +23 +24 +25 +26 +27 +28 +29 +2A +2B +2C +2D +2E +2F 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 • • • • • • • • • • • • • • • • B_ +30 +31 +32 +33 +34 +35 +36 +37 +38 +39 +3A +3B +3C +3D +3E +3F 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191

2-byte Latin Latin Latin Latin Latin Latin Latin IPA IPA IPA accents accents Greek Greek C_ 0000 0040 0080 00C0 0100 0140 0180 01C0 0200 0240 0280 02C0 0300 0340 0380 03C0 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207

2-byte Cyril Cyril Cyril Cyril Cyril Armeni Hebrew Hebrew Arabic Arabic Arabic Arabic Syriac Arabic Thaana N'Ko D_ 0400 0440 0480 04C0 0500 0540 0580 05C0 0600 0640 0680 06C0 0700 0740 0780 07C0 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223

3-byte Indic Misc. Kana, CJK CJK CJK CJK CJK CJK CJK Asian Hangul Hangul PUA Forms E_ 0800* 1000 2000 3000 4000 5000 6000 7000 8000 9000 A000 B000 C000 D000 E000 F000 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239

4‑byte SMP, SIP SSP, SPUA SPUA-B 5-byte 5-byte 5-byte 5-byte 6-byte 6-byte F_ 10000* 40000 80000 C0000 100000 140000 180000 1C0000 200000* 1000000 2000000 3000000 4000000* 40000000 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255

Orange cells with a large dot are continuation bytes. The hexadecimal number shown after a "+" plus sign is the value of the six bits they add.

White cells are the leading bytes for a sequence of multiple bytes, the length shown at the left edge of the row. The text shows the Unicode blocks encoded by sequences starting with this byte, and the hexadecimal code point shown in the cell is the lowest character value encoded using that leading byte.

Red cells must never appear in a valid UTF-8 sequence. The first two (C0 and C1) could only be used for an invalid "overlong encoding" of ASCII characters (i.e., trying to encode a 7-bit ASCII value between 0 and 127 using two bytes instead of one; see below). The remaining red cells indicate leading bytes of sequences that could only encode numbers larger than the 0x10FFFF limit of Unicode, or that were also never used in the original design for 31 bits (FE and FF).

Pink cells are the leading bytes for a sequence of multiple bytes, of which some, but not all, possible continuation sequences are valid. E0 and F0 could start overlong encodings, in this case the lowest non-overlong-encoded code point is shown, marked by an asterisk "*". F4 can start code points greater than 0x10FFFF which are invalid. ED can start the encoding of a code point in the range 0xD800–0xDFFF; these are invalid since they are reserved for UTF-16 surrogate halves.

Overlong encodings

In principle, it would be possible to inflate the number of bytes in an encoding by padding the code point with leading 0s. To encode the Euro sign € from the above example in four bytes instead of three, it could be padded with leading 0s until it was 21 bits long – 000 000010 000010 101100, and encoded as 11110000 10000010 10000010 10101100 (or F0 82 82 AC in hexadecimal). This is called an overlong encoding.

The standard specifies that the correct encoding of a code point use only the minimum number of bytes required to hold the significant bits of the code point. Longer encodings are called overlong and are not valid UTF-8 representations of the code point. This rule maintains a one-to-one correspondence between code points and their valid encodings, so that there is a unique valid encoding for each code point. This ensures that string comparisons and searches are well-defined.

Modified UTF-8 uses the two-byte overlong encoding of U+0000 (the NUL character), 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00). This allows the byte 00 to be used as a string terminator.

Invalid byte sequences

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 leading byte not followed by enough continuation bytes (can happen in simple string truncation, when a string is too long to fit when copying it) an overlong encoding as described above a sequence that decodes to an invalid code point as described below

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, , or quotes. Invalid UTF-8 has been used to bypass security validations in high-profile products including 's IIS [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] This can turn what would otherwise be harmless errors (producing a message such as "no such file") into a denial of service bug. Early versions of Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8,[15] making it impossible to handle such errors.

