FLTK 1.4.0
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Unicode and UTF-8 Support

This chapter explains how FLTK handles international text via Unicode and UTF-8.

Unicode support was added to FLTK starting with version 1.3.0 and is still incomplete but mostly functional. This chapter is Work in Progress, reflecting the current state of Unicode support.

About Unicode, ISO 10646 and UTF-8

The summary of Unicode, ISO 10646 and UTF-8 given below is deliberately brief and provides just enough information for the rest of this chapter.

For further information, please see:

The Unicode Standard

The Unicode Standard was originally developed by a consortium of mainly US computer manufacturers and developers of multi-lingual software. It has now become a defacto standard for character encoding and is supported by most of the major computing companies in the world.

Before Unicode, many different systems, on different platforms, had been developed for encoding characters for different languages, but no single encoding could satisfy all languages. Unicode provides access to over 130,000 characters used in all the major languages written today, and is independent of platform and language.

Unicode also provides higher-level concepts needed for text processing and typographic publishing systems, such as algorithms for sorting and comparing text, composite character and text rendering, right-to-left and bi-directional text handling.

There are currently no plans to add this extra functionality to FLTK.
ISO 10646

The International Organisation for Standardization (ISO) had also been trying to develop a single unified character set. Although both ISO and the Unicode Consortium continue to publish their own standards, they have agreed to coordinate their work so that specific versions of the Unicode and ISO 10646 standards are compatible with each other.

The international standard ISO 10646 defines the Universal Character Set (UCS) which contains the characters required for almost all known languages. The standard also defines three different implementation levels specifying how these characters can be combined.

There are currently no plans for handling the different implementation levels or the combining characters in FLTK.

In UCS, characters have a unique numerical code and an official name, and are usually shown using 'U+' and the code in hexadecimal, e.g. U+0041 is the "Latin capital letter A". The UCS characters U+0000 to U+007F correspond to US-ASCII, and U+0000 to U+00FF correspond to ISO 8859-1 (Latin1).

ISO 10646 was originally designed to handle a 31-bit character set from U+00000000 to U+7FFFFFFF, but the current idea is that 21 bits will be sufficient for all future needs, giving characters up to U+10FFFF. The complete character set is sub-divided into planes. Plane 0, also known as the Basic Multilingual Plane (BMP), ranges from U+0000 to U+FFFD and consists of the most commonly used characters from previous encoding standards. Other planes contain characters for specialist applications.

FLTK 1.3 and later supports the full Unicode range (21 bits), but there are a few exceptions, for instance binary shortcut values in menus (Fl_Shortcut) can only be used with characters from the BMP (16 bits). This may be extended in a future FLTK version.

The UCS also defines various methods of encoding characters as a sequence of bytes. UCS-2 encodes Unicode characters into two bytes, which is wasteful if you are only dealing with ASCII or Latin1 text, and insufficient if you need characters above U+00FFFF. UCS-4 uses four bytes, which lets it handle higher characters, but this is even more wasteful for ASCII or Latin1.


The Unicode standard defines various UCS Transformation Formats (UTF). UTF-16 and UTF-32 are based on units of two and four bytes. UCS characters requiring more than 16 bits are encoded using "surrogate pairs" in UTF-16.

UTF-8 encodes all Unicode characters into variable length sequences of bytes. Unicode characters in the 7-bit ASCII range map to the same value and are represented as a single byte, making the transformation to Unicode quick and easy.

All UCS characters above U+007F are encoded as a sequence of several bytes. The top bits of the first byte are set to show the length of the byte sequence, and subseqent bytes are always in the range 0x80 to 0xBF. This combination provides some level of synchronisation and error detection.

Unicode range Byte sequences
U+00000000 - U+0000007F 0xxxxxxx
U+00000080 - U+000007FF 110xxxxx 10xxxxxx
U+00000800 - U+0000FFFF 1110xxxx 10xxxxxx 10xxxxxx
U+00010000 - U+001FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
U+00200000 - U+03FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
U+04000000 - U+7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
This table contains theoretical values outside the valid Unicode range (U+000000 - U+10FFFF). Such values can only be returned by conversion functions for illegal input values (see Illegal Unicode and UTF-8 Sequences).

Moving from ASCII encoding to Unicode will allow all new FLTK applications to be easily internationalized and used all over the world. By choosing UTF-8 encoding, FLTK remains largely source-code compatible to previous iterations of the library.

Unicode in FLTK

Work through the code and this documentation to harmonize the [OksiD] and [fltk2] functions.

FLTK will be entirely converted to Unicode using UTF-8 encoding. If a different encoding is required by the underlying operating system, FLTK will convert the string as needed.

It is important to note that the initial implementation of Unicode and UTF-8 in FLTK involves three important areas:

  • provision of Unicode character tables and some simple related functions;
  • conversion of char* variables and function parameters from single byte per character representation to UTF-8 variable length sequences;
  • modifications to the display font interface to accept general Unicode character or UCS code numbers instead of just ASCII or Latin1 characters.

