UTF-8

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UTF-8 (UCS Transformation Format—8-bit[1]) is a variable-width encoding that can represent every character in the Unicode character set. It was designed for backward compatibility with ASCII and to avoid the complications of endianness and byte order marks in UTF-16 and UTF-32. UTF-8 has become the dominant character encoding for the World Wide Web, accounting for more than half of all Web pages.[2][3][4] The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8.[5] UTF-8 is also increasingly being used as the default character encoding in operating systems, programming languages, APIs, and software applications.[citation needed]

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.

The official IANA code for the UTF-8 character encoding is UTF-8.[6]

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, but did introduce the notion that bytes in the range of 0–127 continue representing the ASCII characters in UTF, thereby providing backward compatibility with ASCII.

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. This original proposal, the File System Safe UCS Transformation Format (FSS-UTF), was similar in concept to UTF-8, but lacked the crucial property of self-synchronization.[7][8]

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 made a small but crucial modification to the encoding, making it very slightly less bit-efficient than the previous proposal but allowing it to be self-synchronizing, meaning that it was no longer 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. 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.[7]

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

Google reported that in 2008 UTF-8 (misleadingly labelled "Unicode") became the most common encoding for HTML files.[9][10]

Description[edit]

The design of UTF-8 can be seen in this table of the scheme as originally proposed by Dave Prosser and subsequently modified by Ken Thompson (the x characters are replaced by the bits of the code point):

Bits of
code point
First
code point
Last
code point
Bytes in
sequence
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6
  7 U+0000 U+007F 1 0xxxxxxx
11 U+0080 U+07FF 2 110xxxxx 10xxxxxx
16 U+0800 U+FFFF 3 1110xxxx 10xxxxxx 10xxxxxx
21 U+10000 U+1FFFFF 4 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
26 U+200000 U+3FFFFFF 5 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
31 U+4000000 U+7FFFFFFF 6 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx

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 all 5- and 6-byte sequences, and about half of the 4-byte sequences.

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.
  • 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.
  • Self synchronization: Single bytes, leading bytes, and continuation bytes 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).
  • Clear indication of code 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, so that the length of the sequence can be determined without examining the continuation bytes.
  • 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 lead byte, lower-order bits in succeeding continuation bytes. The number of bytes in the encoding is 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 and Tāna 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[11]). 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]

Let us consider how to encode 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 0010000010101100. 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 it is a three-byte encoding, the leading byte starts with three 1s, then a 0 (1110 ...)
  5. The remaining bits of this byte are taken from the code point (11100010), leaving ... 000010101100.
  6. Each of the continuation bytes starts with 10 and takes six bits of the code point (so 10000010, then 10101100).

The three bytes 11100010 10000010 10101100 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 0100100 00100100 24
¢ U+00A2 000 10100010 11000010 10100010 C2 A2
U+20AC 00100000 10101100 11100010 10000010 10101100 E2 82 AC
𤭢 U+24B62 00010 01001011 01100010 11110000 10100100 10101101 10100010 F0 A4 AD A2

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_
 
2-byte
inval

(0000)
192
2-byte
inval

(0040)
193
Latin-1
0080
194
Latin-1
00C0
195
Latin
Ext-A

0100
196
Latin
Ext-A

0140
197
Latin
Ext-B

0180
198
Latin
Ext-B

01C0
199
Latin
Ext-B

0200
200
IPA
0240
201
IPA
0280
202
Spaci
Modif

02C0
203
Combi
Diacr

0300
204
Combi
Diacr

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
Armen
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
Surr

D000
237
Priv Use
E000
238
Forms
F000
239
 
4-byte
F_
 
Ancient
Sym,CJK

10000*
240
unall
40000
241
unall
80000
242
Tags
Priv

C0000
243
Priv
Use

100000
244
4-byte
inval

140000
245
4-byte
inval

180000
246
4-byte
inval

1C0000
247
5-byte
inval

200000*
248
5-byte
inval

1000000
249
5-byte
inval

2000000
250
5-byte
inval

3000000
251
6-byte
inval

4000000*
252
6-byte
inval

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 are the start 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 start byte. When a start byte could form both overlong and valid encodings, the lowest non-overlong-encoded code point 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 (i.e., trying to encode a 7-bit ASCII value between 0 and 127 using 2 bytes instead of 1). 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 would also be invalid if the subsequent bytes attempted to encode a value higher than 0x10FFFF.