More recent converters translate the first byte of an invalid sequence to a replacement character and continue parsing with the next byte. These error bytes will always have the high bit set. This avoids denial-of-service bugs, and it is very common in text rendering such as browser display, since mangled text is probably more useful than nothing for helping the user figure out what the string was supposed to contain. Popular replacements include:

The replacement character "�" (U+FFFD) (or EF BF BD in UTF-8) The invalid Unicode code points U+DC80–U+DCFF where the low eight bits are the byte's value.[16] Sometimes it is called UTF-8B[17] The Unicode code points U+0080–U+00FF with the same value as the byte, thus interpreting the bytes according to ISO-8859-1 The Unicode code point for the character represented by the byte in CP1252, which is similar to using ISO-8859-1, except that some characters in the range 0x80–0x9F are mapped into different Unicode code points. For example, 0x80 becomes the Euro sign, U+20AC.

These replacement algorithms are "lossy", as more than one sequence is translated to the same code point. This means that it would not be possible to reliably convert back to the original encoding, therefore losing information. Reserving 128 code points (such as U+DC80–U+DCFF) to indicate errors, and defining the UTF-8 encoding of these points as invalid so they convert to 3 errors, would seem to make conversion lossless. But this runs into practical difficulties: the converted text cannot be modified such that errors are arranged so they convert back into valid UTF-8, which means if the conversion is UTF-16, it cannot also be used to store arbitrary invalid UTF-16, which is usually needed on the same systems that need invalid UTF-8. U+DC80–U+DCFF are reserved for UTF-16 surrogates, so that when they are used for UTF-8 in this way, and the string is converted to UTF-16 this can lead to bugs or the string being rejected.

The large number of invalid byte sequences provides the advantage of making it easy to have a program accept both UTF-8 and legacy encodings such as ISO-8859-1. Software can check for UTF-8 correctness, and if that fails assume the input to be in the legacy encoding. It is technically true that this may detect an ISO-8859-1 string as UTF-8, but this is very unlikely if it contains any 8-bit bytes as they all have to be in unusual patterns of two or more in a row, such as "£".

Invalid code points

Since RFC 3629 (November 2003), the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) and code points not encodable by UTF-16 (those after U+10FFFF) are not legal Unicode values, and their UTF-8 encoding must be treated as an invalid byte sequence.

Not decoding surrogate halves makes it impossible to store invalid UTF-16, such as Windows filenames, as UTF-8. Therefore, detecting these as errors is often not implemented and there are attempts to define this behavior formally (see WTF-8 and CESU below).

Official name and variants

The official Internet Assigned Numbers Authority (IANA) code for the encoding is "UTF-8".[18] 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 IANA list (which include CSS, HTML, XML, and HTTP headers),[19] as the declaration is case insensitive.[18]

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.[13] 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.

Unofficially, UTF-8-BOM and UTF-8-NOBOM are sometimes used to refer to text files which respectively contain and lack a byte order mark (BOM). In Japan especially, UTF-8 encoding without BOM is sometimes called "UTF-8N".[20][21]

In PCL, UTF-8 is called Symbol-ID "18N" (PCL supports 183 character encodings, called Symbol Sets, which potentially could be reduced to one, 18N, that is UTF- 8).[22]

Derivatives

The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification.

CESU-8

Many programs added UTF-8 conversions for UCS-2 data and did not alter this UTF-8 conversion when UCS-2 was replaced with the surrogate-pair using UTF-16. In such programs each half of a UTF-16 surrogate pair is encoded as its own three-byte UTF-8 encoding, resulting in six-byte sequences rather than four bytes for characters outside the Basic Multilingual Plane. Oracle and MySQL databases use this, as well as Java and Tcl as described below, and probably many Windows programs where the programmers were unaware of the complexities of UTF-16. Although this non-optimal encoding is generally not deliberate, a supposed benefit is that it preserves UTF-16 binary sorting order when CESU-8 is binary sorted.

