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Programming languages

C language

The C language is a low level language, close to the hardware. It has a builtin :ref:`character string <str>` type (:c:type:`wchar_t*`), but only few libraries support this type. It is usually used as the first "layer" between the kernel (system calls, e.g. open a file) and applications, higher level libraries and other programming languages. This first layer uses the same type as the kernel: except :ref:`Windows`, all kernels use :ref:`byte strings <bytes>`.

There are higher level libraries, like :ref:`glib <glib>` or :ref:`Qt <qt>`, offering a Unicode API, even if the underlying kernel uses byte strings. Such libraries use a codec to :ref:`encode <encode>` data to the kernel and to :ref:`decode <decode>` data from the kernel. The codec is usually the current :ref:`locale encoding <locale encoding>`.

Because there is no Unicode standard library, most third-party libraries chose the simple solution: use :ref:`byte strings <str>`. For example, the OpenSSL library, an open source cryptography toolkit, expects :ref:`filenames <filename>` as byte strings. On Windows, you have to encode Unicode filenames to the current :ref:`ANSI code page <codepage>`, which is a small subset of the Unicode charset.

.. todo:: "Because there is no Unicode standard library": add historical/compatibilty reasons

Byte API (char)

.. c:type:: char

    For historical reasons, :c:type:`char` is the C type for a character ("char" as
    "character"). In pratical, it's only true for 7 and 8 bits encodings like :ref:`ASCII`
    or :ref:`ISO-8859-1`. With multibyte encodings, a :c:type:`char` is only one byte. For example, the
    character "é" (U+00E9) is encoded as two bytes (``0xC3 0xA9``) in :ref:`UTF-8`.

    :c:type:`char` is a 8 bits integer, it is signed or not depending on the
    operating system and the compiler. On Linux, the GNU compiler (gcc) uses a
    signed type for Intel CPU. It defines :c:macro:`__CHAR_UNSIGNED__` if
    :c:type:`char` type is unsigned. Check if the :c:macro:`CHAR_MAX` constant
    from ``<limits.h>`` is equal to 255 to check if :c:type:`char` is unsigned.

    A literal byte is written between apostrophes, e.g. ``'a'``. Some control
    characters can be written with an backslash plus a letter (e.g. ``'\n'`` = 10).
    It's also possible to write the value in octal (e.g. ``'\033'`` = 27) or
    hexadecimal (e.g. ``'\x20'`` = 32). An apostrophe can be written ``'\''`` or
    ``'\x27'``. A backslash is written ``'\\'``.

    ``<ctype.h>`` contains functions to manipulate bytes, like
    :c:func:`toupper` or :c:func:`isprint`.
.. todo:: toupper() and isprint() are locale dependent

Byte string API (char*)

.. todo:: :c:type:`char*` points to char, not char*
.. c:type:: char*

   :c:type:`char*` is a a :ref:`byte string <bytes>`. This type is used
   in many places in the C standard library. For example, :c:func:`fopen` uses
   :c:type:`char*` for the filename.

   ``<string.h>`` is the byte string library. Most functions starts with "str"
   (string) prefix: :c:func:`strlen`, :c:func:`strcat`, etc. ``<stdio.h>`` contains useful string
   functions like :c:func:`snprintf` to format a message.

   The length of a string is stored directly in the string as a nul byte at the end. This
   is a problem with encodings using nul bytes (e.g. :ref:`UTF-16 <utf16>` and :ref:`UTF-32 <utf32>`): :c:func:`strlen()`
   cannot be used to get the length of the string, whereas most C functions
   suppose that :c:func:`strlen` gives the length of the string. To support such
   encodings, the length should be stored differently (e.g. in another variable or
   function argument) and :c:func:`str*` functions should be replaced by :c:type:`mem*`
   functions (e.g. replace ``strcmp(a, b) == 0`` by ``memcmp(a, b) == 0``).

   A literal byte strings is written between quotes, e.g. ``"Hello World!"``. As byte
   literal, it's possible to add control characters and characters in octal or
   hexadecimal, e.g. ``"Hello World!\n"``.
.. todo:: Create a section for NUL byte/character

Character API (wchar_t)

.. c:type:: wchar_t

   With ISO C99 comes :c:type:`wchar_t`: the :ref:`character <character>` type.
   It can be used to store Unicode characters. As :c:type:`char`, it has a
   library: ``<wctype.h>`` contains functions like :c:func:`towupper` or
   :c:func:`iswprint` to manipulate characters.