Overlong encodings[edit]

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. Allowing multiple encodings would make testing for string equality difficult to define.

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. The leading byte prefix for a four-byte encoding is 11110, and so the complete, overlong encoding is 11110000 10000010 10000010 10101100 (or F0 82 82 AC in hexadecimal).

Although overlong encodings are forbidden in UTF-8, at least one derivative makes use of the form. Modified UTF-8 requires the Unicode code point U+0000 (the NUL character) to be encoded in the overlong form 11000000 10000000 (hex C0 80), rather than 00000000 (hex 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 start byte not followed by enough continuation bytes
  • an Overlong Encoding as described above
  • A 4-byte sequence (starting with 0xF4) 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[12] and Apache's Tomcat servlet container.[13]

RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[14] 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.[15] 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 or environment variables contained invalid UTF-8,[16] so it was impossible for any Python program to detect and recover from such an error.

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. These error bytes will always have the high bit set. Popular replacements include:

  • The replacement character "�" (U+FFFD)
  • The invalid Unicode code points U+DC80..U+DCFF where the low 8 bits are the byte's value.
  • 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. This 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": an invalid string is converted to the same sequence of code points that a valid UTF-8 string could have.

Many programs are specified to allow input in one of several encodings, for example UTF-8, UTF-16 or ISO-8859-1. In that case, the software would first check for UTF-8 correctness. If incorrect then it would check if it is UTF-16, and if not interpret it as entirely ISO-8859-1.

Invalid code points[edit]

According to the UTF-8 definition (RFC 3629) the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) are not legal Unicode values, and their UTF-8 encoding should be treated as an invalid byte sequence.

Whether an actual application should do this is debatable, as it makes it impossible to store invalid UTF-16 (that is, UTF-16 with unpaired surrogate halves) in a UTF-8 string. This is necessary to store unchecked UTF-16 such as Windows filenames as UTF-8. It is also incompatible with CESU encoding (described below).

Official name and variants[edit]

The official name is "UTF-8". 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 Internet Assigned Numbers Authority (IANA) list (which include CSS, HTML, XML, and HTTP headers),[17] 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.[14] 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 or UTF-8-NOBOM are sometimes used to refer to text files which contain or lack a byte order mark (BOM). In Japan especially, UTF-8 encoding without BOM is sometimes called "UTF-8N".[19][20]

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 3-byte UTF-8 encoding, resulting in 6-byte sequences rather than 4 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,[21] the null character (U+0000) is encoded as 0xC0,0x80; this is not valid UTF-8[22] 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,[23] 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,[24] for the Java Native Interface,[25] and for embedding constant strings in class files.[26] The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[27] Tcl also uses the same modified UTF-8[28] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

Extending from 31 bit to 36 bit range[edit]

Extending the accepted input pattern from 6 bytes to 7 bytes would allow over 70 billion code points including all 32-bit integers to be encoded;[29] however, this would require an initial byte value of 0xFE to be accepted as a 7-byte sequence indicator (see under Advantages in section "Compared to single-byte encodings").

Byte order mark[edit]

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 strange characters (e.g. "" in software interpreting the document as ISO 8859-1 or Windows-1252) at the start of the document.

The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8.[30] 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.
  • 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).

If compatibility with existing programs is not important, the BOM could be used to identify UTF-8 encoding, but such use should not be necessary as UTF-8 can be identified with very high reliability since other encodings are extremely unlikely to contain valid UTF-8 byte sequences.

