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UTF-8 is a variable-length
character encoding Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values tha ...
used for electronic communication. Defined by the
Unicode Standard Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
, the name is derived from ''Unicode'' (or ''Universal Coded Character Set'') ''Transformation Format 8-bit''. UTF-8 is capable of encoding all 1,112,064 valid character
code point In character encoding terminology, a code point, codepoint or code position is a numerical value that maps to a specific character. Code points usually represent a single grapheme—usually a letter, digit, punctuation mark, or whitespace—but ...
s in
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
using one to four one-
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
(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 an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especiall ...
with
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
: 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. UTF-8 was designed as a superior alternative to
UTF-1 UTF-1 is a method of transforming ISO/IEC 10646/Unicode into a stream of bytes. Its design does not provide self-synchronization, which makes searching for substrings and error recovery difficult. It reuses the ASCII printing characters for mult ...
, a proposed variable-length 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. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B programmi ...
and Rob Pike produced the first implementation for the Plan 9 operating system in September 1992. This led to its adoption by X/Open as its specification for ''FSS-UTF'', which would first be officially presented at USENIX in January 1993 and subsequently adopted by the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...
(IETF) in () for future internet standards work, replacing Single Byte Character Sets such as Latin-1 in older RFCs. UTF-8 is the dominant encoding for the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web ...
(and internet technologies), accounting for 98.0% of all web pages, and up to 100.0% for many languages, as of 2022.


Naming

The official
Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Inte ...
(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 Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone technolo ...
,
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaS ...
,
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
, and HTTP headers), 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 can understand them, and so standards intended to describe existing practice (such as HTML5) may effectively require their recognition. "UTF-8-BOM" and "UTF-8-NOBOM" are sometimes used for text files which contain or don't contain a
byte order mark The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of t ...
(BOM), respectively. In Japan especially, UTF-8 encoding without a BOM is sometimes called "UTF-8N". In
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for se ...
UTF-8 is codepage 65001 (i.e. CP_UTF8 in source code). In HP PCL, UTF-8 is called Symbol-ID "18N".


Encoding

UTF-8 encodes code points in one to four bytes, depending on the value of the code point. The characters are replaced by the bits of the code point: The first 128 code points (ASCII) need one byte. The next 1,920 code points need two bytes to encode, which covers the remainder of almost all
Latin-script alphabet A Latin-script alphabet (Latin alphabet or Roman alphabet) is an alphabet that uses letters of the Latin script. The 21-letter archaic Latin alphabet and the 23-letter classical Latin alphabet belong to the oldest of this group. The 26-letter ...
s, and also
IPA extensions IPA Extensions is a block (U+0250–U+02AF) of the Unicode standard that contains full size letters used in the International Phonetic Alphabet (IPA). Both modern and historical characters are included, as well as former and proposed IPA sign ...
,
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
,
Cyrillic The Cyrillic script ( ), Slavonic script or the Slavic script, is a writing system used for various languages across Eurasia. It is the designated national script in various Slavic, Turkic, Mongolic, Uralic, Caucasian and Iranic-speaking co ...
, Coptic, Armenian,
Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
,
Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...
,
Syriac Syriac may refer to: *Syriac language, an ancient dialect of Middle Aramaic *Sureth, one of the modern dialects of Syriac spoken in the Nineveh Plains region * Syriac alphabet ** Syriac (Unicode block) ** Syriac Supplement * Neo-Aramaic languages a ...
,
Thaana Thaana, Taana or Tāna (  ) is the present writing system of the Maldivian language spoken in the Maldives. Thaana has characteristics of both an abugida (diacritic, vowel-killer strokes) and a true alphabet (all vowels are written), ...
and
N'Ko N'Ko () is a script devised by Solomana Kante in 1949, as a modern writing system for the Mandé languages of West Africa. The term ''N'Ko'', which means ''I say'' in all Mandé languages, is also used for the Mandé literary standard written ...
alphabets, as well as
Combining Diacritical Marks Combining Diacritical Marks is a Unicode block containing the most common combining characters. It also contains the character " Combining Grapheme Joiner", which prevents canonical reordering of combining characters, and despite the name, actu ...
. Three bytes are needed for the rest of the
Basic Multilingual Plane In the Unicode standard, a plane is a continuous group of 65,536 (216) code points. There are 17 planes, identified by the numbers 0 to 16, which corresponds with the possible values 00–1016 of the first two positions in six position hexadeci ...
, which contains virtually all code points in common use, including most Chinese, Japanese and Korean characters. Four bytes are needed for code points in the other planes of Unicode, which include less common
CJK characters In internationalization, CJK characters is a collective term for the Chinese, Japanese, and Korean languages, all of which include Chinese characters and derivatives in their writing systems, sometimes paired with other scripts. Collectively, ...
, various historic scripts, mathematical symbols, and
emoji An emoji ( ; plural emoji or emojis) is a pictogram, logogram, ideogram or smiley embedded in text and used in electronic messages and web pages. The primary function of emoji is to fill in emotional cues otherwise missing from typed conv ...
(pictographic symbols). A "character" can take more than 4 bytes because it is made of more than one code point. For instance a national flag character takes 8 bytes since it's "constructed from a pair of Unicode scalar values" both from outside the BMP.


