UTF-8

Consider the encoding of the Euro sign, €:

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

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

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

UTF-8's use of six bits per byte to represent the actual characters being encoded means that octal notation (which uses 3-bit groups) can aid in the comparison of UTF-8 sequences with one another.[31]

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

  Blue cells are 7-bit (single-byte) sequences. They must not be followed by a continuation byte.[32]

  Orange cells with a large dot are a continuation byte.[33] The hexadecimal number shown after the + symbol is the value of the 6 bits they add. This character never occurs as the first byte of a multi-byte sequence.

  White cells are the leading bytes for a sequence of multiple bytes,[34] the length shown at the left edge of the row. The text shows the Unicode blocks encoded by sequences starting with this byte, and the hexadecimal code point shown in the cell is the lowest character value encoded using that leading byte.

  Red cells must never appear in a valid UTF-8 sequence. The first two red cells (C0 and C1) could be used only for a 2-byte encoding of a 7-bit ASCII character which should be encoded in 1 byte; as described below, such "overlong" sequences are disallowed.[35] To understand why this is, consider the character 128, hex 80, binary 1000 0000. To encode it as 2 characters, the low six bits are stored in the second character as 128 itself 10 000000, but the upper two bits are stored in the first character as 110 00010, making the minimum first character C2. The red cells in the F_ row (F5 to FD) indicate leading bytes of 4-byte or longer sequences that cannot be valid because they would encode code points larger than the U+10FFFF limit of Unicode (a limit derived from the maximum code point encodable in UTF-16 [36]). FE and FF do not match any allowed character pattern and are therefore not valid start bytes.[37]

  Pink cells are the leading bytes for a sequence of multiple bytes, of which some, but not all, possible continuation sequences are valid. E0 and F0 could start overlong encodings, in this case the lowest non-overlong-encoded code point is shown. F4 can start code points greater than U+10FFFF which are invalid. ED can start the encoding of a code point in the range U+D800–U+DFFF; these are invalid since they are reserved for UTF-16 surrogate halves.[38]

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

The standard specifies that the correct encoding of a code point 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.

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 web server[39] and Apache's Tomcat servlet container.[40] RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[41] 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 (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.

Some implementations of decoders throw exceptions on errors.[42] 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. For instance early versions of Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8.[43] An alternative practice is to replace errors with a replacement character. Since Unicode 6[44] (October 2010), the standard (chapter 3) has recommended a "best practice" where the error ends as soon as a disallowed byte is encountered. In these decoders E1,A0,C0 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.[45] The standard also recommends replacing each error with the replacement character "�" (U+FFFD).

If the UTF-16 Unicode byte order mark (BOM) character is at the start of a UTF-8 file, the first three bytes will be 0xEF, 0xBB, 0xBF.

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.[46] 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. 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).

In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. A modification by Ken Thompson of the Plan 9 operating system group at Bell Labs made it somewhat less bit-efficient than the previous proposal but crucially allowed it to be self-synchronizing, letting a reader start anywhere and immediately detect byte sequence boundaries. It also abandoned the use of biases and instead added the rule that only the shortest possible encoding is allowed; the additional loss in compactness is relatively insignificant, but readers now have to look out for invalid encodings to avoid reliability and especially security issues. Thompson's design was outlined on September 2, 1992, on a placemat in a New Jersey diner with Rob Pike. In the following days, Pike and Thompson implemented it and updated Plan 9 to use it throughout, and then communicated their success back to X/Open, which accepted it as the specification for FSS-UTF.[57]

UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25 to 29, 1993. The Internet Engineering Task Force 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.[59]

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

• 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).
• 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 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 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.

• 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 written in the same file without specialised 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, 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 Shift JIS 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 operations. UTF-8 does not require slower mathematical operations such as multiplication or division (unlike Shift JIS, GB 2312 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.

• 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 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.
  • 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.

Unicode Technical Report #26[65] 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 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 UTF8 character set uses CESU-8 encoding, and is deprecated. The AL32UTF8 character set uses standards-compliant UTF-8 encoding, and is preferred.[66][67]

CESU-8 is prohibited for use in HTML5 documents.[68][69][70]

In MySQL, the utf8mb3 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 are supported. Unicode characters in supplementary planes are explicitly not supported. utf8mb3 is deprecated in favor of the utf8mb4 character set, which uses standards-compliant UTF-8 encoding. utf8 is an alias for utf8mb3, but is intended to become an alias to utf8mb4 in a future release of MySQL.[71] It is possible, though unsupported, to store CESU-8 encoded data in utf8mb3, by handling UTF-16 data with supplementary characters as though it is UCS-2.

Modified UTF-8 (MUTF-8) originated in the Java programming language. In Modified UTF-8, the null character (U+0000) uses the two-byte overlong encoding 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00).[72] Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000,[73] 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 InputStreamReader and OutputStreamWriter (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object serialization[74] among other applications of DataInput and DataOutput, for the Java Native Interface,[75] and for embedding constant strings in class files.[76]

The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[77] Tcl also uses the same modified UTF-8[78] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

In WTF-8 (Wobbly Transformation Format, 8-bit) unpaired surrogate halves (U+D800 through U+DFFF) are allowed.[79] 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.[80]

(The term "WTF-8" has also been used humorously to refer to erroneously doubly-encoded UTF-8[81][82] sometimes with the implication that CP1252 bytes are the only ones encoded)[83]

Version 3 of the Python programming language treats each byte of an invalid UTF-8 bytestream as an error; this gives 128 different possible errors. Extensions have been created to allow any byte sequence that is assumed to be UTF-8 to be lossless 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.[84] Another encoding called MirBSD OPTU-8/16 converts them to U+EF80...U+EFFF in a Private Use Area.[85] 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;[86] as the Windows filesystem API uses UTF-16, the need to support invalid UTF-8 is less there.[84]

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.[86]