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Operating systems

.. todo:: write an intro for all OS?


Since Windows 2000, Windows offers a nice Unicode API and supports :ref:`non-BMP characters <bmp>`. It uses :ref:`Unicode strings <str>` implemented as :c:type:`wchar_t*` strings (LPWSTR). :c:type:`wchar_t` is 16 bits long on Windows and so it uses :ref:`UTF-16 <utf16>`: :ref:`non-BMP <bmp>` characters are stored as two :c:type:`wchar_t` (a :ref:`surrogate pair <surrogates>`), and the length of a string is the number of UTF-16 units and not the number of characters.

Windows 95, 98 an Me had also Unicode strings, but were limited to :ref:`BMP characters <bmp>`: they used :ref:`UCS-2 <ucs2>` instead of UTF-16.

.. todo:: And Windows CE?

Code pages

A Windows application has two encodings, called code pages (abbreviated "cp"): ANSI and OEM code pages. The ANSI code page, :c:macro:`CP_ACP`, is used for the ANSI version of the :ref:`Windows API <win_api>` to decode :ref:`byte strings <bytes>` to :ref:`character strings <str>` and has a number between 874 and 1258. The OEM code page or "IBM PC" code page, :c:macro:`CP_OEMCP`, comes from MS-DOS, is used for the :ref:`Windows console <win_console>`, contains glyphs to create text interfaces (draw boxes) and has a number between 437 and 874. Example of a French setup: ANSI is :ref:`cp1252` and OEM is cp850.

There are code page constants:


.. c:function:: UINT GetACP()

   Get the ANSI code page number.

.. c:function:: UINT GetOEMCP()

   Get the OEM code page number.

.. c:function:: BOOL SetThreadLocale(LCID locale)

   Set the locale. It can be used to change the ANSI code page of current
   thread (:c:macro:`CP_THREAD_ACP`).

.. seealso::

   Wikipedia article:
   `Windows code page <>`_.

Encode and decode functions

Encode and decode functions of <windows.h>.

.. c:function:: MultiByteToWideChar()

   :ref:`Decode <decode>` a :ref:`byte string <bytes>` from a code page to a
   :ref:`character string <str>`. Use :c:macro:`MB_ERR_INVALID_CHARS` flag to
   :ref:`return an error <strict>` on an :ref:`undecodable byte sequence

   The default behaviour (flags=0) depends on the Windows version:

    - Windows Vista and later: :ref:`replace <replace>` :ref:`undecodable bytes
    - Windows 2000, XP and 2003: :ref:`ignore <ignore>` :ref:`undecodable bytes

   In strict mode (:c:macro:`MB_ERR_INVALID_CHARS`), the :ref:`UTF-8 <utf8>`
   decoder (:c:macro:`CP_UTF8`) returns an error on :ref:`surrogate characters
   <surrogates>` on Windows Vista and later. On Windows XP, the :ref:`UTF-8
   decoder is not strict <strict utf8 decoder>`: surrogates can be decoded in
   any mode.

   The :ref:`UTF-7 <utf7>` decoder (:c:macro:`CP_UTF7`) only supports flags=0.

   Examples on any Windows version:

   | Flags                  | default (0)         | MB_ERR_INVALID_CHARS |
   | ``0xE9 0x80``, cp1252  | é€ {U+00E9, U+20AC} | é€ {U+00E9, U+20AC}  |
   | ``0xC3 0xA9``, CP_UTF8 | é {U+00E9}          | é {U+00E9}           |
   | ``0xFF``, cp932        | {U+F8F3}            | *decoding error*     |
   | ``0xFF``, CP_UTF7      | {U+FF}              | *invalid flags*      |

   Examples on Windows Vista and later:

   | Flags                       | default (0)              | MB_ERR_INVALID_CHARS |
   | ``0x81 0x00``, cp932        | {U+30FB, U+0000}         | *decoding error*     |
   | ``0xFF``, CP_UTF8           | {U+FFFD}                 | *decoding error*     |
   | ``0xED 0xB2 0x80``, CP_UTF8 | {U+FFFD, U+FFFD, U+FFFD} | *decoding error*     |

   Examples on Windows 2000, XP, 2003:

   | Flags                       | default (0)      | MB_ERR_INVALID_CHARS |
   | ``0x81 0x00``, cp932        | {U+0000}         | *decoding error*     |
   | ``0xFF``, CP_UTF8           | *decoding error* | *decoding error*     |
   | ``0xED 0xB2 0x80``, CP_UTF8 | {U+DC80}         | {U+DC80}             |

   .. note::

      The U+30FB character is the Katakana middle dot (・). U+F8F3 code point
      is part of a Unicode range reserved for private use (U+E000—U+F8FF).

