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1. Introduction

1.1 Why Unicode?

People in different countries use different characters to represent the words of their native languages. Nowadays most applications, including email systems and web browsers, are 8-bit clean, i.e. they can operate on and display text correctly provided that it is represented in an 8-bit character set, like ISO-8859-1.

There are far more than 256 characters in the world - think of cyrillic, hebrew, arabic, chinese, japanese, korean and thai -, and new characters are being invented now and then. The problems that come up for users are:

The solution of this problem is the adoption of a world-wide usable character set. This character set is Unicode http://www.unicode.org/. For more info about Unicode, do `man 7 unicode' (manpage contained in the man-pages-1.20 package).

1.2 Unicode encodings

This reduces the user's problem of dealing with character sets to a technical problem: How to transport Unicode characters using the 8-bit bytes? 8-bit units are the smallest addressing units of most computers and also the unit used by TCP/IP network connections. The use of 1 byte to represent 1 character is, however, an accident of history, caused by the fact that computer development started in Europe and the U.S. where 96 characters were found to be sufficient for a long time.

There are basically four ways to encode Unicode characters in bytes:

UTF-8

128 characters are encoded using 1 byte (the ASCII characters). 1920 characters are encoded using 2 bytes (Roman, Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic characters). 63488 characters are encoded using 3 bytes (Chinese and Japanese among others). The other 2147418112 characters (not assigned yet) can be encoded using 4, 5 or 6 characters. For more info about UTF-8, do `man 7 utf-8' (manpage contained in the man-pages-1.20 package).

UCS-2

Every character is represented as two bytes. This encoding can only represent the first 65536 Unicode characters.

UTF-16

This is an extension of UCS-2 which can represent 1112064 Unicode characters. The first 65536 Unicode characters are represented as two bytes, the other ones as four bytes.

UCS-4

Every character is represented as four bytes.

The space requirements for encoding a text, compared to encodings currently in use (8 bit per character for European languages, more for Chinese/Japanese/Korean), is as follows. This has an influence on disk storage space and network download speed (when no form of compression is used).

UTF-8

No change for US ASCII, just a few percent more for ISO-8859-1, 50% more for Chinese/Japanese/Korean, 100% more for Greek and Cyrillic.

UCS-2 and UTF-16

No change for Chinese/Japanese/Korean. 100% more for US ASCII and ISO-8859-1, Greek and Cyrillic.

UCS-4

100% more for Chinese/Japanese/Korean. 300% more for US ASCII and ISO-8859-1, Greek and Cyrillic.

Given the penalty for US and European documents caused by UCS-2, UTF-16, and UCS-4, it seems unlikely that these encodings have a potential for wide-scale use. The Microsoft Win32 API supports the UCS-2 encoding since 1995 (at least), yet this encoding has not been widely adopted for documents - SJIS remains prevalent in Japan.

UTF-8 on the other hand has the potential for wide-scale use, since it doesn't penalize US and European users, and since many text processing programs don't need to be changed for UTF-8 support.

In the following, we will describe how to change your Linux system so it uses UTF-8 as text encoding.

Footnotes for C/C++ developers

The Microsoft Win32 approach makes it easy for developers to produce Unicode versions of their programs: You "#define UNICODE" at the top of your program and then change many occurrences of `char' to `TCHAR', until your program compiles without warnings. The problem with it is that you end up with two versions of your program: one which understands UCS-2 text but no 8-bit encodings, and one which understands only old 8-bit encodings.

Moreover, there is an endianness issue with UCS-2 and UCS-4. The IANA character set registry http://www.isi.edu/in-notes/iana/assignments/character-sets says about ISO-10646-UCS-2: "this needs to specify network byte order: the standard does not specify". Network byte order is big endian. And RFC 2152 is even clearer: "ISO/IEC 10646-1:1993(E) specifies that when characters the UCS-2 form are serialized as octets, that the most significant octet appear first." Whereas Microsoft, in its C/C++ development tools, recommends to use machine-dependent endianness (i.e. little endian on ix86 processors) and either a byte-order mark at the beginning of the document, or some statistical heuristics(!).

The UTF-8 approach on the other hand keeps `char*' as the standard C string type. As a result, your program will handle US ASCII text, independently of any environment variables, and will handle both ISO-8859-1 and UTF-8 encoded text provided the LANG environment variable is set accordingly.

1.3 Related resources

Markus Kuhn's very up-to-date resource list:

Roman Czyborra's overview of Unicode, UTF-8 and UTF-8 aware programs: http://czyborra.com/utf/#UTF-8

Some example UTF-8 files:

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