UTF-8

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UTF-8 is a character encoding capable of encoding all possible characters, or code points, defined by Unicode 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 to 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]

Shows the usage of the main encodings on the web from 2001 to 2012 as recorded by Google. Note that the ASCII only figure includes web pages with any declared header if they are restricted to ASCII characters.

UTF-8 is the dominant character encoding for the World Wide Web, accounting for 87.4% of all Web pages in August 2016 (with the most popular East Asian encodings, GB 2312, at 0.8% and Shift JIS at 1.1%).[3][4][5] The Internet Mail Consortium (IMC) recommends that all e-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 valid code points in the Unicode code space (1,114,112 code points minus 2,048 surrogate code points) using one to four 8-bit bytes (a group of 8 bits is known as an octet in the Unicode Standard). Code points with lower numerical values (i.e., earlier code positions in the Unicode character set, 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, making valid ASCII text valid UTF-8-encoded Unicode as well. And ASCII bytes do not occur when encoding non-ASCII code points into UTF-8, making UTF-8 safe to use within most programming and document languages that interpret certain ASCII characters in a special way, e.g. as end of string.

History[edit]

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.

Number
of bytes
First
code point
Last
code point
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5
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.[8][9][10][11]

Number
of bytes
Bits for
code point
First
code point
Last
code point
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5
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 operating system 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.[10]

Number
of bytes
Bits for
code point
First
code point
Last
code point
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6
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 (misleadingly labelled "Unicode"[12]) became the most common encoding for HTML files.[13]

Description[edit]

This table shows UTF-8 as it is since 2003 (the x characters are replaced by the bits of the code point):

Number
of bytes
Bits for
code point
First
code point
Last
code point
Byte 1 Byte 2 Byte 3 Byte 4
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+10FFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

The salient features of this scheme are as follows:

  • Backward compatibility: One-byte codes are used only for the ASCII values 0 through 127. In this case the UTF-8 code has the same value as the ASCII code. The high-order bit of these codes is always 0. This means that ASCII text is valid UTF-8, and UTF-8 can be used for parsers expecting 8-bit extended ASCII even if they are not designed for UTF-8.
  • Clear distinction between multi-byte and single-byte characters: Code points larger than 127 are represented by multi-byte sequences, composed of a leading byte and one or more continuation bytes. The leading byte has two or more high-order 1s followed by a 0, while continuation bytes all have '10' in the high-order position. Thus, no bytes representing ASCII characters appear in multi-byte sequences.
  • Self synchronization: The high order bits of every byte determine the type of byte; single bytes (0xxxxxxx), leading bytes (11xxxxxx), and continuation bytes (10xxxxxx) do not share values. This makes the scheme self-synchronizing, allowing the start of a character to be found by backing up at most five bytes (three bytes in actual UTF‑8 per RFC 3629 restriction, see above). The first byte of a valid character sequence will be either a single byte or leading byte.
  • Clear indication of byte sequence length: The number of high-order 1s in the leading byte of a multi-byte sequence indicates the number of bytes in the sequence. When reading from a stream, a reader can process all fully received sequences without first having to wait for either the leading byte of a next sequence or an end-of-stream indication.
  • Code structure: The remaining bits of the encoding are used for the bits of the code point being encoded, padded with high-order 0s if necessary. The high-order bits go in the leading byte, lower-order bits in subsequent continuation bytes. The number of bytes in the encoding must be the minimum required to hold all the significant bits of the code point.

The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode. This covers the remainder of almost all Latin alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac, Thaana and N'Ko alphabets, as well as Combining Diacritical Marks. Three bytes are needed for characters in the rest of the Basic Multilingual Plane, which contains virtually all characters in common use[14] 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 emoji (pictographic symbols).

Examples[edit]

Consider the encoding of the Euro sign, €.

  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. Hexadecimal 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.