.. c:function:: WideCharToMultiByte()

   :ref:`Encode <encode>` a :ref:`character string <str>` to a :ref:`byte
   string <bytes>`. The behaviour on :ref:`unencodable characters
   <unencodable>` depends on the code page, the Windows version and the flags.

   | Code page | Windows version      | Flags                | Behaviour                    |
   |           | 2000, XP, 2003       | 0                    | Encode surrogates            |
   |           +----------------------+----------------------+------------------------------+
   | CP_UTF8   |                      | 0                    | Replace surrogates by U+FFFD |
   |           | Vista or later       +----------------------+------------------------------+
   |           |                      | WC_ERR_INVALID_CHARS | Strict                       |
   | CP_UTF7   | *all versions*       | 0                    | Encode surrogates            |
   | Others    |                      | 0                    | Replace by similar glyph     |
   |           | *all versions*       +----------------------+------------------------------+
   |           |                      | WC_NO_BEST_FIT_CHARS | Replace by ? (1)             |

   (1) : Strict if you check for pusedDefaultChar pointer.

   pusedDefaultChar is not supported by CP_UTF7 or CP_UTF8.

   Use :c:macro:`WC_NO_BEST_FIT_CHARS` flag (or :c:macro:`WC_ERR_INVALID_CHARS`
   flag for :c:macro:`CP_UTF8`) to have a strict encoder: :ref:`return an error
   <strict>` on :ref:`unencodable character <unencodable>`. By default, if
   :ref:`a character cannot be encoded <unencodable>`, it is :ref:`replaced by
   a character with a similar glyph <translit>` or by "?" (U+003F). For
   example, with :ref:`cp1252`, Ł (U+0141) is replaced by L (U+004C).

   On Windows Vista or later with :c:macro:`WC_ERR_INVALID_CHARS` flag, the
   :ref:`UTF-8 <utf8>` encoder (:c:macro:`CP_UTF8`) returns an error on
   :ref:`surrogate characters <surrogates>`. The default behaviour (flags=0)
   depends on the Windows version: surrogates are replaced by U+FFFD on Windows
   Vista and later, and are encoded to UTF-8 on older Windows versions. The
   :c:macro:`WC_NO_BEST_FIT_CHARS` flag is not supported by the UTF-8 encoder.

   The :c:macro:`WC_ERR_INVALID_CHARS` flag is only supported by
   :c:macro:`CP_UTF8` and only on Windows Vista or later.

   The :ref:`UTF-7 <utf7>` encoder (:c:macro:`CP_UTF7`) only supports flags=0.
   It is not strict: it encodes :ref:`surrogate characters <surrogates>`.

   Examples (on any Windows version):

   | Flags              | default (0)                          | WC_NO_BEST_FIT_CHARS |
   | ÿ (U+00FF), cp932  | ``0x79`` (y)                         | ``0x3F`` (?)         |
   | Ł (U+0141), cp1252 | ``0x4C`` (L)                         | ``0x3F`` (?)         |
   | € (U+20AC), cp1252 | ``0x80``                             | ``0x80``             |
   | U+DC80, CP_UTF7    | ``0x2b 0x33 0x49 0x41 0x2d`` (+3IA-) | *invalid flags*      |

   Examples on Windows Vista an later:

   | Flags              | default (0)        | WC_ERR_INVALID_CHARS | WC_NO_BEST_FIT_CHARS |
   | U+DC80, CP_UTF8    | ``0xEF 0xBF 0xBD`` | *encoding error*     | *invalid flags*      |

   Examples on Windows 2000, XP, 2003:

   | Flags              | default (0)        | WC_ERR_INVALID_CHARS | WC_NO_BEST_FIT_CHARS |
   | U+DC80, CP_UTF8    | ``0xED 0xB2 0x80`` | *invalid flags*      | *invalid flags*      |

.. todo:: Document NormalizeString()

.. todo:: Document the replacement character?

Windows API: ANSI and wide versions

Windows has two versions of each function of its API: the ANSI version using :ref:`byte strings <bytes>` (A suffix) and the :ref:`ANSI code page <codepage>`, and the wide version (W suffix) using :ref:`character strings <str>`. There are also functions without suffix using :c:type:`TCHAR*` strings: if the :ref:`C <c>` define :c:macro:`_UNICODE` is defined, :c:type:`TCHAR` is replaced by :c:type:`wchar_t` and the Unicode functions are used; otherwise :c:type:`TCHAR` is replaced by :c:type:`char` and the ANSI functions are used. Example:

Always prefer the Unicode version to avoid encoding/decoding errors, and use directly the W suffix to avoid compiling issues.