The current implementation of Unicode / UTF-8 in FLTK will impose the following limitations:

  • An implementation note in the [OksiD] code says that all functions are LIMITED to 24 bit Unicode values, but also says that only 16 bits are really used under linux and win32. [Can we verify this?]
  • The [fltk2] fl_utf8encode() and fl_utf8decode() functions are designed to handle Unicode characters in the range U+000000 to U+10FFFF inclusive, which covers all UTF-16 characters, as specified in RFC 3629. Note that the user must first convert UTF-16 surrogate pairs to UCS.
  • FLTK will only handle single characters, so composed characters consisting of a base character and floating accent characters will be treated as multiple characters.
  • FLTK will only compare or sort strings on a byte by byte basis and not on a general Unicode character basis.
  • FLTK will not handle right-to-left or bi-directional text.

    Verify 16/24 bit Unicode limit for different character sets? OksiD's code appears limited to 16-bit whereas the FLTK2 code appears to handle a wider set. What about illegal characters? See comments in fl_utf8fromwc() and fl_utf8toUtf16().

Illegal Unicode and UTF-8 Sequences

Three pre-processor variables are defined in the source code [1] that determine how fl_utf8decode() handles illegal UTF-8 sequences:

  • if ERRORS_TO_CP1252 is set to 1 (the default), fl_utf8decode() will assume that a byte sequence starting with a byte in the range 0x80 to 0x9f represents a Microsoft CP1252 character, and will return the value of an equivalent UCS character. Otherwise, it will be processed as an illegal byte value as described below.
  • if STRICT_RFC3629 is set to 1 (not the default!) then UTF-8 sequences that correspond to illegal UCS values are treated as errors. Illegal UCS values include those above U+10FFFF, or corresponding to UTF-16 surrogate pairs. Illegal byte values are handled as described below.
  • if ERRORS_TO_ISO8859_1 is set to 1 (the default), the illegal byte value is returned unchanged, otherwise 0xFFFD, the Unicode REPLACEMENT CHARACTER, is returned instead.

[1] Since FLTK 1.3.4 you may set these three pre-processor variables on your compile command line with -D"variable=value" (value: 0 or 1) to avoid editing the source code.

fl_utf8encode() is less strict, and only generates the UTF-8 sequence for 0xFFFD, the Unicode REPLACEMENT CHARACTER, if it is asked to encode a UCS value above U+10FFFF.

Many of the [fltk2] functions below use fl_utf8decode() and fl_utf8encode() in their own implementation, and are therefore somewhat protected from bad UTF-8 sequences.

The [OksiD] fl_utf8len() function assumes that the byte it is passed is the first byte in a UTF-8 sequence, and returns the length of the sequence. Trailing bytes in a UTF-8 sequence will return -1.

  • WARNING: fl_utf8len() can not distinguish between single bytes representing Microsoft CP1252 characters 0x80-0x9f and those forming part of a valid UTF-8 sequence. You are strongly advised not to use fl_utf8len() in your own code unless you know that the byte sequence contains only valid UTF-8 sequences.
  • WARNING: Some of the [OksiD] functions below still use fl_utf8len() in their implementations. These may need further validation.

Please see the individual function description for further details about error handling and return values.

FLTK Unicode and UTF-8 Functions

This section provides a brief overview of the functions. For more details, consult the main text for each function via its link.

int fl_utf8locale() FLTK2

fl_utf8locale() returns true if the "locale" seems to indicate that UTF-8 encoding is used.
It is highly recommended that you change your system so this does return true!

int fl_utf8test(const char *src, unsigned len) FLTK2

fl_utf8test() examines the first len bytes of src. It returns 0 if there are any illegal UTF-8 sequences; 1 if src contains plain ASCII or if len is zero; or 2, 3 or 4 to indicate the range of Unicode characters found.

int fl_utf_nb_char(const unsigned char *buf, int len) OksiD

Returns the number of UTF-8 characters in the first len bytes of buf.

int fl_unichar_to_utf8_size(Fl_Unichar)
int fl_utf8bytes(unsigned ucs)

Returns the number of bytes needed to encode ucs in UTF-8.

int fl_utf8len(char c) OksiD

If c is a valid first byte of a UTF-8 encoded character sequence, fl_utf8len() will return the number of bytes in that sequence. It returns -1 if c is not a valid first byte.

unsigned int fl_nonspacing(unsigned int ucs) OksiD

Returns true if ucs is a non-spacing character.

const char* fl_utf8back(const char *p, const char *start, const char *end) FLTK2
const char* fl_utf8fwd(const char *p, const char *start, const char *end) FLTK2

If p already points to the start of a UTF-8 character sequence, these functions will return p. Otherwise fl_utf8back() searches backwards from p and fl_utf8fwd() searches forwards from p, within the start and end limits, looking for the start of a UTF-8 character.

