by

C Game Dev Unicode

I posted a question on Unicode support in getargs.c last month (working on a different project), but now that I'm trying to support unicode-based APIs more seriously I find that it leaves even more to be.

P: n/a
* wizardyhnr:
i want to try ANSI C99's unicode fuctions. so i write a test program.
the function is simple, but i cannot compile it with dev c++ 4.9.9.2
under windows xp sp2, since the compiler always think that the
initialization of the wchar_t string is illegal. here is my function:
#include <stdio.h>
#include <stdlib.h>
#include <wchar.h>
#include <wctype.h>
#include <string.h>
int main(int argc, char *argv[])
{
wchar_t *cur_buff=L'X';
wprintf(cur_buff);
return 0;
}
in the function, the initialization of wchar_t *cur_buff is L'X', if X
is an ascii character, then all things function well. But if X is
non-ascii charater such as a Chinese character, compiler would alert
that this is a illegal byte sequence. The source file is saved as ascci
code, and the character set is gb2312. i wonder why this happens?
Don't know about C, but in C++ you'd have to put a 'const' in there,
wchar_t const* curr_buff = L'X';
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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, itis depressingly unlikely that he or she will be able to read it. If you writesoftware, the burden of this sad state of affairs rests on your shoulders.

Given modern hardware resources, it is unacceptable that we can't yetroutinely communicate text in different scripts or containing technicalsymbols. Fortunately, we are getting there.

After reading a lot on the subject and incorporating Unicode compatibilityinto some of my software, I decided to prepare this quick and highlypragmatic guide to digital text in the 21st century (for C programmers,of course). I don't mind adding my voice to the numerous articles thatalready exist on this subject, since the world needs as many programmersas possible to pick up these skills as soon as possible.

I. Encoding text

Given the variety of human languages on this planet, text is a complexsubject. Many are scared away from dealing with world scripts, because theythink of the numerous related software problems in the area instead of focusingon what they can actually do with their code to help.

The first thing to know is that you do not have to worry about mostproblems with digital text. The most difficult work is handled below theapplication layer, in OSes, UI libraries, and the C library. To give youan idea of what goes on though, here is a summary of software problemssurrounding text:

  • Encoding
    Mapping characters to numbers. Many such mappings exist; once youknow the encoding of a piece of text, you know what character is meantby a particular number. Unicode is one such mapping, and a popular one sinceit incorporates more characters than any other at this time.
  • Display
    Once you know what character is meant, you have to find a font that hasthe character and render it. This task is much complicated by the need todisplay both left-to-right and right-to-left text, the existence ofcombining characters that modify previous characters and have zero width,the fact that some languages require wider character cells than others, andcontext-sensitive letterforms.
  • Input
    An input method is a way to map keystrokes (most likely severalkeystrokes on a typical keyboard) to characters. Input is also complicatedby bidirectional text.
  • Internationalization (i18n)
    This refers to the practice of translating a program into multiple languages,effectively by translating all of the program's strings.
  • Lexicography
    Code that processes text as more than just binary data might have to becomea lot smarter. The problems of searching, sorting, and modifying letter case(upper/lower) vary per-language. If your application doesn't need to performsuch tasks, consider yourself lucky. If you do need these operations, youcan probably find a UI toolkit or i18n library that already implements them.
If you are savvy with just the first issue (encoding), thenOS-vendor-supplied input methods and display routines should magically workwith your program. Whether you want to or are able to translate yoursoftware is another matter, and compared to proper handling of characterencodings it is almost optional (corrupting data is worse thanhaving an unintelligible UI).

The encoding I'll talk about is called Unicode. Unicode officially encodes1,114,112 characters, from 0x000000 to 0x10FFFF. (The idea that Unicode isa 16-bit encoding is completely wrong.) For maximum compatibility, individualUnicode values are usually passed around as 32-bit integers (4 bytes percharacter), even though this is more than necessary. For convenience, thefirst 128 Unicode characters are the same as those in the familiar ASCIIencoding.