There is a third version of the API: the MBCS API (multibyte character string). Use the TCHAR functions and define :c:macro:`_MBCS` to use the MBCS functions. For example, :c:func:`_tcsrev` is replaced by :c:func:`_mbsrev` if :c:macro:`_MBCS` is defined, by :c:func:`_wcsrev` if :c:macro:`_UNICODE` is defined, or by :c:func:`_strrev` otherwise.

Windows string types


Windows stores filenames as Unicode in the filesystem. Filesystem wide character POSIX-like API:

.. c:function:: int _wfstat(const wchar_t* filename, struct _stat *statbuf)

   Unicode version of :c:func:`stat()`.

.. c:function:: FILE *_wfopen(const wchar_t* filename, const wchar_t *mode)

   Unicode version of :c:func:`fopen`.

.. c:function:: int _wopen(const wchar_t *filename, int oflag[, int pmode])

   Unicode version of :c:func:`open`.

POSIX functions, like :c:func:`fopen()`, use the :ref:`ANSI code page <codepage>` to encode/decode strings.

Windows console

Console functions.

.. c:function:: GetConsoleCP()

   Get the code page of the standard input (stdin) of the console.

.. c:function:: GetConsoleOutputCP()

   Get the code page of the standard output (stdout and stderr) of the console.

.. c:function:: WriteConsoleW()

   Write a :ref:`character string <str>` into the console.

.. todo:: document ReadConsoleW()?

To improve the :ref:`Unicode support <support>` of the console, set the console font to a TrueType font (e.g. "Lucida Console") and use the wide character API

If the console is unable to render a character, it tries to use a :ref:`character with a similar glyph <translit>`. For example, with OEM :ref:`code page <codepage>` 850, Ł (U+0141) is replaced by L (U+0041). If no replacment character can be found, "?" (U+003F) is displayed instead.

In a console (cmd.exe), chcp command can be used to display or to change the :ref:`OEM code page <codepage>` (and console code page). Changing the console code page is not a good idea because the ANSI API of the console still expects characters encoded to the previous console code page.

.. seealso::

   `Conventional wisdom is retarded, aka What the @#%&* is _O_U16TEXT?
   <>`_ (Michael
   S.  Kaplan, 2008) and the Python bug report #1602: `windows console doesn't
   print or input Unicode <>`_.


Set the console :ref:`code page <codepage>` to cp65001 (:ref:`UTF-8`) doesn't improve Unicode support, it is the opposite: non-ASCII are not rendered correctly and type non-ASCII characters (e.g. using the keyboard) doesn't work correctly, especially using raster fonts.

File mode

:c:func:`_setmode` and :c:func:`_wsopen` are special functions to set the encoding of a file:

:c:func:`fopen` can use these modes using ccs= in the file mode:

.. todo:: Consequences on TTY and pipes?

Mac OS X

Mac OS X uses :ref:`UTF-8` for the filenames. If a filename is an invalid UTF-8 byte string, Mac OS X :ref:`returns an error <strict>`. The filenames are :ref:`decomposed <normalization>` to an incompatible variant of the Normal Form D (NFD). Extract of the Technical Q&A QA1173: "For example, HFS Plus uses a variant of Normal Form D in which U+2000 through U+2FFF, U+F900 through U+FAFF, and U+2F800 through U+2FAFF are not decomposed."


To support different languages and encodings, UNIX and BSD operating systems have "locales". Locales are process-wide: if a thread or a library change the locale, the whole process is impacted.

Locale categories

Locale categories:

:c:macro:`LC_ALL` is a special category: if you set a locale using this category, it sets the locale for all categories.

Each category has its own environment variable with the same name. For example, LC_MESSAGES=C displays error messages in English. To get the value of a locale category, LC_ALL, LC_xxx (e.g. LC_CTYPE) or LANG environment variables are checked: use the first non empty variable. If all variables are unset, fallback to the C locale.


The gettext library reads LANGUAGE, LC_ALL and LANG environment variables (and some others) to get the user language. The LANGUAGE variable is specific to gettext and is not related to locales.

The C locale

When a program starts, it does not get directly the user locale: it uses the default locale which is called the "C" locale or the "POSIX" locale. It is also used if no locale environment variable is set. For :c:macro:`LC_CTYPE`, the C locale usually means :ref:`ASCII`, but not always (see the locale encoding section). For :c:macro:`LC_MESSAGES`, the C locale means to speak the original language of the program, which is usually English.