Modified UTF-8

In Modified UTF-8 (MUTF-8),[23] the null character (U+0000) uses the two-byte overlong encoding 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00). Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000,[24] 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 (https://docs.oracle.com/javase/ 8/docs/api/java/io/InputStreamReader.) and OutputStreamWriter (https://docs.oracle.com/javase/8/docs/api/java/io/OutputStreamWriter.html) (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object serialization[25] among other applications of DataInput (https://docs.oracle.c /javase/8/docs/api/java/io/DataInput.html) and DataOutput (https://docs.oracle.com/javase/8/docs/api/java/io/DataOutput.html), for the Java Native Interface,[26] and for embedding constant strings in class files.[27] The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[28] Tcl also uses the same modified UTF-8[29] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

WTF-8

WTF-8 (Wobbly Transformation Format – 8-bit) is an extension of UTF-8 where the encodings of the surrogate halves (U+D800 through U+DFFF) are allowed.[30] This is necessary to store possibly-invalid UTF-16, such as Windows filenames. Many systems that deal with UTF-8 work this way without considering it a different encoding, as it is simpler.

WTF-8 has been used to refer to erroneously doubly-encoded UTF-8.[31][32][33]

Byte order mark

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. A BOM can also appear if another encoding with a BOM is translated to UTF-8 without stripping it. Software that is not aware of multibyte encodings will display the BOM as three garbage characters at the start of the document, e.g. "" in software interpreting the document as ISO 8859-1 or Windows-1252 or "" if interpreted as , a default for certain older Windows console applications.

The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but does allow the character to be at the start of a file.[34] The presence of the UTF-8 BOM may cause 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 shebang syntax. Another example is Internet Explorer which will render pages in standards mode only when it starts with a document type declaration. Programs that insert information at the start of a file will break use of the BOM to identify UTF-8 (one example is offline browsers that add the originating URL to the start of the file).

History

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, among other problems, and the biggest problem was probably that it did not have a clear separation between ASCII and non-ASCII: new UTF-1 tools would be backward compatible with ASCII- encoded text, but UTF-1-encoded text could confuse existing code expecting ASCII (or extended ASCII), because it could contain continuation bytes in the range 0x21–0x7E that meant something else in ASCII, e.g., 0x2F for '/', the Unix path directory separator, and this example is reflected in the name and introductory text of its replacement. The table below was derived from a textual description in the annex.

UTF-1 Number First Last Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 of bytes code point code point 1 U+0000 U+009F 00–9F 2 U+00A0 U+00FF A0 A0–FF 2 U+0100 U+4015 A1–F5 21–7E, A0–FF 3 U+4016 U+38E2D F6–FB 21–7E, A0–FF 21–7E, A0–FF 5 U+38E2E U+7FFFFFFF FC–FF 21–7E, A0–FF 21–7E, A0–FF 21–7E, A0–FF 21–7E, A0–FF

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. The name File System Safe UCS Transformation Format (FSS-UTF) and most of the text of this proposal were later preserved in the final specification.[35][36][37][38]

FSS-UTF proposal (1992) Number Bits for First Last Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 of bytes code point code point code point 1 7 U+0000 U+007F 0xxxxxxx 2 13 U+0080 U+207F 10xxxxxx 1xxxxxxx 3 19 U+2080 U+8207F 110xxxxx 1xxxxxxx 1xxxxxxx 4 25 U+82080 U+208207F 1110xxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 5 31 U+2082080 U+7FFFFFFF 11110xxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx

In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. A modification by Ken Thompson of the Plan 9 group at Bell Labs made it somewhat less bit-efficient than the previous proposal but crucially allowed it to be self-synchronizing, letting a reader start anywhere and immediately detect byte sequence boundaries. It also abandoned the use of biases and instead added the rule that only the shortest possible encoding is allowed; the additional loss in compactness is relatively insignificant, but readers now have to look out for invalid encodings to avoid reliability and especially security issues. Thompson's design was outlined on September 2, 1992, on a placemat in a New Jersey diner with Rob Pike. In 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, which accepted it as the specification for FSS-UTF.[37]

FSS-UTF (1992) / UTF-8 (1993)[1] Number Bits for First Last Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 of bytes code point code point code point 1 7 U+0000 U+007F 0xxxxxxx 2 11 U+0080 U+07FF 110xxxxx 10xxxxxx 3 16 U+0800 U+FFFF 1110xxxx 10xxxxxx 10xxxxxx 4 21 U+10000 U+1FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx 5 26 U+200000 U+3FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 6 31 U+4000000 U+7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx

UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25 to 29, 1993.