The consensus is that storing four bytes per character is wasteful, so avariety of representations have sprung up for Unicode characters. The mostinteresting one for C programmers is called UTF-8. UTF-8 is a 'multi-byte'encoding scheme, meaning that it requires a variable number of bytes torepresent a single Unicode value. Given a so-called 'UTF-8 sequence', you canconvert it to a Unicode value that refers to a character.

UTF-8 has the property that all existing 7-bit ASCII strings are still valid.UTF-8 only affects the meaning of bytes greater than 127, which it uses torepresent higher Unicode characters. A character might require 1, 2, 3, or 4bytes of storage depending on its value;more bytes are needed as values get larger. To store the full range of possible32-bit characters, UTF-8 would require a whopping 6 bytes. But again, Unicodeonly defines characters up to 0x10FFFF, so this should never happen inpractice.

UTF-8 is a specific scheme for mapping a sequence of 1-4 bytes to a numberfrom 0x000000 to 0x10FFFF:

The x's are bits to be extracted from the sequence and glued together toform the final number.

It is fair to say that UTF-8 is taking over the world. It is already used forfilenames in Linux and is supported by all mainstream web browsers. Thisis not surprising considering its many nice properties:

  1. It can represent all 1,114,112 Unicode characters.
  2. Most C code that deals with strings on a byte-by-byte basis still works,since UTF-8 is fully compatible with 7-bit ASCII.
  3. Characters usually require fewer than four bytes.
  4. String sort order is preserved. In other words, sorting UTF-8 stringsper-byte yields the same order as sorting them per-character by logicalUnicode value.
  5. A missing or corrupt byte in transmission can only affect a singlecharacter—you can always find the start of the sequence for the nextcharacter just by scanning a couple bytes.
  6. There are no byte-order/endianness issues, since UTF-8 data is abyte stream.
The only price to pay for all this is that there is no longer aone-to-one correspondence between bytes and characters in a string. Findingthe nth character of a string requires iterating over the string from thebeginning.

SeeWhat is UTF-8?for more information about UTF-8.

Side note:Some consider UTF-8 to be discriminatory, since it allows English text to bestored efficiently at one byte per character while other world scripts requiretwo bytes or more. This is a troublesome point, but it should not get in theway of Unicode adoption. First of all, UTF-8 was not really designed topreferentially encode English text. It was designed to preserve compatibilitywith the large body of existing code that scans for special characters suchas line breaks, spaces, NUL terminators, and so on. Furthermore, theencoding used internally by a program has little impact on the user as longas it is able to represent their data without loss.UTF-8 is a great boon, especially for C programming. Think of itthis way: if it allows you to internationalize an application that would havebeen difficult to convert otherwise, it is much less discriminatory than thealternative.

II. The C library

All recent implementations of the standard C library have lots of functionsfor manipulating international strings. Before reading up on them, it helpsto know some vocabulary:

'Multibyte character' or 'multibyte string' refers to text inone of the many (possibly language-specific) encodings that exist throughoutthe world. A multibyte character does not necessarily require more than onebyte to store; the term is merely intended to be broad enough to encompassencodings where this is the case. UTF-8 is in fact only one suchencoding; the actual encoding of user input is determined by the user's currentlocale setting (selected as an option in a system dialog or stored as anenvironment variable in UNIX). Strings you get from the user will be inthis encoding, and strings you pass to printf() are supposed to be as well.Strings within your program can of course be in any encoding you want, butyou might have to convert them for proper display.

'Wide character' or 'wide character string' refers to text whereeach character is the same size (usually a 32-bit integer)and simply represents a Unicode charactervalue ('code point'). This format is a known common currency that allows youto get at character values if you want to. The wprintf() family is able towork with wide character format strings, and the '%ls' format specifier fornormal printf() will print wide character strings (converting them to thecorrect locale-specific multibyte encoding on the way out).

The C library also provides functions like towupper() that can convert awide character from any language to uppercase (if applicable). strftime()can format a date and time string appropriately for the current locale, andstrcoll() can do international sorting. These and other functions thatdepend on locale must be initialized at the beginning of your program using

You don't have to do anything with the locale string returned by setlocale(),but you can use it to query your user's locale settings (more on this later).