Locale encoding

For Unicode, the most important locale category is LC_CTYPE: it is used to set the "locale encoding".

To get the locale encoding:

  • Copy the current locale: setlocale(LC_CTYPE, NULL)
  • Set the current locale encoding to the user preference: setlocale(LC_CTYPE, "")
  • Use nl_langinfo(CODESET) if available
  • or setlocale(LC_CTYPE, NULL)
.. todo:: write a full example in C

For the C locale, nl_langinfo(CODESET) returns :ref:`ASCII`, or an alias to this encoding (e.g. "US-ASCII" or "646"). But on FreeBSD, Solaris and :ref:`Mac OS X <osx>`, codec functions (e.g. :c:func:`mbstowcs`) use :ref:`ISO-8859-1` even if nl_langinfo(CODESET) announces ASCII encoding. AIX uses :ref:`ISO-8859-1` for the C locale (and nl_langinfo(CODESET) returns "ISO8859-1").

Locale functions

<locale.h> functions.

.. c:function:: char* setlocale(category, NULL)

   Get the value of the specified locale category.

.. c:function:: char* setlocale(category, name)

   Set the value of the specified locale category.

.. todo:: setlocale("") means user preference

<langinfo.h> functions.

.. c:function::  char* nl_langinfo(CODESET)

   Get the name of the locale encoding.

<stdlib.h> functions.

.. c:function:: size_t mbstowcs(wchar_t *dest, const char *src, size_t n)

   :ref:`Decode <decode>` a :ref:`byte string <bytes>` from the :ref:`locale encoding <locale
   encoding>` to a :ref:`character string <str>`. The decoder is :ref:`strict
   <strict>`: it returns an error on :ref:`undecodable byte sequence
   <undecodable>`. If available, prefer the reentrant version:

.. c:function:: size_t wcstombs(char *dest, const wchar_t *src, size_t n)

   :ref:`Encode <encode>` a :ref:`character string <str>` to a :ref:`byte string <bytes>` in
   the :ref:`locale encoding <locale encoding>`. The encoder is :ref:`strict
   <strict>` : it returns an error if :ref:`a character cannot by encoded
   <unencodable>`.  If available, prefer the reentrant version:

mbstowcs() and wcstombs() are :ref:`strict <strict>` and don't support :ref:`error handlers <errors>`.


"mbs" stands for "multibyte string" (byte string) and "wcs" stands for "wide character string".

On Windows, the "locale encoding" are the :ref:`ANSI and OEM code pages <codepage>`. A Windows program uses the user preferred code pages at startup, whereas a program starts with the C locale on UNIX.

Filesystems (filenames)


CD-ROM uses the ISO 9660 filesystem which stores filenames as :ref:`byte strings <bytes>`. This filesystem is very restrictive: only A-Z, 0-9, _ and "." are allowed. Microsoft has developed the Joliet extension: store filenames as :ref:`UCS-2 <ucs2>`, up to 64 characters (:ref:`BMP <bmp>` only). It was first supported by Windows 95. Today, all operating systems are able to read it.

UDF (Universal Disk Format) is the filesystem of DVD: it stores filenames as character strings.

.. todo:: UDF encoding?

Microsoft: FAT and NTFS filesystems

MS-DOS uses the FAT filesystems (FAT 12, FAT 16, FAT 32): filenames are stored as :ref:`byte strings <bytes>`. Filenames are limited to 8+3 characters (8 for the name, 3 for the extension) and displayed differently depending on the :ref:`code page <codepage>` (:ref:`mojibake issue <mojibake>`).

Microsoft extended its FAT filesystem in Windows 95: the Virtual FAT (VFAT) supports "long filenames", filenames are stored as :ref:`UCS-2 <ucs2>`, up to 255 characters (BMP only). Starting at Windows 2000, :ref:`non-BMP characters <bmp>` can be used: :ref:`UTF-16 <utf16>` replaces UCS-2 and the limit is now 255 UTF-16 units.

The NTFS filesystem stores filenames using UTF-16 encoding.

Apple: HFS and HFS+ filesystems

HFS stores filenames as byte strings.

HFS+ stores filenames as :ref:`UTF-16 <utf16>`: the maximum length is 255 UTF-16 units.


JFS and ZFS also use Unicode.

The ext family (ext2, ext3, ext4) store filenames as byte strings.

.. todo:: Linux: mount options (FAT, NFSv3)
.. todo:: USB keys, camera, memory cards
.. todo:: Network fileystems like NFS (NFS4 supports Unicode?)