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 Binary code point Binary UTF-8 Hexadecimal UTF-8
$ U+0024 010 0100 00100100 24
¢ U+00A2 000 1010 0010 11000010 10100010 C2 A2
U+20AC 0010 0000 1010 1100 11100010 10000010 10101100 E2 82 AC
𐍈 U+10348 0 0001 0000 0011 0100 1000 11110000 10010000 10001101 10001000 F0 90 8D 88

Codepage layout[edit]

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
2-byte
C_

0000
192

0040
193
Latin
0080
194
Latin
00C0
195
Latin
0100
196
Latin
0140
197
Latin
0180
198
Latin
01C0
199
Latin
0200
200
IPA
0240
201
IPA
0280
202
IPA
02C0
203
accents
0300
204
accents
0340
205
Greek
0380
206
Greek
03C0
207
2-byte
D_
Cyril
0400
208
Cyril
0440
209
Cyril
0480
210
Cyril
04C0
211
Cyril
0500
212
Armeni
0540
213
Hebrew
0580
214
Hebrew
05C0
215
Arabic
0600
216
Arabic
0640
217
Arabic
0680
218
Arabic
06C0
219
Syriac
0700
220
Arabic
0740
221
Thaana
0780
222
N'Ko
07C0
223
3-byte
E_
Indic
0800*
224
Misc.
1000
225
Symbol
2000
226
Kana, CJK
3000
227
CJK
4000
228
CJK
5000
229
CJK
6000
230
CJK
7000
231
CJK
8000
232
CJK
9000
233
Asian
A000
234
Hangul
B000
235
Hangul
C000
236
Hangul
D000
237
PUA
E000
238
Forms
F000
239
4‑byte
F_
SMP, SIP
10000*
240

40000
241

80000
242
SSP, SPUA
C0000
243
SPUA-B
100000
244

140000
245

180000
246

1C0000
247
5-byte
200000*
248
5-byte
1000000
249
5-byte
2000000
250
5-byte
3000000
251
6-byte
4000000*
252
6-byte
40000000
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 cast 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 surrogate half that cannot be encoded in UTF-16 and are also invalid.

Overlong encodings[edit]

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[edit]

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, slash, or quotes. Invalid UTF-8 has been used to bypass security validations in high-profile products including Microsoft's IIS web server[15] and Apache's Tomcat servlet container.[16]

RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[17] 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.[18] 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,[19] 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.[20] Sometimes it is called UTF-8B[21]
  • The Unicode code points U+0080–U+00FF with the same value as the byte, thus interpreting the bytes according to ISO-8859-1[citation needed]
  • The Unicode code point for the character represented by the byte in CP1252,[citation needed] 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.

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[edit]

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[edit]

The official Internet Assigned Numbers Authority (IANA) code for the encoding is "UTF-8".[22] All letters are upper-case, and the name is hyphenated. This spelling is used in all the Unicode Consortium 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),[23] as the declaration is case insensitive.[22]

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.[17] 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).[citation needed] In Japan especially, UTF-8 encoding without BOM is sometimes called "UTF-8N".[24][25]

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).[26]

Derivatives[edit]

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

CESU-8[edit]

Main article: 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[edit]

In Modified UTF-8 (MUTF-8),[27] 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,[28] 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 (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object serialization[29] among other applications of DataInput and DataOutput, for the Java Native Interface,[30] and for embedding constant strings in class files.[31] The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[32] Tcl also uses the same modified UTF-8[33] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

WTF-8[edit]

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 even when not paired.[34] 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.[35][36]

Byte order mark[edit]

Main article: 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 code page 437, default for 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.[37] 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 Unix 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).

Many programmers think that it is impossible to reliably detect UTF-8 without testing for a leading BOM. This is not true, the fact that a file is UTF-8 can be determined with surprisingly high probability by searching for valid multi-byte characters. If the bytes are random, the chances of a byte with the high bit set starting a valid UTF-8 character is only 6.64%. The chances of finding 7 of these without finding an invalid sequence is actually lower than the chance of the first three bytes randomly being the UTF-8 BOM.[citation needed]

Advantages and disadvantages[edit]

General[edit]

Advantages[edit]

  • UTF-8 is the only encoding for XML entities that does not require a BOM or an indication of the encoding.[38]
  • 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.[39] 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 (mojibake) 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.