The C library pretty much assumes you will be using multibyte stringsthroughout your program (since that's what you get as input). Since multibytestrings are opaque, a lot of functions beginning with 'mb' are provided todeal with them. Personally, I don't like not knowing what encoding my stringsuse. One concrete problem with the multibyte thing is file I/O— agiven file could be in any encoding, independent of locale. When you write afile or send data over a network, keeping the multibyte encoding mightbe a bad idea. (Even if all software uses only the properlocale-independent C library functions, and all platforms support allencodings internally, there is still no single standard for communicating theencoding of a piece of text; email messages and HTML tags do it in variousways.) You also might be ableto do more efficient processing, or avoid rewriting code, if you knew theencoding your strings used.

Your encoding options

You are free to choose a string encoding for internal use in your program.The choice pretty much boils down to either UTF-8, wide (4-byte) characters,or multibyte.Each has its advantages and disadvantages:

Game Dev Tycoon

    UTF-8
    • Pro: compatible with all existing strings and most existing code
    • Pro: takes less space
    • Pro: widely used as an interchange format (e.g. in XML)
    • Con: more complex processing, O(n) string indexing
    Wide characters
    • Pro: easy to process
    • Con: wastes space
    • Pro/Con: although you can use the syntaxto easily include wide-character strings in C programs, the size of widecharacters is not consistent across platforms (some incorrectly use 2-bytewide characters)
    • Con: should not be used for output, since spurious zero bytes and otherlow-ASCII characters with common meanings (such as '/' and 'n') will likelybe sprinkled throughout the data.
    Multibyte
    • Pro: no conversions ever needed on input and output
    • Pro: built-in C library support
    • Pro: provides the widest possible internationalization, since in rarecases conversion between local encodings and Unicode does not work well
    • Con: strings are opaque
    • Con: perpetuates incompatibilities. For example, there are three majorencodings for Russian. If one Russian sends data to another through yourprogram, the recipient will not be able to read the message if his or hercomputer is configured for a different Russian encoding. But if your programalways converts to UTF-8, the text is effectively normalized so that it willbe widely legible (especially in the future) no matter what encoding itstarted in.

In this article I will advocate and give explicit instruction on usingUTF-8 as an internal string encoding. Many Linux users already set theirenvironment to a UTF-8 locale, in which case you won't even have to doany conversions. Otherwise you will have to convert multibyte towide to UTF-8 on input, and back to multibyte on output. Nevertheless,UTF-8 has its advantages.

III. What to do right now

Below I'll outline concrete steps any C programmer could take tobring his or her code up to date with respect to text encoding. I'll also bepresenting a simple C library that provides the routines you need tomanipulate UTF-8.

Here's your to-do list:

  1. 'char' no longer means character
    I hereby recommend referring to character codes in C programs using a32-bit unsigned integer type. Many platforms provide a 'wchar_t' (widecharacter) type, but unfortunately it is to be avoided since some compilersallot it only 16 bits—not enough to represent Unicode. Wherever youneed to pass around an individual character, change 'char' to 'unsigned int'or similar. The only remaining use for the 'char' type is to mean 'byte'.
  2. Get UTF-8-clean
    To take advantage of UTF-8, you'll have to treat bytes higher than 127 asperfectly ordinary characters. For example, say you have a routine thatrecognizes valid identifier names for a programming language. Your existingstandard might be that identifiers begin with a letter:If you use UTF-8, you can extend this to allow letters from other languagesas follows:A UTF-8 sequence can only start with values 0xC0 or greater, so that's whatI used for checking the start of an identifier. Within an identifier, youwould also want to allow characters >= 0x80, which is the range ofUTF-8 continuation bytes.

    Most C string library routines still work with UTF-8, since they only scanfor terminating NUL characters. A notable exception is strchr(), whichin this context is more aptly named 'strbyte()'. Since you will be passingcharacter codes around as 32-bit integers, you need to replace this witha routine such as my u8_strchr() that can scan UTF-8 for a given character.The traditional strchr() returns a pointer to the location of the foundcharacter, and u8_strchr() follows suit. However, you might want to knowthe index of the found character, and since u8_strchr() has to scan throughthe string anyway, it keeps a count and returns a character index as well.