   :c:type:`wchar_t` is a 16 or 32 bits integer, signed or not. Linux uses 32
   bits signed integer. Mac OS X uses 32 bits integer. Windows and AIX use 16 bits
   integer (:ref:`BMP <bmp>` only). Check if the :c:macro:`WCHAR_MAX` constant
   from ``<wchar.h>`` is equal to 0xFFFF to check if :c:type:`wchar_t` is a 16
   bits unsigned integer.

   A literal character is written between apostrophes with the ``L`` prefix, e.g.
   ``L'a'``. As byte literal, it's possible to write control character with an
   backslash and a character with its value in octal or hexadecimal. For codes
   bigger than 255, ``'\uHHHH'`` syntax can be used. For codes bigger than 65535,
   ``'\UHHHHHHHH'`` syntax can be used with 32 bits :c:type:`wchar_t`.
.. todo:: towupper() and iswprint() are locale dependent
.. todo:: is wchar_t signed on Windows and Mac OS X?
.. todo:: can wchar_t be signed?

Character string API (wchar_t*)

.. c:type:: wchar_t*

   With ISO C99 comes :c:type:`wchar_t*`: the :ref:`character string <str>`
   type. The standard library ``<wchar.h>`` contains character string functions
   like :c:func:`wcslen` or :c:func:`wprintf`, and constants like
   :c:macro:`WCHAR_MAX`. If :c:type:`wchar_t` is 16 bits long, :ref:`non-BMP
   <bmp>` characters are encoded to :ref:`UTF-16 <utf16>` as :ref:`surrogate
   pairs <surrogates>`.

   A literal character strings is written between quotes with the ``L``
   prefix, e.g. ``L"Hello World!\n"``. As character literals, it supports also control
   character, codes written in octal, hexadecimal, ``L"\uHHHH"`` and ``L"\UHHHHHHHH"``.

POSIX.1-2001 has no function ignoring case to compare character strings. POSIX.1-2008, a recent standard, adds :c:func:`wcscasecmp`: the GNU libc has it as an extension (if :c:macro:`_GNU_SOURCE` is defined). Windows has the :c:func:`_wcsnicmp` function.

:ref:`Windows` uses (:ref:`UTF-16 <utf16>`) wchar_t* strings for its Unicode API.

printf functions family

.. c:function:: int printf(const char* format, ...)
.. c:function:: int wprintf(const wchar_t* format, ...)

Formats of string arguments for the printf functions:

printf("%ls") is :ref:`strict <strict>`: it stops immediatly if a :ref:`character string <str>` argument :ref:`cannot be encoded <unencodable>` to the :ref:`locale encoding <locale encoding>`. For example, the following code prints the truncated string "Latin capital letter L with stroke: [" if Ł (U+0141) cannot be encoded to the locale encoding.

printf("Latin capital letter L with stroke: [%ls]\n", L"\u0141");

wprintf("%s") and wprintf("%.<length>s") are :ref:`strict <strict>`: they stop immediatly if :ref:`a byte string <bytes>` argument :ref:`cannot be decoded <undecodable>` from the :ref:`locale encoding <locale encoding>`. For example, the following code prints the truncated string "Latin capital letter L with stroke: [" if 0xC5 0x81 (U+0141 encoded to :ref:`UTF-8`) cannot be decoded from the :ref:`locale encoding <locale encoding>`.

wprintf(L"Latin capital letter L with stroke): [%s]\n", "\xC5\x81");
wprintf(L"Latin capital letter L with stroke): [%.10s]\n", "\xC5\x81");

wprintf("%ls") :ref:`replaces <replace>` :ref:`unencodable <unencodable>` :ref:`character string <str>` arguments by ? (U+003F). For example, the following example print "Latin capital letter L with stroke: [?]" if Ł (U+0141) cannot be encoded to the :ref:`locale encoding <locale encoding>`:

wprintf(L"Latin capital letter L with stroke: [%s]\n", L"\u0141");

So to avoid truncated strings, try to use only :c:func:`wprintf` with character string arguments.

.. todo:: how are non-ASCII characters handled in the format string?

Note

There is also "%S" format which is a deprecated alias to the "%ls" format, don't use it.

.. todo:: locale encoding should be initialized.

C++

To initialize the :ref:`locales <locales>`, equivalent to setlocale(LC_ALL, ""), use:

#include <locale>
std::locale::global(std::locale(""));

If you use also C and C++ functions (e.g. :c:func:`printf` and std::cout) to access the standard streams, you may have issues with :ref:`non-ASCII <ascii>` characters. To avoid these issues, you can disable the automatic synchronization between C (std*) and C++ (std::c*) streams using:

#include <iostream>
std::ios_base::sync_with_stdio(false);

Note

Use typedef basic_ostringstream<wchar_t> wostringstream; if wostringstream is not available.