Examples

Consider the encoding of the
euro sign The euro sign () is the currency sign used for the euro, the official currency of the eurozone and unilaterally adopted by Kosovo and Montenegro. The design was presented to the public by the European Commission on 12 December 1996. It consists o ...
, €: # The Unicode code point for € is U+20AC. # As this code point lies between U+0800 and U+FFFF, this will take three bytes to encode. #
Hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, he ...
is binary . The two leading zeros are added because a three-byte encoding needs exactly sixteen bits from the code point. # Because the encoding will be three bytes long, its leading byte starts with three 1s, then a 0 () # The four most significant bits of the code point are stored in the remaining low order four bits of this byte (), leaving 12 bits of the code point yet to be encoded (). # 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 is stored in the high order two bits to mark it as a continuation byte (so ). # Finally the last six bits of the code point are stored in the low order six bits of the final byte, and again is stored in the high order two bits (). The three bytes can be more concisely written in
hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, he ...
, as . The following table summarizes 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.


Octal

UTF-8's use of six bits per byte to represent the actual characters being encoded means that
octal The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...
notation (which uses 3-bit groups) can aid in the comparison of UTF-8 sequences with one another and in manual conversion. With octal notation, the arbitrary octal digits, marked with x, y, z or w in the table, will remain unchanged when converting to or from UTF-8. :Example: Á = U+00C1 = (in octal) is encoded as in UTF-8 (C3 81 in hex). :Example: € = U+20AC = is encoded as in UTF-8 (E2 82 AC in hex).


Codepage layout

The following table summarizes usage of UTF-8 ''code units'' (individual
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s or
octets Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compo ...
) in a ''code'' page format. The upper half is for bytes used only in single-byte codes, so it looks like a normal code page; the lower half is for continuation bytes and leading bytes and is explained further in the legend below.


Overlong encodings

In principle, it would be possible to inflate the number of bytes in an encoding by padding the code point with leading 0s. To encode the euro sign € from the above example in four bytes instead of three, it could be padded with leading 0s until it was 21 bits long , and encoded as (or in hexadecimal). This is called an ''overlong encoding''. The standard specifies that the correct encoding of a code point uses 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.


Invalid sequences and error handling

Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for: * invalid bytes * an unexpected continuation byte * a non-continuation byte before the end of the character * the string ending before the end of the character (which can happen in simple string truncation) * an overlong encoding * a sequence that decodes to an invalid code point Many of the first UTF-8 decoders would decode these, ignoring incorrect bits and accepting overlong results. 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 IIS may refer to: Organizations * Indian Information Service, of the Government of India * Institute of Information Scientists, a professional association now merged into the Chartered Institute of Library and Information Professionals, UK * Inst ...
web server and Apache's Tomcat servlet container. states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences." ''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." Since RFC 3629 (November 2003), the high and low surrogate halves used by
UTF-16 UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as cod ...
(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 unpaired surrogate halves makes it impossible to store invalid UTF-16 (such as Windows filenames or UTF-16 that has been split between the surrogates) as UTF-8, while it is possible with WTF-8. Some implementations of decoders throw exceptions on errors. This has the disadvantage that it can turn what would otherwise be harmless errors (such as a "no such file" error) into a
denial of service In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host conne ...
. For instance early versions of Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8. An alternative practice is to replace errors with a replacement character. Since Unicode 6 (October 2010), the standard () has recommended a "best practice" where the error ends as soon as a disallowed byte is encountered. In these decoders is two errors (2 bytes in the first one). This means an error is no more than three bytes long and never contains the start of a valid character, and there are 21,952 different possible errors. The standard also recommends replacing each error with the replacement character "�" (U+FFFD).