unsigned int fl_utf8decode(const char *p, const char *end, int *len) FLTK2
int fl_utf8encode(unsigned ucs, char *buf) FLTK2

fl_utf8decode() attempts to decode the UTF-8 character that starts at p and may not extend past end. It returns the Unicode value, and the length of the UTF-8 character sequence is returned via the len argument. fl_utf8encode() writes the UTF-8 encoding of ucs into buf and returns the number of bytes in the sequence. See the main documentation for the treatment of illegal Unicode and UTF-8 sequences.

unsigned int fl_utf8froma(char *dst, unsigned dstlen, const char *src, unsigned srclen) FLTK2
unsigned int fl_utf8toa(const char *src, unsigned srclen, char *dst, unsigned dstlen) FLTK2

fl_utf8froma() converts a character string containing single bytes per character (i.e. ASCII or ISO-8859-1) into UTF-8. If the src string contains only ASCII characters, the return value will be the same as srclen.
fl_utf8toa() converts a string containing UTF-8 characters into single byte characters. UTF-8 characters that do not correspond to ASCII or ISO-8859-1 characters below 0xFF are replaced with '?'.
Both functions return the number of bytes that would be written, not counting the null terminator. dstlen provides a means of limiting the number of bytes written, so setting dstlen to zero is a means of measuring how much storage would be needed before doing the real conversion.

char* fl_utf2mbcs(const char *src) OksiD

converts a UTF-8 string to a local multi-byte character string. [More info required here!]

unsigned int fl_utf8fromwc(char *dst, unsigned dstlen, const wchar_t *src, unsigned srclen) FLTK2
unsigned int fl_utf8towc(const char *src, unsigned srclen, wchar_t *dst, unsigned dstlen) FLTK2
unsigned int fl_utf8toUtf16(const char *src, unsigned srclen, unsigned short *dst, unsigned dstlen) FLTK2

These routines convert between UTF-8 and wchar_t or "wide character" strings. The difficulty lies in the fact that sizeof(wchar_t) is 2 on Windows and 4 on Linux and most other systems. Therefore some "wide characters" on Windows may be represented as "surrogate pairs" of more than one wchar_t.
fl_utf8fromwc() converts from a "wide character" string to UTF-8. Note that srclen is the number of wchar_t elements in the source string and on Windows this might be larger than the number of characters. dstlen specifies the maximum number of bytes to copy, including the null terminator.
fl_utf8towc() converts a UTF-8 string into a "wide character" string. Note that on Windows, some "wide characters" might result in "surrogate pairs" and therefore the return value might be more than the number of characters. dstlen specifies the maximum number of wchar_t elements to copy, including a zero terminating element. [Is this all worded correctly?]
fl_utf8toUtf16() converts a UTF-8 string into a "wide character" string using UTF-16 encoding to handle the "surrogate pairs" on Windows. dstlen specifies the maximum number of wchar_t elements to copy, including a zero terminating element. [Is this all worded correctly?]
These routines all return the number of elements that would be required for a full conversion of the src string, including the zero terminator. Therefore setting dstlen to zero is a way of measuring how much storage would be needed before doing the real conversion.

unsigned int fl_utf8from_mb(char *dst, unsigned dstlen, const char *src, unsigned srclen) FLTK2
unsigned int fl_utf8to_mb(const char *src, unsigned srclen, char *dst, unsigned dstlen) FLTK2

These functions convert between UTF-8 and the locale-specific multi-byte encodings used on some systems for filenames, etc. If fl_utf8locale() returns true, these functions don't do anything useful. [Is this all worded correctly?]

int fl_tolower(unsigned int ucs) OksiD
int fl_toupper(unsigned int ucs) OksiD
int fl_utf_tolower(const unsigned char *str, int len, char *buf) OksiD
int fl_utf_toupper(const unsigned char *str, int len, char *buf) OksiD

fl_tolower() and fl_toupper() convert a single Unicode character from upper to lower case, and vice versa. fl_utf_tolower() and fl_utf_toupper() convert a string of bytes, some of which may be multi-byte UTF-8 encodings of Unicode characters, from upper to lower case, and vice versa.
Warning: to be safe, buf length must be at least 3*len [for 16-bit Unicode]

int fl_utf_strcasecmp(const char *s1, const char *s2) OksiD
int fl_utf_strncasecmp(const char *s1, const char *s2, int n) OksiD

fl_utf_strcasecmp() is a UTF-8 aware string comparison function that converts the strings to lower case Unicode as part of the comparison. flt_utf_strncasecmp() only compares the first n characters [bytes?]

FLTK Unicode Versions of System Calls

  • more doc on unicode, add links
  • write something about filename encoding on OS X...
  • explain the fl_utf8_... commands
  • explain issues with Fl_Preferences

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