Python

Python supports Unicode since its version 2.0 released in October 2000. :ref:`Byte <bytes>` and :ref:`Unicode <str>` strings store their length, so it's possible to embed nul byte/character.

Python can be compiled in two modes: narrow (:ref:`UTF-16 <utf16>`) and wide (:ref:`UCS-4 <ucs2>`). sys.maxunicode constant is 0xFFFF in narrow build, and 0x10FFFF in wide build. Python is compiled in narrow mode on Windows, because :c:type:`wchar_t` is also 16 bits on Windows and so it is possible to use Python Unicode strings as :c:type:`wchar_t*` strings without any (expensive) conversion.

.. seealso::

   `Python Unicode HOWTO <http://docs.python.org/howto/unicode.html>`_.

Python 2

str is the :ref:`byte string <bytes>` type and unicode is the :ref:`character string <str>` type. Literal byte strings are written b'abc' (syntax compatible with Python 3) or 'abc' (legacy syntax), \xHH can be used to write a byte by its hexadecimal value (e.g. b'\x80' for 128). Literal Unicode strings are written with the prefix u: u'abc'. Code points can be written as hexadecimal: \xHH (U+0000—U+00FF), \uHHHH (U+0000—U+FFFF) or \UHHHHHHHH (U+0000—U+10FFFF), e.g. 'euro sign:\u20AC'.

In Python 2, str + unicode gives unicode: the byte string is :ref:`decoded <decode>` from the default encoding (:ref:`ASCII`). This coercion was a bad design idea because it was the source of a lot of confusion. At the same time, it was not possible to switch completely to Unicode in 2000: computers were slower and there were fewer Python core developers. It took 8 years to switch completely to Unicode: Python 3 was relased in December 2008.

Narrow build of Python 2 has a partial support of :ref:`non-BMP <bmp>` characters. The unichr() function raises an error for code bigger than U+FFFF, whereas literal strings support non-BMP characters (e.g. '\U0010FFFF'). Non-BMP characters are encoded as :ref:`surrogate pairs <surrogates>`. The disavantage is that len(u'\U00010000') is 2, and u'\U0010FFFF'[0] is u'\uDBFF' (lone surrogate character).

Note

DO NOT CHANGE THE DEFAULT ENCODING! Calling sys.setdefaultencoding() is a very bad idea because it impacts all libraries which suppose that the default encoding is ASCII.

Python 3

bytes is the :ref:`byte string <bytes>` type and str is the :ref:`character string <str>` type. Literal byte strings are written with the b prefix: b'abc'. \xHH can be used to write a byte by its hexadecimal value, e.g. b'\x80' for 128. Literal Unicode strings are written 'abc'. Code points can be used directly in hexadecimal: \xHH (U+0000—U+00FF), \uHHHH (U+0000—U+FFFF) or \UHHHHHHHH (U+0000—U+10FFFF), e.g. 'euro sign:\u20AC'. Each item of a byte string is an integer in range 0—255: b'abc'[0] gives 97, whereas 'abc'[0] gives 'a'.

Python 3 has a full support of :ref:`non-BMP <bmp>` characters, in narrow and wide builds. But as Python 2, chr(0x10FFFF) creates a string of 2 characters (a :ref:`UTF-16 surrogate pair <surrogates>`) in a narrow build. chr() and ord() supports non-BMP characters in both modes.

Python 3 uses U+DC80—U+DCFF character range to store :ref:`undecodable bytes <undecodable>` with the surrogateescape error handler, described in the PEP 383 (Non-decodable Bytes in System Character Interfaces). It is used for filenames and environment variables on UNIX and BSD systems. Example: b'abc\xff'.decode('ASCII', 'surrogateescape') gives 'abc\uDCFF'.

Differences between Python 2 and Python 3

str + unicode gives unicode in Python 2 (the byte string is decoded from the default encoding, :ref:`ASCII`) and it raises a TypeError in Python 3. In Python 3, comparing bytes and str gives False, emits a BytesWarning warning or raises a BytesWarning exception depending of the bytes warning flag (-b or -bb option passed to the Python program). In Python 2, the byte string is :ref:`decoded <decode>` from the default encoding (ASCII) to Unicode before being compared.

:ref:`UTF-8` decoder of Python 2 accept :ref:`surrogate characters <surrogates>`, even if there are invalid, to keep backward compatibility with Python 2.0. In Python 3, the :ref:`UTF-8 decoder is strict <strict utf8 decoder>`: it rejects surrogate characters.

