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UTF-8 is a variable-width
character encoding Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing them to be Data storage, stored, Data communication, transmi ...
used for electronic communication. Defined by the
Unicode Standard Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's writ ...
, the name is derived from ''Unicode'' (or ''Universal Coded Character Set'') ''Transformation Format 8-bit''. UTF-8 is capable of encoding all 1,112,06417 planes times 216 code points per plane, minus 211 technically-invalid
surrogates ''Surrogates'' is a 2009 American science fiction File:Imagination 195808.jpg, Space exploration, as predicted in August 1958 in the science fiction magazine ''Imagination (magazine), Imagination.'' Science fiction (sometimes shortened to sci ...
.
valid character
code point In character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardw ...
s in
Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requireme ...

Unicode
using one to four one-
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
(8-bit) code units. Code points with lower numerical values, which tend to occur more frequently, are encoded using fewer bytes. It was designed for
backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of a system, product, or technology that allows for interoperability Interoperability is a characteristic of a product or system, whose interfaces are completely ...
with
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the stu ...
: the first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII, so 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 / (
slash Slash may refer to: * Slash (punctuation), the "/" character Arts and entertainment Fictional characters * Slash (Marvel Comics) * Slash (Teenage Mutant Ninja Turtles), Slash (''Teenage Mutant Ninja Turtles'') Music * Slash (musician), stage n ...
) in filenames, \ (
backslash The backslash is a typographical mark used mainly in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and develop ...
) in
escape sequences An escape sequence is a combination of Character (computing), characters that has a meaning other than the literal characters contained therein; it is marked by one or more preceding (and possibly terminating) characters. Examples * In C (progra ...
, and % in
printf printf format string refers to a control parameter used by a class of in the input/output libraries of and many other . The string is written in a simple : characters are usually copied literally into the function's output, but format specif ...

printf
. UTF-8 was designed as a superior alternative to
UTF-1 UTF-1 is a method of transforming ISO 10646 The International Organization for Standardization (ISO; ) is an international standard are technical standards developed by international organizations (intergovernmental organizations), such as Cod ...
, a proposed variable-width encoding with partial ASCII compatibility which lacked some features including self-synchronization and fully ASCII-compatible handling of characters such as slashes.
Ken Thompson Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techni ...

Ken Thompson
and
Rob Pike Robert "Rob" C. Pike (born 1956) is a Canadian programmer A computer programmer, sometimes called a software developer, a programmer or more recently a coder (especially in more informal contexts), is a person who creates computer software ...
produced the first implementation for the Plan 9 operating system in September 1992. This led to its adoption by
X/OpenX/Open Company, Ltd., originally the Open Group for Unix Systems, was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of information technology. More specifically, the or ...
as its specification for ''FSS-UTF'', which would first be officially presented at
USENIX USENIX is an American 501(c)(3) nonprofit membership organization A membership organization is any organization that allows people to subscribe, and often requires them to pay a membership fee or "subscription". Membership organizations typicall ...
in January 1993 and subsequently adopted by the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is an open standards organization, which develops and promotes voluntary Internet standards, in particular the technical standards that comprise the Internet protocol suite (TCP/IP). It has no formal ...
(IETF) in () for future Internet standards work, replacing Single Byte Character Sets such as
Latin-1 ISO/IEC 8859-1:1998, ''Information technology — 8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, data units are those that are 8 bits wide (1 octet). Also, 8-bit c ...
in older RFCs. UTF-8 is by far the most common encoding for the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system An information system (IS) is a formal, sociotechnical Sociotechnical systems (STS) in organizational development is an approach to complex organizational ...
, accounting for 98% of all web pages, and up to 100.0% for some languages, as of 2021.


Naming

The official
Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network A computer ne ...
(IANA) code for the encoding 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. However, the name "utf-8" may be used by all standards conforming to the IANA list (which include
CSS #REDIRECT CSS #REDIRECT CSS Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation Introduction A presentation conveys information from a speaker to an audience. Presentations are typically demonst ...

CSS
,
HTML The HyperText Markup Language, or HTML is the standard markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacticall ...

HTML
,
XML Extensible Markup Language (XML) is a markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntactically distinguishable fro ...