Byte order mark

If the Unicode
byte order mark The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of t ...
(BOM, U+FEFF) character is at the start of a UTF-8 file, the first three bytes will be , , . The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but warns that it may be encountered at the start of a file trans-coded from another encoding. While ASCII text encoded using UTF-8 is backward compatible with ASCII, this is not true when Unicode Standard recommendations are ignored and a BOM is added. A BOM can confuse software that isn't prepared for it but can otherwise accept UTF-8, e.g. programming languages that permit non-ASCII bytes in
string literal A string literal or anonymous string is a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally " bracketed delimiters", as in x = "foo", where "foo" is a string ...
s but not at the start of the file. Nevertheless, there was and still is software that always inserts a BOM when writing UTF-8, and refuses to correctly interpret UTF-8 unless the first character is a BOM (or the file only contains ASCII).


Adoption

Many standards only support UTF-8, e.g. open
JSON JSON (JavaScript Object Notation, pronounced ; also ) is an open standard file format and data interchange format that uses human-readable text to store and transmit data objects consisting of attribute–value pairs and arrays (or other se ...
exchange requires it (without a byte order mark (BOM)). UTF-8 is also the recommendation from the
WHATWG The Web Hypertext Application Technology Working Group (WHATWG) is a community of people interested in evolving HTML and related technologies. The WHATWG was founded by individuals from Apple Inc., the Mozilla Foundation and Opera Software, l ...
for HTML and DOM specifications, and the
Internet Mail Consortium The Internet Mail Consortium (IMC) was an organization between 1996 and 2002 that claimed to be the only international organization focused on cooperatively managing and promoting the rapidly expanding world of electronic mail on the Internet. The ...
recommends that all e-mail programs be able to display and create mail using UTF-8. The
World Wide Web Consortium The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working ...
recommends UTF-8 as the default encoding in XML and HTML (and not just using UTF-8, also declaring it in metadata), "even when all characters are in the ASCII range .. Using non-UTF-8 encodings can have unexpected results". Lots of software has the ability to read/write UTF-8, and for some functions UTF-8 is the only option. In some cases it may though require the user to change options from the normal settings, or may require a BOM (byte order mark) as the first character to read the file. Examples of software supporting UTF-8 include
Google Drive Google Drive is a file storage and synchronization service developed by Google. Launched on April 24, 2012, Google Drive allows users to store files in the cloud (on Google's servers), synchronize files across devices, and share files. In add ...
and
LibreOffice LibreOffice () is a free and open-source office productivity software suite, a project of The Document Foundation (TDF). It was forked in 2010 from OpenOffice.org, an open-sourced version of the earlier StarOffice. The LibreOffice suite co ...
, most databases support UTF-8. UTF-8 has been the most common encoding for the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web ...
since 2008. , UTF-8 accounts for on average 98.0% of all web pages (and 990 of the top 1,000 highest ranked web pages). Although many pages only use ASCII characters to display content, few websites now declare their encoding to only be ASCII instead of UTF-8. Over a third of the languages tracked have 100% UTF-8 use. For local text files UTF-8 usage is less prevalent, where a few legacy single-byte (and a few CJK multi-byte) encodings remain in use to some degree. The primary cause for this is text editors refusing to use UTF-8 when processing files unless the first character of the file is a byte order mark (BOM). However, as most text editors do not expect a BOM, they do not insert a BOM at the start of files they save, causing compatibility issues with such editors. To support editors that expect a BOM, a BOM must be added manually to the start of the file. Many other text editors simply assume a UTF-8 encoding for all files due to its nigh-ubiquity. As of Windows 10, Windows Notepad defaults to writing UTF-8 without a BOM (a change since
Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on July 22, 2009, and became generally available on October 22, ...
), bringing it into line with most other text editors. With regard to system files, some system files on Windows 11 require UTF-8 with no requirement for a BOM, and almost all files on macOS and Linux are required to be UTF-8 without a BOM.
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
18 defaults to reading and writing files as UTF-8, and in older versions (e.g. LTS versions) only the
NIO are two wrathful and muscular guardians of the Buddha standing today at the entrance of many Buddhist temples in East Asian Buddhism in the form of frightening wrestler-like statues. They are dharmapala manifestations of the bodhisattva Vajrap ...
API was changed to do so. Many other programming languages default to UTF-8 for I/O, including
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
3.0 and R 4.2.2. All currently supported versions of Python support UTF-8 for I/O, even on Windows (where it is opt-in for the open() function), and plans exist to make UTF-8 I/O the default in Python 3.15 on all platforms. Usage of UTF-8 within software is also lower than in other areas as
UTF-16 UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as cod ...
is often used instead. This occurs particularly in Windows, but also in
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
, Python, Qt, and many other cross-platform software libraries. Compatibility with the
Windows API The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to several different platform implementations th ...
is the primary reason for this, though the belief that direct indexing of the BMP improves speed was also a factor. More recent software has started to use UTF-8 almost exclusively: the default string primitive in Go, Julia,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
, Swift 5, and PyPy uses UTF-8; a future version of Python is planned to store strings as UTF-8; and modern versions of
Microsoft Visual Studio Visual Studio is an integrated development environment (IDE) from Microsoft. It is used to develop computer programs including websites, web apps, web services and mobile apps. Visual Studio uses Microsoft software development platforms such ...
use UTF-8 internally (though still requiring a command-line switch to read or write UTF-8). UTF-8 is the "only text encoding mandated to be supported by the C++ standard" in
C++20 C20 or C-20 may refer to: Science and technology * Carbon-20 (C-20 or 20C), an isotope of carbon * C20, the smallest possible fullerene (a carbon molecule) * C20 (engineering), a mix of concrete that has a compressive strength of 20 newtons per sq ...
. All currently supported Windows versions support UTF-8 in some way (including
Xbox Xbox is a video gaming brand created and owned by Microsoft. The brand consists of five video game consoles, as well as applications (games), streaming services, an online service by the name of Xbox network, and the development arm by the ...
); partial support has existed since at least
Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was release to manufacturing, released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Wind ...
. As of May 2019, Microsoft has reversed its previous position of only recommending UTF-16; the capability to set UTF-8 as the encoding for the Windows API was introduced. As of 2020, Microsoft recommends programmers use UTF-8.