It is possible to make Python 2 behave more like Python 3 with from __future__ import unicode_literals.

Codecs

The codecs and encodings modules provide text encodings. They support a lot of encodings. Some examples: ASCII, ISO-8859-1, UTF-8, UTF-16-LE, ShiftJIS, Big5, cp037, cp950, EUC_JP, etc.

UTF-8, UTF-16-LE, UTF-16-BE, UTF-32-LE and UTF-32-BE don't use :ref:`BOM <bom>`, whereas UTF-8-SIG, UTF-16 and UTF-32 use BOM. mbcs is only available on Windows: it is the :ref:`ANSI code page <codepage>`.

Python provides also many :ref:`error handlers <errors>` used to specify how to handle :ref:`undecodable byte sequences <undecodable>` and :ref:`unencodable characters <unencodable>`:

  • strict (default): raise a UnicodeDecodeError or a UnicodeEncodeError
  • replace: replace undecodable bytes by � (U+FFFD) and unencodable characters by ? (U+003F)
  • ignore: ignore undecodable bytes and unencodable characters
  • backslashreplace (only encode): replace unencodable bytes by \xHH

Python 3 has three more error handlers:

  • surrogateescape: replace undecodable bytes (non-ASCII: 0x800xFF) by :ref:`surrogate characters <surrogates>` (in U+DC80—U+DCFF) on decoding, replace characters in range U+DC80—U+DCFF by bytes in 0x800xFF on encoding. Read the PEP 383 (Non-decodable Bytes in System Character Interfaces) for the details.
  • surrogatepass, specific to UTF-8 codec: allow encoding/decoding surrogate characters in :ref:`UTF-8`. It is required because UTF-8 decoder of Python 3 rejects surrogate characters by default.
  • backslashreplace (for decode): replace undecodable bytes by \xHH

Decoding examples in Python 3:

  • b'abc\xff'.decode('ASCII') uses the strict error handler and raises an UnicodeDecodeError
  • b'abc\xff'.decode('ASCII', 'ignore') gives 'abc'
  • b'abc\xff'.decode('ASCII', 'replace') gives 'abc\uFFFD'
  • b'abc\xff'.decode('ASCII', 'surrogateescape') gives 'abc\uDCFF'

Encoding examples in Python 3:

  • '\u20ac'.encode('UTF-8') gives b'\xe2\x82\xac'
  • 'abc\xff'.encode('ASCII') uses the strict error handler and raises an UnicodeEncodeError
  • 'abc\xff'.encode('ASCII', 'backslashreplace') gives b'abc\\xff'

String methods

:ref:`Byte string <bytes>` (str in Python 2, bytes in Python 3) methods:

:ref:`Character string <str>` (unicode in Python 2, str in Python 3) methods:

Filesystem

Python decodes bytes filenames and encodes Unicode filenames using the filesystem encoding, sys.getfilesystemencoding():

Python uses the strict :ref:`error handler <errors>` in Python 2, and surrogateescape (PEP 383) in Python 3. In Python 2, if os.listdir(u'.') cannot decode a filename, it keeps the bytes filename unchanged. Thanks to surrogateescape, decoding a filename never fails in Python 3. But encoding a filename can fail in Python 2 and 3 depending on the filesystem encoding. For example, on Linux with the C locale, the Unicode filename "h\xe9.py" cannot be encoded because the filesystem encoding is ASCII.

In Python 2, use os.getcwdu() to get the current directory as Unicode.

Windows

Encodings used on Windows:

Modules

codecs module:

  • BOM_UTF8, BOM_UTF16_BE, BOM_UTF32_LE, ...: :ref:`Byte order marks (BOM) <bom>` constants
  • lookup(name): get a Python codec. lookup(name).name gets the Python normalized name of a codec, e.g. codecs.lookup('ANSI_X3.4-1968').name gives 'ascii'.
  • open(filename, mode='rb', encoding=None, errors='strict', ...): legacy API to open a binary or text file. To open a file in Unicode mode, use io.open() instead

io module:

  • open(name, mode='r', buffering=-1, encoding=None, errors=None, ...): open a binary or text file in read and/or write mode. For text file, encoding and errors can be used to specify the encoding and the :ref:`error handler <errors>`. By default, it opens text files with the :ref:`locale encoding <locale encoding>` in :ref:`strict <strict>` mode.
  • TextIOWrapper(): wrapper to read and/or write text files, encode from/decode to the specified encoding (and :ref:`error handler <errors>`) and normalize newlines (\r\n and \r are replaced by \n). It requires a buffered file. Don't use it directly to open a text file: use open() instead.