    With the old strchr(), you could use pointer arithmetic to determine thecharacter index. Now, any use of pointer arithmetic on strings is likelyto be broken since characters are no longer bytes. You'll have to find andfix any code that assumes '(char*)b - (char*)a' is the number of charactersbetween a and b (though it is still of course the number of bytesbetween a and b).

  3. Interface with your environment
    Using UTF-8 as an internal encoding is now widespread amongC programmers. However, the environment your program runs in will notnecessarily be nice enough to feed you UTF-8, or expect UTF-8 output.

    The functions mbstowcs() and wcstombs() convert from and to locale-specificencodings, respectively. 'mbs' means multibyte string (i.e. the locale-specificstring), and 'wcs' means wide character string (universal 4-byte characters).Clearly, if you use wide characters internally, you are in luck here. If youuse UTF-8, there is a chance that the user's locale will be set to UTF-8 andyou won't have to do any conversion at all. To take advantage of thatsituation, you will have to specifically detect it (I'll provide a functionfor it). Otherwise, you will have to convert from multibyte to wide to UTF-8.

    Version 1.6 (1.5.x while in development) of theFOX toolkit uses UTF-8 internally,giving your program a nice all-UTF-8-all-the-time environment. GTK2 and Qtalso support UTF-8.

  4. Modify APIs to discourage O(n^2) string processing
    The idea of non-constant-time string indexing may worry you. But whenyou think about it, you rarely need to specifically access the nth characterof a string. Algorithms almost never need to make requests like 'Quick! Getme the 6th character of this piece of text!' Typically, if you're accessingcharacters you're iterating over the whole string or most of it. UTF-8 issimple enough to process that iterating over characters takes essentiallythe same time as iterating over bytes.

    In your own code, you can use my u8_inc() and u8_dec() to move throughstrings. If you develop libraries or languages, be sure to expose some kindof inc() and dec() API so nobody has to move through a string by repeatedlyrequesting the nth character.

IV. Some UTF-8 routines

Various libraries are available for internationalizationand converting betweendifferent text encodings. However, I couldn't find a straightforward setof C routines providing the minimal support needed for using UTF-8 asan internal encoding (although this functionality is often embedded in largeUI toolkits and such). I decided to create a small library that could be usedto bring UTF-8 to arbitrary C programs.

This library is quite incomplete; you might want to look atrelated FSF offerings andlibutf8.libutf8 provides the multibyte and wide character C library routines mentionedabove, in case your C library doesn't have them.

C Game Dev Unicode Online

Since performance is sometimes a concern with UTF-8, I made my routines asfast and lightweight as possible. They perform minimal error checking—in particular, they do not bother to determine whether a sequence is validUTF-8, which can actually be a security problem. I justify this decision byreiterating that the intention of the library is to manipulate an internalencoding; you can enforce that all strings you store in memory bevalid UTF-8, enabling the library to make that assumption. Routines forvalidating and converting from/to UTF-8 areavailable free from Unicode, Inc.

Note that my routines do not need to support the many encodings of theworld—the C library can handle that. If the current locale is notUTF-8, you call mbstowcs() on user input to convert any encoding(whatever it is) to a wide character string, then use my u8_toutf8() toconvert it to the UTF-8 your program is comfortable with. Here's anexample input routine wrapping readline():

The first call to mbstowcs() uses the special parameter value NULL to findthe number of characters in the opaque multibyte string.

C Game Dev Unicode Online

Anyway, on with the routines. They are divided into four groups:

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Group 1: conversions

Note that the library uses 'unsigned int' as its wide character type.
You canconvert a known number of bytes, or a NUL-terminated string. The length ofa UTF-8 string is often communicated as a byte count, since that's whatreally matters. Recall that you can usually treat a UTF-8 string like anormal C-string with N characters (where N is the number of bytes in theUTF-8 sequence), with the possibility that some characters are >127.

Group 2: moving through UTF-8 strings

Group 3: Unicode escape sequences
In the absence of Unicode input methods, Unicode characters are oftennotated using special escape sequences beginning with u or U. u expectsup to four hexadecimal digits, and U expects up to eight. With theseroutines your program can accept input and give output using suchsequences if necessary.

Group 4: replacements for standard functions

utf8.c

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utf8.h

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