Compared to single-byte encodings[edit]

Advantages[edit]

  • 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 script 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[edit]

  • 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 Thai and Hindi's Devanagari, 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 if the two halves are not concatenated later.
  • 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.

Compared to other multi-byte encodings[edit]

Advantages[edit]

  • 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. By contrast, in Shift-JIS 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 escape sequence, incorrectly influencing the interpretation of subsequent bytes.[40]
  • 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 beginning of the next valid character and resume processing. 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[edit]

  • 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.

Compared to UTF-16[edit]

Advantages[edit]

  • 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 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[edit]

  • 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 Hindi will take more space in UTF-8 if there are more of these characters than there are ASCII characters. This happens for pure text[nb 2] but actual documents often contain enough spaces and line terminators, numbers (digits 0–9), and HTML or XML or wiki markup characters, that they are shorter in UTF-8. For example, both the Japanese UTF-8 and the Hindi Unicode articles on Wikipedia take more space in UTF-16 than in UTF-8.[nb 3]

See also[edit]

Notes[edit]

  1. ^ 1920 valid two-byte UTF-8 characters over 128 × 128 possible two-byte sequences.
  2. ^ Although the difference may not be great: the 2010-11-22 version of यूनिकोड (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.
  3. ^ 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 यूनिकोड (Unicode in Hindi) required 119 KB in UTF-16 and 76 KB in UTF-8.

References[edit]