XML
, and
HTTP headers HTTP header fields are components of the header section of request and response messages in the Hypertext Transfer Protocol The Hypertext Transfer Protocol (HTTP) is an application layer protocol for distributed, collaborative, hypermedia ...
), as the declaration is case-insensitive. Other variants, such as those that omit the hyphen or replace it with a space, i.e. "utf8" or "UTF 8", are not accepted as correct by the governing standards. Despite this, most
web browsers A web browser (commonly referred to as a browser) is application software Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typical ...

web 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 for text files which contain or don't contain a byte order mark (BOM), respectively. In Japan especially, UTF-8 encoding without a BOM is sometimes called "UTF-8N".
Windows XP Windows XP is a major release of the Windows NT Windows NT is a proprietary {{Short pages monitor more than 48% of the four-byte sequences and all five- and six-byte sequences.


Standards

There are several current definitions of UTF-8 in various standards documents: * / STD 63 (2003), which establishes UTF-8 as a standard Internet protocol element * defines UTF-8 Unicode equivalence, NFC for Network Interchange (2008) * ISO/IEC 10646:2014 §9.1 (2014) * ''The Unicode Standard, Version 11.0'' (2018) They supersede the definitions given in the following obsolete works: * ''The Unicode Standard, Version 2.0'', Appendix A (1996) * ISO/IEC 10646-1:1993 Amendment 2 / Annex R (1996) * (1996) * (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'' (2001) * ''The Unicode Standard, Version 5.0'' (2006) * ''The Unicode Standard, Version 6.0'' (2010) 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.


Comparison with other encodings

Some of the important features of this encoding are as follows: * ''Backward compatibility:'' Backward compatibility with ASCII and the enormous amount of software designed to process ASCII-encoded text was the main driving force behind the design of UTF-8. In UTF-8, single bytes with values in the range of 0 to 127 map directly to Unicode code points in the ASCII range. Single bytes in this range represent characters, as they do in ASCII. Moreover, 7-bit bytes (bytes where the most significant bit is 0) never appear in a multi-byte sequence, and no valid multi-byte sequence decodes to an ASCII code-point. A sequence of 7-bit bytes is both valid ASCII and valid UTF-8, and under either interpretation represents the same sequence of characters. Therefore, the 7-bit bytes in a UTF-8 stream represent all and only the ASCII characters in the stream. Thus, many text processors, parsers, protocols, file formats, text display programs, etc., which use ASCII characters for formatting and control purposes, will continue to work as intended by treating the UTF-8 byte stream as a sequence of single-byte characters, without decoding the multi-byte sequences. ASCII characters on which the processing turns, such as punctuation, whitespace, and control characters will never be encoded as multi-byte sequences. It is therefore safe for such processors to simply ignore or pass-through the multi-byte sequences, without decoding them. For example, ASCII whitespace may be used to tokenize a UTF-8 stream into words; ASCII line-feeds may be used to split a UTF-8 stream into lines; and ASCII NUL characters can be used to split UTF-8-encoded data into null-terminated strings. Similarly, many format strings used by library functions like "printf" will correctly handle UTF-8-encoded input arguments. * ''Fallback and auto-detection:'' Only a small subset of possible byte strings are a valid UTF-8 string: the bytes C0, C1, and F5 through FF cannot appear, and bytes with the high bit set must be in pairs, and other requirements. It is extremely unlikely that a readable text in any
extended ASCII Extended ASCII (EASCII or high ASCII) character encoding Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing th ...
is valid UTF-8. Part of the popularity of UTF-8 is due to it providing a form of backward compatibility for these as well. A UTF-8 processor which erroneously receives extended ASCII as input can thus "auto-detect" this with very high reliability. Fallback errors will be false negatives, and these will be rare. Moreover, in many applications, such as text display, the consequence of incorrect fallback is usually slight. A UTF-8 stream may simply contain errors, resulting in the auto-detection scheme producing false positives; but auto-detection is successful in the majority of cases, especially with longer texts, and is widely used. It also works to "fall back" or replace 8-bit bytes using the appropriate code-point for a legacy encoding only when errors in the UTF-8 are detected, allowing recovery even if UTF-8 and legacy encoding is concatenated in the same file. * ''Prefix code:'' The first byte indicates the number of bytes in the sequence. 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 by humans as it is simply the number of high-order 1s in the leading byte. An incorrect character will not be decoded if a stream ends mid-sequence. * ''Self-synchronizing code, Self-synchronization:'' The leading bytes and the continuation bytes do not share values (continuation bytes start with the bits while single bytes start with and longer lead bytes start with ). 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. An incorrect character will not be decoded if a stream starts mid-sequence, and a shorter sequence will never appear inside a longer one. * ''Sorting order:'' The chosen values of the leading bytes means that a list of UTF-8 strings can be sorted in code point order by sorting the corresponding byte sequences.