In November 2003, UTF-8 was restricted by RFC 3629 to match the constraints of the UTF-16 character encoding: explicitly prohibiting code points corresponding to the high and low surrogate characters removed more than 3% of the three-byte sequences, and ending at U+10FFFF removed more than 48% of the four-byte sequences and all five- and six-byte sequences.

Google reported that in 2008, UTF-8 (labelled "Unicode") became the most common encoding for HTML files.[39]

Advantages and disadvantages

General

Advantages

UTF-8 is the only encoding for XML entities that does not require a BOM or an indication of the encoding.[40] 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.[41] Valid UTF-8 cannot contain a lone byte with the high bit set, and the chance that any pair of bytes both with the high bit set is valid UTF-8 is 11.7%[nb 1] and the odds are much lower for longer sequences. This makes it extremely unlikely that text in any other encoding (such as ISO/IEC 8859-1) is valid UTF-8. This is an advantage that most other encodings do not have, and allows UTF-8 to be mixed with a legacy encoding without having to add data to identify which encoding is in use, avoiding errors () typically encountered when trying to change a system to a new default encoding, or introducing data with a different encoding than the default one. Sorting a set of UTF-8 encoded strings as strings of unsigned bytes yields the same order as sorting the corresponding Unicode strings lexicographically by codepoint. UTF-8 is ASCII transparent. As a result, old documents written in English can typically be safely auto-detected as UTF-8, and documents in other Western languages will often pass through with relatively few errors. Even in non-western languages, characters from the ASCII set are common in XML documents because the XML markup depends on them, and because most XML schema use tag names in English. UTF-8 uses the codes 0–127 only for the ASCII characters. This means that UTF-8 is an ASCII extension and can be processed by software that supports 7-bit characters and assigns no meaning to non-ASCII bytes. This means that strings in most programming languages which use single byte characters can use the same native interface. This greatly simplifies adapting software for international use as only the part of the software that deals with user interaction and system software to present glyphs need to be upgraded. By contrast, in Shift-JIS and many other local multi-byte encodings, a byte that can be a 7-bit ASCII character can also be used as part of a multi-byte character; the byte 0x5C, for example, might be part of a multibyte character, but in the context of a string some programming languages or application software would instead interpret it as a backslash ('\') and assume that it marks the beginning of an , incorrectly influencing the interpretation of subsequent bytes.[42] And if using UTF-16 for an upgrade of old software, all code which handle strings must be rewritten. If translating strings using UTF-16 software such as Excel, conversion to UTF-8 is well-defined and done by copy-and-paste into the editor, in opposite to using various local code pages for internationalization.

Comparison with single-byte encodings

Advantages

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 scripts at the same time. For many scripts there have been more than one single-byte encoding in usage, so even knowing the 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 (0377) also eliminates the need to escape this byte in Telnet (and FTP control connection).

Disadvantages

UTF-8 encoded text is larger than specialized single-byte encodings except for plain ASCII characters. In the case of scripts which used 8-bit character sets with non-Latin characters encoded in the upper half (such as most Cyrillic and Greek alphabet code pages), characters in UTF-8 will be double the size. For some scripts, such as and (which is used by various South Asian languages), characters will triple in size. There are even examples where a single byte turns into a composite character in Unicode and is thus six times larger in UTF-8. This has caused objections in India and other countries. It is possible in UTF-8 (or any other multi-byte encoding) to split or truncate a string in the middle of a character. This can result in an invalid string which some software refuse to accept. A good parser should ignore a truncated character at the end, which is easy in UTF-8 but tricky in some other multi-byte encodings. If the code points are all the same size, measurements of a fixed number of them is easy. Due to ASCII-era documentation where "character" is used as a synonym for "byte" this is often considered important. However, by measuring string positions using bytes instead of "characters" most algorithms can be easily and efficiently adapted for UTF-8. Searching for a string within a long string can for example be done byte by byte. Some software, such as text editors, will refuse to correctly display or interpret UTF-8 unless the text starts with a byte order mark, and will insert such a mark. This has the effect of making it impossible to use UTF-8 with any older software that can handle ASCII-like encodings but cannot handle the byte order mark. This, however, is no problem of UTF-8 itself but one of bad software implementations.