locale module (:ref:`locales <locales>`):

sys module:

  • getdefaultencoding(): get the default encoding, e.g. used by 'abc'.encode(). In Python 3, the default encoding is fixed to 'utf-8', in Python 2, it is 'ascii' by default.
  • getfilesystemencoding(): get the filesystem encoding used to decode and encode filenames
  • maxunicode: biggest Unicode code point storable in a single Python Unicode character, 0xFFFF in narrow build or 0x10FFFF in wide build.

unicodedata module:

.. todo:: cleanup Python 2/3 here (open)

PHP

In PHP 5, a literal string (e.g. "abc") is a :ref:`byte string <bytes>`. PHP has no :ref:`character string <str>` type, only a "string" type which is a :ref:`byte string <bytes>`.

PHP has "multibyte" functions to manipulate byte strings using their encoding. These functions have an optional encoding argument. If the encoding is not specified, PHP uses the default encoding (called "internal encoding"). Some multibyte functions:

Perl compatible regular expressions (PCRE) have an u flag ("PCRE8") to process byte strings as UTF-8 encoded strings.

.. todo:: u flag: instead of which encoding?

PHP also includes a binding for the :ref:`iconv <iconv>` library.

.. todo:: Document utf8_encode() and utf8_decode() functions?

PHP 6 was a project to improve Unicode support of Unicode. This project died at the beginning of 2010. Read The Death of PHP 6/The Future of PHP 6 (May 25, 2010 by Larry Ullman) and Future of PHP6 (March 2010 by Johannes Schlüter) for more information.

.. todo:: PHP6 creation date?

Perl

Write a character using its code point written in hexadecimal:

  • chr(0x1F4A9)
  • "\x{2639}"
  • "\N{U+A0}"

Using use charnames qw( :full );, you can use a Unicode character in a string using "\N{name}" syntax. Example:

say "\N{long s} \N{ae} \N{Omega} \N{omega} \N{UPWARDS ARROW}"

Declare that filehandles opened within this lexical scope but not elsewhere are in UTF-8, until and unless you say otherwise. The :std adds in STDIN, STDOUT, and STDERR. This critical step implicitly decodes incoming data and encodes outgoing data as UTF-8:

use open qw( :encoding(UTF-8) :std );

If PERL_UNICODE environment variable is set to AS, the following data will use UTF-8:

  • @ARGV
  • STDIN, STDOUT, STDERR

If you have a DATA handle, you must explicitly set its encoding. If you want this to be UTF-8, then say:

binmode(DATA, ":encoding(UTF-8)");

Misc:

use feature qw< unicode_strings >;
use Unicode::Normalize qw< NFD NFC >;
use Encode qw< encode decode >;
@ARGV = map { decode("UTF-8", $_) } @ARGV;
open(OUTPUT, "> :raw :encoding(UTF-16LE) :crlf", $filename);

Misc:

  • Encode
  • Unicode::Normalize
  • Unicode::Collate
  • Unicode::Collate::Locale
  • Unicode::UCD
  • DBM_Filter::utf8

History:

Read perluniintro, perlunicode and perlunifaq manuals.

See Tom Christiansen’s Materials for OSCON 2011 for more information.

Java

char is a character able to store Unicode :ref:`BMP <bmp>` only characters (U+0000—U+FFFF), whereas Character is a wrapper of the char with static helper functions. Character methods:

  • .getType(ch): get the :ref:`category <unicode categories>` of a character
  • .isWhitespace(ch): test if a character is a whitespace according to Java
  • .toUpperCase(ch): convert to uppercase
  • .codePointAt(CharSequence, int): return the code point at the given index of the CharSequence
.. todo:: explain isWhitespace()

String is a :ref:`character string <str>` implemented using a char array and :ref:`UTF-16 <utf16>`. String methods:

As :ref:`Python` compiled in narrow mode, :ref:`non-BMP <bmp>` characters are stored as :ref:`UTF-16 surrogate pairs <surrogates>` and the length of a string is the number of UTF-16 units, not the number of Unicode characters.

Java, as the Tcl language, uses a variant of :ref:`UTF-8` which encodes the nul character (U+0000) as the :ref:`overlong byte sequence <strict utf8 decoder>` 0xC0 0x80, instead of 0x00. So it is possible to use :ref:`C <c>` functions like :c:func:`strlen` on :ref:`byte string <bytes>` with embeded nul characters.

Go and D

The Go and D languages use :ref:`UTF-8` as internal encoding to store :ref:`Unicode strings <str>`.