Single-byte

* UTF-8 can encode any Universal Character Set characters, 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). * 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 The Cyrillic script ( ) is a writing system used for various languages across Eurasia and is used as the national script in various Slavic languages, Slavic, Turkic languages, Turkic, Mongolic languages, Mongolic, Uralic languages, Uralic, Caucas ...
and Greek alphabet code pages), characters in UTF-8 will be double the size. For some scripts, such as Thai alphabet, Thai and Devanagari (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 variable-length encoding) to split or Data truncation, truncate a string in the middle of a character. If the two pieces are not re-appended later before interpretation as characters, this can introduce an invalid sequence at both the end of the previous section and the start of the next, and some decoders will not preserve these bytes and result in data loss. Because UTF-8 is self-synchronizing this will however never introduce a different valid character, and it is also fairly easy to move the truncation point backward to the start of a character. * 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; the self-synchronization property prevents false positives.


Other multi-byte

* UTF-8 can encode any
Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requireme ...

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 written in the same file without specialized markup or manual settings that specify an 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 data corruption, 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 such as are much harder to resynchronize. This also means that byte-oriented protocol, byte-oriented string-searching algorithms can be used with UTF-8 (as a character is the same as a "word" made up of that many bytes), optimized versions of byte searches can be much faster due to hardware support and lookup tables that have only 256 entries. Self-synchronization does however require that bits be reserved for these markers in every byte, increasing the size. * Efficient to encode using simple bitwise operations. UTF-8 does not require slower mathematical operations such as multiplication or division (unlike , and other encodings). * 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.


UTF-16

* Byte encodings and UTF-8 are represented by byte arrays in programs, and often nothing needs to be done to a function when converting source code from a byte encoding to UTF-8.
UTF-16 UTF-16 (16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers ...
is represented by 16-bit word arrays, and converting to UTF-16 while maintaining compatibility with existing
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the stu ...
-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. If backward compatibility is not needed, all string handling still must be modified. * 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. It is often true even for languages like Chinese, due to the large number of spaces, newlines, digits, and HTML markup in typical files. * Most communication (e.g. HTML and IP) and storage (e.g. for Unix) was designed for a Bitstream#Definition of bytestream, 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.


Derivatives

The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification and may be rejected by conforming UTF-8 applications.


CESU-8

Unicode Technical Report #26 assigns the name CESU-8 to a nonstandard variant of UTF-8, in which Unicode characters in Plane (Unicode), supplementary planes are encoded using six bytes, rather than the four bytes required by UTF-8. CESU-8 encoding treats each half of a four-byte UTF-16 surrogate pair as a two-byte UCS-2 character, yielding two three-byte UTF-8 characters, which together represent the original supplementary character. Unicode characters within the
Basic Multilingual Plane In the Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, conventi ...
appear as they would normally in UTF-8. The Report was written to acknowledge and formalize the existence of data encoded as CESU-8, despite the Unicode Consortium discouraging its use, and notes that a possible intentional reason for CESU-8 encoding is preservation of UTF-16 binary collation. CESU-8 encoding can result from converting UTF-16 data with supplementary characters to UTF-8, using conversion methods that assume UCS-2 data, meaning they are unaware of four-byte UTF-16 supplementary characters. It is primarily an issue on operating systems which extensively use UTF-16 internally, such as Microsoft Windows. In Oracle Database, the character set uses CESU-8 encoding, and is deprecated. The character set uses standards-compliant UTF-8 encoding, and is preferred. CESU-8 is prohibited for use in HTML5 documents.