Advantages and disadvantages[edit]

General[edit]

Advantages[edit]

  • 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 byte 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.[31]
  • 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.[32] The probability of a random string of bytes which is not pure ASCII being valid UTF-8 is 3.9% for a two-byte sequence,[33] and decreases exponentially for longer sequences. 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 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.
  • 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.

Disadvantages[edit]

  • 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.[14]

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. This has caused objections in India and other countries.[citation needed]
  • 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.
  • 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.[citation needed]
  • 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 is considered an incorrect implementation of the text editor, not the older software.

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 7-bit ASCII characters are allowed as part of a multi-byte character, such as with 0x5C character, so this does not work, as a byte matching the ASCII '\' or '"' may appear as part of the text and change the handling of subsequent bytes.[34]
  • 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]

  • For certain scripts UTF-8 will take more space than an older multi-byte encoding. East Asian scripts generally have two bytes per character in their multi-byte encodings yet take three bytes per character in UTF-8.

Compared to UTF-16[edit]

Advantages[edit]

  • A text byte stream cannot be losslessly converted to UTF-16, due to the possible presence of errors in the byte stream encoding. This causes unexpected and often severe problems attempting to use existing data in a system that uses UTF-16 as an internal encoding. Results are security bugs, DoS if bad encoding throws an exception, and data loss when different byte streams convert to the same UTF-16. Due to the ASCII compatibility and high degree of pattern recognition in UTF-8, random byte streams can be passed losslessly through a system using it, as interpretation can be deferred until display.
  • 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 programs (such as was done with Windows) requires every API and data structure that takes a string to be duplicated. Invalid encodings make the duplicated APIs not exactly map to each other, often making it impossible to do some action with one of them.
  • Characters outside the basic multilingual plane are not a special case. UTF-16 is often mistaken to be the obsolete constant-length UCS-2 encoding, leading to code that works for most text but suddenly fails for non-BMP characters.
  • 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 of all modern European languages. As HTML markup characters, numbers (digits 0-9), spaces and line terminators are all code points below U+0080, this is often true even for Asian scripts.
  • 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. If any partial character is removed the corruption is always recognizable.

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 could take more space in UTF-8 if there are more of these characters than there are ASCII characters. This happens for pure text[35] but rarely for HTML documents or documents in XML based formats such as .docx or .odt. For example, both the Japanese UTF-8 and the Hindi Unicode articles on Wikipedia take more space in UTF-16 than in UTF-8.[36]

See also[edit]

References[edit]

