Programming languages

C language

The C language is a low level language, close to the hardware. It has a builtin character string <str> type (:cwchar_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 Windows, all kernels use byte strings <bytes>.

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

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

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

Byte API (char)

toupper() and isprint() are locale dependent

Byte string API (char*)

:cchar* points to char, not char*

Create a section for NUL byte/character

Character API (wchar_t)

towupper() and iswprint() are locale dependent

is wchar_t signed on Windows and Mac OS X?

can wchar_t be signed?

Character string API (wchar_t*)

POSIX.1-2001 has no function ignoring case to compare character strings. POSIX.1-2008, a recent standard, adds :cwcscasecmp: the GNU libc has it as an extension (if :c_GNU_SOURCE is defined). Windows has the :c_wcsnicmp function.

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

printf functions family

Formats of string arguments for the printf functions:

"%s": literal byte string (:cchar*)

"%ls": literal character string (:cwchar_t*)

printf("%ls") is strict <strict>: it stops immediatly if a character string <str> argument cannot be encoded <unencodable> to the 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 strict <strict>: they stop immediatly if a byte string <bytes> argument cannot be decoded <undecodable> from the 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 UTF-8) cannot be decoded from the 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") replaces <replace> unencodable <unencodable> 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 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 :cwprintf with character string arguments.

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.

locale encoding should be initialized.

C++

std::wstring: character string <str> using the :cwchar_t type, Unicode version of std::string (byte string <bytes>)

std::wcin, std::wcout and std::wcerr: standard input, output and error output; Unicode version of std::cin, std::cout and std::cerr

std::wostringstream: character stream buffer; Unicode version of std::ostringstream.

To initialize the 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. :cprintf and std::cout) to access the standard streams, you may have issues with 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. Byte <bytes> and Unicode <str> strings store their length, so it's possible to embed nul byte/character.

Python can be compiled in two modes: narrow (UTF-16 <utf16>) and wide (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 :cwchar_t is also 16 bits on Windows and so it is possible to use Python Unicode strings as :cwchar_t* strings without any (expensive) conversion.

Python Unicode HOWTO.

Python 2

str is the byte string <bytes> type and unicode is the 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 decoded <decode> from the default encoding (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 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 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 byte string <bytes> type and str is the 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 non-BMP <bmp> characters, in narrow and wide builds. But as Python 2, chr(0x10FFFF) creates a string of 2 characters (a 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 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, 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 decoded <decode> from the default encoding (ASCII) to Unicode before being compared.

UTF-8 decoder of Python 2 accept surrogate characters <surrogates>, even if there are invalid, to keep backward compatibility with Python 2.0. In Python 3, the 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 BOM <bom>, whereas UTF-8-SIG, UTF-16 and UTF-32 use BOM. mbcs is only available on Windows: it is the ANSI code page <codepage>.

Python provides also many error handlers <errors> used to specify how to handle undecodable byte sequences <undecodable> and 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: 0x80—0xFF) by surrogate characters <surrogates> (in U+DC80—U+DCFF) on decoding, replace characters in range U+DC80—U+DCFF by bytes in 0x80—0xFF 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 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

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

.decode(encoding, errors='strict'): decode <decode> from the specified encoding and (optional) error handler <errors>.

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

.encode(encoding, errors='strict'): encode <encode> to the specified encoding with an (optional) error handler <errors>

.isprintable(): False if the character category <unicode categories> is other (Cc, Cf, Cn, Co, Cs) or separator (Zl, Zp, Zs), True otherwise. There is an exception: even if U+0020 is a separator, ' '.isprintable() gives True.

.toupper(): convert to uppercase

Filesystem

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

mbcs (ANSI code page <codepage>) on Windows

UTF-8 on Mac OS X <osx>

locale encoding <locale encoding> otherwise

Python uses the strict 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:

locale.getpreferredencoding(): ANSI code page <codepage>

'mbcs' codec: ANSI code page <codepage>

sys.stdout.encoding, sys.stderr.encoding: encoding of the Windows console <win_console>.

sys.argv, os.environ, subprocess.Popen(args): native Unicode support (no encoding)

Modules

codecs module:

BOM_UTF8, BOM_UTF16_BE, BOM_UTF32_LE, ...: 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 error handler <errors>. By default, it opens text files with the locale encoding <locale encoding> in strict <strict> mode.

TextIOWrapper(): wrapper to read and/or write text files, encode from/decode to the specified encoding (and 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 (locales <locales>):

LC_ALL, LC_CTYPE, ...: locale categories <locale categories>

getlocale(category): get the value of a locale category <locale categories> as the tuple (language code, encoding name)

getpreferredencoding(): get the locale encoding <locale encoding>

setlocale(category, value): set the value of a locale category

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:

category(char): get the category <unicode categories> of a character

name(char): get the name of a character

normalize(string): normalize <normalization> a string to the NFC, NFD, NFKC or NFKD form

cleanup Python 2/3 here (open)

PHP

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

mb_internal_encoding(): get or set the internal encoding

mb_substitute_character(): change how to handle <errors> unencodable characters <unencodable>:

"none": ignore <ignore> unencodable characters

"long": escape as hexadecimal <escape> value, e.g. "U+E9" or "JIS+7E7E"

"entity": escape as HTML entities <escape>, e.g. "é"

mb_convert_encoding(): decode <decode> from an encoding and encode <encode> to another encoding

mb_ereg(): search a pattern using a regular expression

mb_strlen(): get the length in characters

mb_detect_encoding(): guess the encoding <guess> of a byte string <bytes>

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

u flag: instead of which encoding?

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

iconv(): decode <decode> a byte string <bytes> from an encoding and encode <encode> to another encoding, you can use //IGNORE or //TRANSLIT suffix to choose the error handler <errors>

iconv_mime_decode(): decode a MIME header field

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.

PHP6 creation date?

Perl

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:

Perl 5.6 (2000): initial Unicode support, support character strings <str>

Perl 5.8 (2002): regex supports Unicode

use "use utf8;" pragma to specify that your Perl script is encoded to UTF-8

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 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 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

explain isWhitespace()

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

String(bytes, encoding): decode <decode> a byte string <bytes> from the specified encoding. The decoder is strict <strict>: throw a CharsetDecoder exception if a byte sequence cannot be decoded <undecodable>.

.getBytes(encoding): encode <encode> to the specified encoding, throw a CharsetEncoder exception if a character cannot be encoded <undecodable>.

.length(): get the length in UTF-16 units.

As Python compiled in narrow mode, non-BMP <bmp> characters are stored as 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 UTF-8 which encodes the nul character (U+0000) as the overlong byte sequence <strict utf8 decoder> 0xC0 0x80, instead of 0x00. So it is possible to use C <c> functions like :cstrlen on byte string <bytes> with embeded nul characters.

Go and D

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

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programming_languages.rst

programming_languages.rst

Programming languages

C language

Byte API (char)

Byte string API (char*)

Character API (wchar_t)

Character string API (wchar_t*)

printf functions family

C++

Python

Python 2

Python 3

Differences between Python 2 and Python 3

Codecs

String methods

Filesystem

Windows

Modules

PHP

Perl

Java

Go and D

Files

programming_languages.rst

Latest commit

History

programming_languages.rst

File metadata and controls

Programming languages

C language

Byte API (char)

Byte string API (char*)

Character API (wchar_t)

Character string API (wchar_t*)

printf functions family

C++

Python

Python 2

Python 3

Differences between Python 2 and Python 3

Codecs

String methods

Filesystem

Windows

Modules

PHP

Perl

Java

Go and D