  1. ^ Email Subject: UTF-8 history, From: "Rob 'Commander' Pike", Date: Wed, 30 Apr 2003..., ...UTF-8 was designed, in front of my eyes, on a placemat in a New Jersey diner one night in September or so 1992...So that night Ken wrote packing and unpacking code and I started tearing into the C and graphics libraries. The next day all the code was done...
  2. ^ "Chapter 2. General Structure". The Unicode Standard (6.0 ed.). Mountain View, California, USA: The Unicode Consortium. ISBN 978-1-936213-01-6. 
  3. ^ Davis, Mark (2010-01-28). "Unicode nearing 50% of the web". Official Google Blog. Google. Retrieved 2010-12-05. 
  4. ^ "Usage Statistics of Character Encodings for Websites, (updated daily)". W3Techs. Retrieved 2016-02-21. 
  5. ^ "UTF-8 Usage Statistics". BuiltWith. Retrieved 2011-03-28. 
  6. ^ "Using International Characters in Internet Mail". Internet Mail Consortium. 1998-08-01. Retrieved 2007-11-08. 
  7. ^ "Specifying the document's character encoding", HTML5, World Wide Web Consortium, 2014-06-17, retrieved 2014-07-30 
  8. ^ "Appendix F. FFS-UTF File System Save UCS Transformation format" (PDF). The Unicode Standard 1.1. Archived (PDF) from the original on 2016-06-07. Retrieved 2016-06-07. 
  9. ^ Whistler, Kenneth (2001-06-12). "FSS-UTF, UTF-2, UTF-8, and UTF-16". Archived from the original on 2016-06-07. Retrieved 2006-06-07. 
  10. ^ a b Pike, Rob (2003-04-30). "UTF-8 history". Retrieved 2012-09-07. 
  11. ^ Pike, Rob (2012-09-06). "UTF-8 turned 20 years old yesterday". Retrieved 2012-09-07. 
  12. ^ Goodger, David (2008-05-06). "Unicode misinformation". Retrieved 2013-03-01. 
  13. ^ Davis, Mark (2008-05-05). "Moving to Unicode 5.1". Retrieved 2013-03-01. 
  14. ^ Allen, Julie D.; Anderson, Deborah; Becker, Joe; Cook, Richard, eds. (2012). "The Unicode Standard, Version 6.1". Mountain View, California: Unicode Consortium. The Basic Multilingual Plane (BMP, or Plane 0) contains the common-use characters for all the modern scripts of the world as well as many historical and rare characters. By far the majority of all Unicode characters for almost all textual data can be found in the BMP. 
  15. ^ Marin, Marvin (2000-10-17). "Web Server Folder Traversal MS00-078". 
  16. ^ "National Vulnerability Database - Summary for CVE-2008-2938". 
  17. ^ a b Yergeau, F. (2003). "RFC 3629 - UTF-8, a transformation format of ISO 10646". Internet Engineering Task Force. Retrieved 2015-02-03. 
  18. ^ decode() method of Java UTF8 object
  19. ^ "Non-decodable Bytes in System Character Interfaces". python.org. 2009-04-22. Retrieved 2014-08-13. 
  20. ^ Kuhn, Markus (2000-07-23). "Substituting malformed UTF-8 sequences in a decoder". Retrieved 2014-09-25. 
  21. ^ Sittler, B. (2006-04-02). "Binary vs. UTF-8, and why it need not matter". Retrieved 2014-09-25. 
  22. ^ a b "Character Sets". Internet Assigned Numbers Authority. 2013-01-23. Retrieved 2013-02-08. 
  23. ^ Dürst, Martin. "Setting the HTTP charset parameter". W3C. Retrieved 2013-02-08. 
  24. ^ "BOM - suikawiki" (in Japanese). Retrieved 2013-04-26. 
  25. ^ Davis, Mark. "Forms of Unicode". IBM. Archived from the original on 2005-05-06. Retrieved 2013-09-18. 
  26. ^ PCL Symbol Sets
  27. ^ "Java SE documentation for Interface java.io.DataInput, subsection on Modified UTF-8". Oracle Corporation. 2015. Retrieved 2015-10-16. 
  28. ^ "The Java Virtual Machine Specification, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Oracle Corporation. 2015. Retrieved 2015-10-16. Java virtual machine UTF-8 strings never have embedded nulls. 
  29. ^ "Java Object Serialization Specification, chapter 6: Object Serialization Stream Protocol, section 2: Stream Elements". Oracle Corporation. 2010. Retrieved 2015-10-16. […] encoded in modified UTF-8. 
  30. ^ "Java Native Interface Specification, chapter 3: JNI Types and Data Structures, section: Modified UTF-8 Strings". Oracle Corporation. 2015. Retrieved 2015-10-16. The JNI uses modified UTF-8 strings to represent various string types. 
  31. ^ "The Java Virtual Machine Specification, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Oracle Corporation. 2015. Retrieved 2015-10-16. […] differences between this format and the "standard" UTF-8 format. 
  32. ^ "ART and Dalvik". Android Open Source Project. Retrieved 2013-04-09. [T]he dex format encodes its string data in a de facto standard modified UTF-8 form, hereafter referred to as MUTF-8. 
  33. ^ "Tcler's Wiki: UTF-8 bit by bit (Revision 6)". 2009-04-25. Retrieved 2009-05-22. In orthodox UTF-8, a NUL byte (\x00) is represented by a NUL byte. […] But […] we […] want NUL bytes inside […] strings […] 
  34. ^ Sapin, Simon (2016-03-11) [2014-09-25]. "The WTF-8 encoding". Archived from the original on 2016-05-24. Retrieved 2016-05-24. 
  35. ^ "WTF-8.com". 2006-05-18. Retrieved 2016-06-21. 
  36. ^ Speer, Rob (2015-05-21). "ftfy (fixes text for you) 4.0: changing less and fixing more". Retrieved 2016-06-21. 
  37. ^ "The Unicode Standard - Chapter 2" (PDF). p. 30. 
  38. ^ "Extensible Markup Language (XML) 1.0 (Fifth Edition)". W3C. 2008-11-26. Retrieved 2013-02-08. 
  39. ^ Dürst, Martin. "Multilingual Forms". W3C. Retrieved 2013-02-08. 
  40. ^ "#418058 - iconv: half-smart on ascii compatible code conversion (shift-jis) - Debian Bug report logs". Bugs.debian.org. 2007-04-06. Retrieved 2014-06-13. 

External links[edit]

There are several current definitions of UTF-8 in various standards documents:

They supersede the definitions given in the following obsolete works:

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.