  1. ^ "Chapter 2. General Structure". The Unicode Standard (6.0 ed.). Mountain View, California, USA: The Unicode Consortium. ISBN 978-1-936213-01-6. 
  2. ^ Davis, Mark (28 January 2010). "Unicode nearing 50% of the web". Official Google Blog. Google. Retrieved 5 December 2010. 
  3. ^ "UTF-8 Usage Statistics". BuiltWith. Retrieved March 28, 2011. 
  4. ^ "Usage of character encodings for websites". W3Techs. Retrieved March 30, 2010. 
  5. ^ "Using International Characters in Internet Mail". Internet Mail Consortium. August 1, 1998. Retrieved November 8, 2007. 
  6. ^ "CHARACTER SETS". Internet Assigned Numbers Authority. November 4, 2010. Retrieved 5 December 2010. 
  7. ^ a b Pike, Rob (30 Apr 2003). "UTF-8 history". Retrieved September 7, 2012. 
  8. ^ Pike, Rob (September 6, 2012). "UTF-8 turned 20 years old yesterday". Retrieved September 7, 2012. 
  9. ^ Davis, Mark (5 May 2008). "Moving to Unicode 5.1". Retrieved 2013-03-01. 
  10. ^ Goodger, David (6 May 2008). "Unicode misinformation". Retrieved 2013-03-01. 
  11. ^ 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." 
  12. ^ Marin, Marvin (October 17, 2000). "Web Server Folder Traversal MS00-078". 
  13. ^ "National Vulnerability Database - Summary for CVE-2008-2938". 
  14. ^ a b c Yergeau, F. (2003). "UTF-8, a transformation format of ISO 10646". RFC 3629. Internet Engineering Task Force. 
  15. ^ decode() method of Java UTF8 object
  16. ^ "Non-decodable Bytes in System Character Interfaces". 
  17. ^ Dürst, Martin. "Setting the HTTP charset parameter". W3C. Retrieved February 8, 2013. 
  18. ^ "Character Sets". Internet Assigned Numbers Authority. January 23, 2013. Retrieved February 8, 2013. 
  19. ^ "BOM - suikawiki" (in Japanese). Retrieved 2013-04-26. 
  20. ^ Mark Davis. "Forms of Unicode". IBM. Archived from the original on 6 May 2005. Retrieved 18 September 2013. 
  21. ^ "Java SE 6 documentation for Interface java.io.DataInput, subsection on Modified UTF-8". Sun Microsystems. 2008. Retrieved May 22, 2009. 
  22. ^ "Request for Comments 3629: "UTF-8, a transformation format of ISO 10646"". 2003. Retrieved May 22, 2009. "[...] the overlong UTF-8 sequence C0 80 [...]", "[...] the illegal two-octet sequence C0 80 [...]" 
  23. ^ "The Java Virtual Machine Specification, 2nd Edition, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Sun Microsystems. 1999. Retrieved May 24, 2009. "[...] Java virtual machine UTF-8 strings never have embedded nulls." 
  24. ^ "Java Object Serialization Specification, chapter 6: Object Serialization Stream Protocol, section 2: Stream Elements". Sun Microsystems. 2005. Retrieved May 22, 2009. "[...] encoded in modified UTF-8." 
  25. ^ "Java Native Interface Specification, chapter 3: JNI Types and Data Structures, section: Modified UTF-8 Strings". Sun Microsystems. 2003. Retrieved May 22, 2009. "The JNI uses modified UTF-8 strings to represent various string types." 
  26. ^ "The Java Virtual Machine Specification, 2nd Edition, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Sun Microsystems. 1999. Retrieved May 23, 2009. "[...] differences between this format and the "standard" UTF-8 format." 
  27. ^ "dex — Dalvik Executable Format". Retrieved April 9, 2013. "[T]he dex format encodes its string data in a de facto standard modified UTF-8 form, hereafter referred to as MUTF-8." 
  28. ^ "Tcler's Wiki: UTF-8 bit by bit (Revision 6)". April 25, 2009. Retrieved May 22, 2009. "In orthodox UTF-8, a NUL byte(\x00) is represented by a NUL byte. [...] But [...] we [...] want NUL bytes inside [...] strings [...]" 
  29. ^ "Computational Methods in Linguistic Research". University of Pennsylvania Department of Linguistics. 2004. Retrieved February 8, 2013. 
  30. ^ "The Unicode Standard - Chapter 2". p. 30. 
  31. ^ "Extensible Markup Language (XML) 1.0 (Fifth Edition)". W3C. 26 November 26, 2008. Retrieved February 8, 2013. 
  32. ^ Dürst, Martin. "Multilingual Forms". W3C. Retrieved February 8, 2013. 
  33. ^ There are 256 × 256 − 128 × 128 = 49152 2-byte sequences with at least one high bit set, only 1920 encode valid UTF-8 characters (the range U+0080 to U+07FF), 1920 ÷ 49152 = 3.9%
  34. ^ "#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. 
  35. ^ Although the difference may not be great: the 2010-11-22 version of hi: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.
  36. ^ The 2010-10-27 version of ja: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 hi:यूनिकोड (Unicode in Hindi) required 119 KB in UTF-16 and 76 KB in UTF-8.

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.