MySQL utf8mb3

In MySQL, the character set is defined to be UTF-8 encoded data with a maximum of three bytes per character, meaning only Unicode characters in the
Basic Multilingual Plane In the Unicode Unicode, formally the Unicode Standard, is an information technology standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, conventi ...
(i.e. from
UCS-2 The Universal Coded Character Set (UCS, Unicode) is a standard set of characters Character(s) may refer to: Arts, entertainment, and media Literature * Character (novel), ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * Ch ...
) are supported. Unicode characters in Plane (Unicode), supplementary planes are explicitly not supported. is deprecated in favor of the character set, which uses standards-compliant UTF-8 encoding. is an alias for , but is intended to become an alias to in a future release of MySQL. It is possible, though unsupported, to store CESU-8 encoded data in , by handling UTF-16 data with supplementary characters as though it is UCS-2.


Modified UTF-8

''Modified UTF-8'' (MUTF-8) originated in the Java (programming language), Java programming language. In Modified UTF-8, the null character (U+0000) uses the two-byte overlong encoding (hexadecimal ), instead of (hexadecimal ). Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000, 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 language supports standard UTF-8 when reading and writing strings through and (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object Java serialization, serialization among other applications of and , for the Java Native Interface, and for embedding constant strings in Class (file format), class files. The dex format defined by Dalvik (software), Dalvik also uses the same modified UTF-8 to represent string values. Tcl also uses the same modified UTF-8 as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.


WTF-8

In WTF-8 (Wobbly Transformation Format, 8-bit) ''unpaired'' surrogate halves (U+D800 through U+DFFF) are allowed. 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. (The term "WTF-8" has also been used humorously to refer to Mojibake, erroneously doubly-encoded UTF-8 sometimes with the implication that CP1252 bytes are the only ones encoded.)


PEP 383

Version 3 of the Python programming language treats each byte of an invalid UTF-8 bytestream as an error (see also changes with new UTF-8 mode in Python 3.7); this gives 128 different possible errors. Extensions have been created to allow any byte sequence that is assumed to be UTF-8 to be losslessly transformed to UTF-16 or UTF-32, by translating the 128 possible error bytes to reserved code points, and transforming those code points back to error bytes to output UTF-8. The most common approach is to translate the codes to U+DC80...U+DCFF which are low (trailing) surrogate values and thus "invalid" UTF-16, as used by Python (programming language), Python's PEP 383 (or "surrogateescape") approach. Another encoding called MirBSD OPTU-8/16 converts them to U+EF80...U+EFFF in a Private Use Area. In either approach, the byte value is encoded in the low eight bits of the output code point. These encodings are very useful because they avoid the need to deal with "invalid" byte strings until much later, if at all, and allow "text" and "data" byte arrays to be the same object. If a program wants to use UTF-16 internally these are required to preserve and use filenames that can use invalid UTF-8; as the Windows filesystem API uses UTF-16, the need to support invalid UTF-8 is less there. For the encoding to be reversible, the standard UTF-8 encodings of the code points used for erroneous bytes must be considered invalid. This makes the encoding incompatible with WTF-8 or CESU-8 (though only for 128 code points). When re-encoding it is necessary to be careful of sequences of error code points which convert back to valid UTF-8, which may be used by malicious software to get unexpected characters in the output, though this cannot produce ASCII characters so it is considered comparatively safe, since malicious sequences (such as cross-site scripting) usually rely on ASCII characters.


See also

* Alt code * * Comparison of Unicode encodings ** GB 18030 ** UTF-EBCDIC * Iconv * * Specials (Unicode block) * Unicode and email * Unicode and HTML ** Character encodings in HTML


Notes


References


External links


Original UTF-8 paperor pdf
for Plan 9 from Bell Labs * UTF-8 test pages: *
Andreas Prilop
*

*

* Unix/Linux
UTF-8 and Gentoo
* {{Ken Thompson navbox Character encoding Computer-related introductions in 1993 Encodings Unicode Transformation Formats