History

The
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in A ...
(ISO) set out to compose a universal multi-byte character set in 1989. The draft ISO 10646 standard contained a non-required
annex Annex or Annexe refers to a building joined to or associated with a main building, providing additional space or accommodations. It may also refer to: Places * The Annex, a neighbourhood in downtown Toronto, Ontario, Canada * The Annex (New ...
called UTF-1 that provided a byte stream encoding of its
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
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 Extended ASCII is a repertoire of character encodings that include (most of) the original 96 ASCII character set, plus up to 128 additional characters. There is no formal definition of "extended ASCII", and even use of the term is sometimes critic ...
), because it could contain continuation bytes in the range 0x21–0x7E that meant something else in ASCII, e.g., 0x2F for '/', the
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
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. In July 1992, the X/Open committee XoJIG was looking for a better encoding. Dave Prosser of
Unix System Laboratories Unix System Laboratories (USL), sometimes written UNIX System Laboratories to follow relevant trademark guidelines of the time, was an American software laboratory and product development company that existed from 1989 through 1993. At first wh ...
submitted a proposal for one that had faster implementation characteristics and introduced the improvement that 7-bit ASCII characters would only represent themselves; all multi-byte 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.


FSS-UTF

In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. A modification by
Ken Thompson Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B programmi ...
of the Plan 9
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
group at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
made it self-synchronizing, letting a reader start anywhere and immediately detect character boundaries, at the cost of being somewhat less bit-efficient than the previous proposal. 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 A placemat or table mat is a covering or pad designating an individual place setting, unlike the larger tablecloth that covers the entire surface. Placemats are made from many different materials, depending on their purpose: to protect, decorat ...
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. UTF-8 was first officially presented at the USENIX conference in
San Diego San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United States ...
, from January 25 to 29, 1993. The
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...
adopted UTF-8 in its Policy on Character Sets and Languages in RFC 2277 ( BCP 18) for future internet standards work, replacing Single Byte Character Sets such as Latin-1 in older RFCs. In November 2003, UTF-8 was restricted by to match the constraints of the
UTF-16 UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as cod ...
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.