Comparison with other multi-byte encodings

Advantages

UTF-8 can encode any Unicode character. Files in different scripts can be displayed correctly without having to choose the correct code page or font. For instance Chinese and Arabic can be supported (in the same text) without special codes inserted or manual settings to switch the encoding. UTF-8 is self-synchronizing: character boundaries are easily identified by scanning for well-defined bit patterns in either direction. If bytes are lost due to error or corruption, one can always locate the next valid character and resume processing. If there is a need to shorten a string to fit a specified field, the previous valid character can easily be found. Many multi-byte encodings are much harder to resynchronize. Any byte oriented string searching algorithm can be used with UTF-8 data, since the sequence of bytes for a character cannot occur anywhere else. Some older variable-length encodings (such as Shift JIS) did not have this property and thus made string-matching algorithms rather complicated. In Shift JIS the end byte of a character and the first byte of the next character could look like another legal character, something that can't happen in UTF-8. Efficient to encode using simple bit operations. UTF-8 does not require slower mathematical operations such as multiplication or division (unlike the obsolete UTF-1 encoding).

Disadvantages

UTF-8 will take more space than a multi-byte encoding designed for a specific script. East Asian legacy encodings generally used two bytes per character yet take three bytes per character in UTF-8.

Comparison with UTF-16

Advantages

Byte encodings and UTF-8 are represented by byte arrays in programs, and often nothing needs to be done to a function when converting from a byte encoding to UTF-8. UTF-16 is represented by 16-bit word arrays, and converting to UTF-16 while maintaining compatibility with existing ASCII-based programs (such as was done with Windows) requires every API and data structure that takes a string to be duplicated, one version accepting byte strings and another version accepting UTF-16. Text encoded in UTF-8 will be smaller than the same text encoded in UTF-16 if there are more code points below U+0080 than in the range U+0800..U+FFFF. This is true for all modern European languages. Most of the rich text formats (including HTML) contain a large proportion of ASCII characters for the sake of formatting, thus the size usually will be reduced significantly compared with UTF-16, even when the language mostly uses 3-byte long characters in UTF-8.[nb 2] Most communication and storage was designed for a stream of bytes. A UTF-16 string must use a pair of bytes for each code unit: The order of those two bytes becomes an issue and must be specified in the UTF-16 protocol, such as with a byte order mark. If an odd number of bytes is missing from UTF-16, the whole rest of the string will be meaningless text. Any bytes missing from UTF-8 will still allow the text to be recovered accurately starting with the next character after the missing bytes.

Disadvantages

Characters U+0800 through U+FFFF use three bytes in UTF-8, but only two in UTF-16. As a result, text in (for example) Chinese, Japanese or Devanagari will take more space in UTF-8 if there are more of these characters than there are ASCII characters. This is possible when data mainly consist of pure prose, but is lessened by the degree to which the context uses ASCII whitespace, digits, and punctuation.[nb 3]

See also

Alt code Character encodings in HTML Comparison of e-mail clients § Features Comparison of Unicode encodings GB 18030 Iconv – a standardized API used to convert between different character encodings ISO/IEC 8859 () Unicode and e-mail Unicode and HTML Universal Character Set UTF-8 in URIs UTF-EBCDIC UTF-9 and UTF-18 UTF-16/UCS-2

Notes

1. 1920 valid two-byte UTF-8 characters over 128 × 128 possible two-byte sequences. 2. The 2010-10-27 version of UTF-8 generated 169 KB when converted with Notepad to UTF-16, and only 101 KB when converted back to UTF-8. The 2010-11-22 version of यूनकोड (Unicode in Hindi) required 119 KB in UTF-16 and 76 KB in UTF-8. 3. The 2010-11-22 version of यूनकोड (Unicode in Hindi), when the pure text was pasted to Notepad, generated 19 KB when saved as UTF-16 and 22 KB when saved as UTF-8. References