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 NFC for Network Interchange (2008) * ISO/IEC 10646:2014 §9.1 (2014) * ''The Unicode Standard, Version 14.0.0'' (2021) 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: several bytes cannot appear; a byte with the high bit set cannot be alone; and further requirements mean that it is extremely unlikely that a readable text in any
extended ASCII Extended ASCII is a repertoire of character encodings that include (most of) the original 96 ASCII character set, plus up to 128 additional characters. There is no formal definition of "extended ASCII", and even use of the term is sometimes critic ...
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. A UTF-8 stream may simply contain errors, resulting in the auto-detection scheme producing false positives; but auto-detection is successful in the vast 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 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-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 Unicode character, avoiding the need to figure out and set a "
code page In computing, a code page is a character encoding and as such it is a specific association of a set of printable characters and control characters with unique numbers. Typically each number represents the binary value in a single byte. (In some c ...
" 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 The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of t ...
and thus cannot be confused with it. The absence of 0xFF (0377) also eliminates the need to escape this byte in
Telnet Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control i ...
(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 ( ), Slavonic script or the Slavic script, is a writing system used for various languages across Eurasia. It is the designated national script in various Slavic, Turkic, Mongolic, Uralic, Caucasian and Iranic-speaking co ...
and
Greek alphabet The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the earliest known alphabetic script to have distinct letters for vowels as ...
code pages), characters in UTF-8 will be double the size. For some scripts, such as Thai and
Devanagari Devanagari ( ; , , Sanskrit pronunciation: ), also called Nagari (),Kathleen Kuiper (2010), The Culture of India, New York: The Rosen Publishing Group, , page 83 is a left-to-right abugida (a type of segmental writing system), based on the ...
(which is used by various South Asian languages), characters will triple in size. There are even examples where a single byte turns into a composite character in Unicode and is thus six times larger in UTF-8. This has caused objections in India and other countries. * It is possible in UTF-8 (or any other multi-byte encoding) to split or
truncate In mathematics and computer science, truncation is limiting the number of digits right of the decimal point. Truncation and floor function Truncation of positive real numbers can be done using the floor function. Given a number x \in \mathb ...
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,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
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
corruption Corruption is a form of dishonesty or a criminal offense which is undertaken by a person or an organization which is entrusted in a position of authority, in order to acquire illicit benefits or abuse power for one's personal gain. Corruption m ...
, 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 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 operation In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic oper ...
s. 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 Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as cod ...
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. 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 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 The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: * The byte order, or endianness, of t ...
. ** 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 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 standard, a plane is a continuous group of 65,536 (216) code points. There are 17 planes, identified by the numbers 0 to 16, which corresponds with the possible values 00–1016 of the first two positions in six position hexadeci ...
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 The Unicode Consortium (legally Unicode, Inc.) is a 501(c)(3) non-profit organization incorporated and based in Mountain View, California. Its primary purpose is to maintain and publish the Unicode Standard which was developed with the intentio ...
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 Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
. In
Oracle Database Oracle Database (commonly referred to as Oracle DBMS, Oracle Autonomous Database, or simply as Oracle) is a multi-model database management system produced and marketed by Oracle Corporation. It is a database commonly used for running online ...
, 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 HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...
documents.


MySQL utf8mb3

In
MySQL MySQL () is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database ...
, 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 standard, a plane is a continuous group of 65,536 (216) code points. There are 17 planes, identified by the numbers 0 to 16, which corresponds with the possible values 00–1016 of the first two positions in six position hexadeci ...
(i.e. from
UCS-2 The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, ''Information technology — Universal Coded Character Set (UCS)'' (plus amendments to that standard), w ...
) are supported. Unicode characters in 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 is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers ''write once, run anywh ...
. 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 The Compatibility Encoding Scheme for UTF-16: 8-Bit (CESU-8) is a variant of UTF-8 that is described in Unicode Technical Report #26. A Unicode code point from the Basic Multilingual Plane (BMP), i.e. a code point in the range U+0000 to U+FFFF, ...
. 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
serialization In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
among other applications of and , for the Java Native Interface, and for embedding constant strings in class files. The dex format defined by 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 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'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 Cross-site scripting (XSS) is a type of security vulnerability that can be found in some web applications. XSS attacks enable attackers to inject client-side scripts into web pages viewed by other users. A cross-site scripting vulnerability m ...
) usually rely on ASCII characters.


See also

*
Alt code On personal computers with numeric keypads that use Microsoft operating systems, such as Windows, many characters that do not have a dedicated key combination on the keyboard may nevertheless be entered using the Alt code (the Alt numpad input m ...
* * Comparison of Unicode encodings **
GB 18030 GB 18030 is a Chinese government standard, described as ''Information Technology — Chinese coded character set'' and defines the required language and character support necessary for software in China. GB18030 is the registered Internet n ...
** UTF-EBCDIC *
Iconv In Unix and Unix-like operating systems, iconv (an abbreviation of internationalization conversion) is a command-line program and a standardized application programming interface (API) used to convert between different character encodings. "It ...
* *
Specials (Unicode block) Specials is a short Unicode block of characters allocated at the very end of the Basic Multilingual Plane, at U+FFF0–FFFF. Of these 16 code points, five have been assigned since Unicode 3.0: *, marks start of annotated text *, marks start ...
*
Unicode and email Many email clients now offer some support for Unicode. Some clients will automatically choose between a legacy encoding and Unicode depending on the mail's content, either automatically or when the user requests it. Technical requirements for send ...
* Unicode and HTML ** Character encodings in HTML


Notes


References


External links


Original UTF-8 paperor pdf
for
Plan 9 from Bell Labs Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has be ...

History of UTF-8 by Rob Pike
* UTF-8 test pages: *

*

*

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