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Retrieved 2009-05-22. "In orthodox UTF-8, a NUL byte (\x00) is represented by a NUL byte. […] But […] […] want NUL bytes inside […] strings […]" 30. Sapin, Simon (2016-03-11) [2014-09-25]. "The WTF-8 encoding" (https://simonsapin.github.io/wtf-8/). Archived (https://web.archive.org/web/20160524180037/https://simonsapin.github. io/wtf-8/) from the original on 2016-05-24. Retrieved 2016-05-24. 31. "WTF-8.com" (http://wtf-8.com/). 2006-05-18. Retrieved 2016-06-21. 32. Speer, Rob (2015-05-21). "ftfy (fixes text for you) 4.0: changing less and fixing more" (https://blog.luminoso.com/2015/05/21/ftfy-fixes-text-for-you-4-0-changing-less-and-fixing-more/). Retrieved 2016-06-21. 33. "WTF-8, a transformation format of code page 1252" (http://www-uxsup.csx.cam.ac.uk/~fanf2/hermes/doc/qsmtp/draft-fanf-wtf8.html). www-uxsup.csx.cam.ac.uk. Retrieved 2016-10-12. 34. "The Unicode Standard - Chapter 2" (http://www.unicode.org/versions/Unicode6.0.0/ch02.pdf) (PDF). p. 30. 35. "Appendix F. 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External links

There are several current definitions of UTF-8 in various standards documents: Look up UTF-8 in Wiktionary, the free RFC 3629 / STD 63 (2003), which establishes UTF-8 as a standard Internet protocol element dictionary. The Unicode Standard, Version 9.0 (http://www.unicode.org/versions/Unicode9.0.0/), §3.9 D92, §3.10 D95 (http://www.unicode.org/ versions/Unicode9.0.0/ch03.pdf) (2016) ISO/IEC 10646:2014 §9.1 (http://www.iso.org/iso/home/store/catalogue_ics/catalogue_detail_ics.htm?csnumber=63182)

They supersede the definitions given in the following obsolete works:

ISO/IEC 10646-1:1993 Amendment 2 / Annex R (1996) The Unicode Standard, Version 6.0 (http://www.unicode.org/versions/Unicode6.0.0/), §3.9 D92, §3.10 D95 (http://www.unicode.org/versions/Unicode6.0.0/ch03.pd f) (2010) The Unicode Standard, Version 5.0 (http://www.unicode.org/versions/Unicode5.0.0/), §3.9–§3.10 (http://www.unicode.org/versions/Unicode5.0.0/ch03.pdf) (2006) The Unicode Standard, Version 2.0, Appendix A (1996) RFC 2044 (1996) RFC 2279 (1998) The Unicode Standard, Version 3.0, §2.3 (2000) plus Corrigendum #1 : UTF-8 Shortest Form (2000) Unicode Standard Annex #27: Unicode 3.1 (http://www.unicode.org/reports/tr27/tr27-3.html) (2001)

They are all the same in their general mechanics, with the main differences being on issues such as allowed range of code point values and safe handling of invalid input.

Original UTF-8 paper (http://doc.cat-v.org/plan_9/4th_edition/papers/utf) (or pdf (https://web.archive.org/web/20000917055036/http://plan9.bell-labs.com/sys/doc/ utf.pdf)) for Plan 9 from Bell Labs RFC 5198 defines UTF-8 NFC for Network Interchange UTF-8 test pages by Andreas Prilop (http://www.user.uni-hannover.de/nhtcapri/multilingual1.html), Jost Gippert (http://titus.uni-frankfurt.de/indexe.htm?/unicode/ unitest.htm) and the World Wide Web Consortium (http://www.w3.org/2001/06/utf-8-test/UTF-8-demo.html) Unix/Linux: UTF-8/Unicode FAQ (http://www.cl.cam.ac.uk/~mgk25/unicode.html), Linux Unicode HOWTO (http://www.tldp.org/HOWTO/Unicode-HOWTO.html), UTF-8 and Gentoo (http://www.gentoo.org/doc/en/utf-8.xml) The Unicode/UTF-8-character table (http://www.utf8-chartable.de/) displays UTF-8 in a variety of formats (with Unicode and HTML encoding information) Characters, Symbols and the Unicode Miracle – Computerphile (https://www.youtube.com/watch?v=MijmeoH9LT4) on YouTube

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Categories: Encodings Character encoding Unicode Transformation Formats 1993 introductions

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