Wide Characters

THE MAGAZINE OF USENIX & SAGE August 2002 volume 27 • number 4 inside: PROGRAMMING McCluskey: Wide Characters & The Advanced Computing Systems Association & The System Administrators Guild wide characters int main() by Glen { McCluskey printf("sizeof(wchar_t) = %u\n", sizeof(wchar_t)); Glen McCluskey is a } consultant with 20 years of experience When I run this program on my Linux system, the result is: and has focused on programming lan- sizeof(wchar_t) = 4 guages since 1988. He specializes in Java wchar_t is a signed or unsigned integral type big enough to and C++ perfor- mance, testing, and hold all the characters in the local extended character set, and is technical documen- a 32-bit long on my system. tation areas. [email protected] wide character constants are specified similarly to normal character constants, with a preceding L before the constant: #include <stdio.h> We’ve been looking at some of the new features in #include <wchar.h> C99, the standards update to C. In this column we’ll int main() consider features added to the language and library in { support of wide characters, as typically used with for- wchar_t wc1 = L'a'; eign languages. printf("%lx\n", wc1); wchar_t wc2 = L'\377'; Character Sets printf("%lx\n", wc2); Several terms are used in the standard to describe C character sets. The first of these is “basic character set” and refers to a set wchar_t wc3 = L'\x12345678'; of single-byte characters that all C99 implementations must printf("%lx\n", wc3); } support. Roughly speaking, this character set is 7-bit ASCII without some of the control characters. It consists of printable When I run this program, the result is: characters like A–Z and 0–9, along with tab, form feed, and so 61 on. ff The basic character set is divided into source and execution 12345678 character sets, and these differ slightly. For example, the basic In the first two cases, the wide character is stored in the least execution character set is required to have a null character (\0), significant byte of the long, while in the last case, all four bytes used as a string terminator. of the long are used to represent a single wide character. The extended character set is a superset of the basic character This example illustrates a confusing point about wide charac- set, and adds additional locale-specific characters. It, too, is ters – the idea of multiple representations. For example, wc3 is divided into source and execution character sets. initialized with a wide character constant, a constant that A wide character is a character of type wchar_t, and is capable requires 13 bytes to express in the source program. The con- of representing any character in the current locale. In other stant itself is stored in four bytes during execution (in a 32-bit words, a wide character may be a character from either the basic long). And a little later on, we’ll see examples of what is called character set, such as the letter A, or a character from the “state-dependent encoding,”a mechanism used to encode wide extended character set. characters as a stream of bytes (1–6 bytes per wide character, on my system). This encoding is used for writing wide characters Wide Character Constants to a file. Let’s look at some actual examples of wide character usage. The The term “multibyte character” is defined to be a sequence of first demo program prints the size of wchar_t on your local sys- one or more bytes that represents a single character in the tem: extended source or execution environment. A character from #include <stdio.h> the extended character set can have several different representa- #include <wchar.h> tions. These representations may appear in source code, in the execution environment, or in data files. 12 Vol. 27, No. 4 ;login: Here’s another example, showing how wide character strings are #include <assert.h> specified: #include <stdio.h> #include <wchar.h> #include <assert.h> #include <wchar.h> int main() ROGRAMMING { P int main() // write a wide character string to a file { wchar_t* wstr1 = L"testing\x12345678"; FILE* fp = fopen("outfile", "w"); wchar_t wstr2[] = L"testing\x12345678"; assert(fp); fwprintf(fp, L"string is\377: %ls\n", L"TESTing\x1234"); assert(*wstr1 == 't'); fclose(fp); assert(*(wstr1 + 7) == 0x12345678); // read the characters of the string back from the assert(wstr2[0] == 't'); // file assert(wstr2[7] == 0x12345678); } fp = fopen("outfile", "r"); assert(fp); String Operations wint_t c; Many familiar operations are supported on wide character wchar_t buf[25]; strings. For example, here’s a demo that implements a function size_t len = 0; while ((c = getwc(fp)) != WEOF) to convert to lowercase: buf[len++] = c; #include <stdio.h> fclose(fp); #include <wchar.h> buf[len] = 0; #include <wctype.h> // check results wchar_t* tolower(wchar_t* str) if (wcscmp(buf, L"string is\377: TESTing\x1234\n") { == 0) wchar_t* start = str; printf("strings are equal\n"); // convert each wide character to lowercase else printf("strings are unequal\n"); for (; *str; str++) { } if (iswupper(*str)) *str = tolower(*str); Much of this code is identical to what you would use when } reading and writing regular strings of bytes. return start; wint_t is a type that is related to wchar_t in a way similar to the } relationship between int and char; it can hold all possible int main() wchar_t values, as well as one distinguished value that is not { part of the extended character set (WEOF). wchar_t* str = L"TESTing"; wchar_t buf[8]; The only other tricky thing in this example is stream orientation, something that’s implicit in the code. A file stream can be wcscpy(buf, str); either byte or wide oriented. The orientation is determined by tolower(buf); the first operation on the stream, or explicitly via the fwide() call. Since the first write operation in the demo is fwprintf(), and printf("%ls\n", buf); the first read operation is getwc(), and these are wide character } functions, the streams are marked as having wide orientation. Note that the definition of an uppercase character may be locale Why does stream orientation matter? The reason is that an specific. encoding may be applied to wide characters written to a file. Wide Characters and I/O Suppose you are programming with wide characters, you need to do wide character I/O, and your wide characters are four Suppose that you have a wide character string and you’d like to bytes long when using the wchar_t representation. One way of write it to a file and then read it back. How can you do this? writing such characters to a file is to actually write four bytes Here’s one approach: for each character. August 2002 ;login: WIDE CHARACTERS 13 But what happens if, most of the time, the values of your wide len = 1 characters are within the range of 7-bit ASCII? In such a case, 61 three zero bytes will be written for each character. And the len = 2 resulting files will not be readable by tools that expect ASCII. c3 bf This problem exists today, for example, with tools that write 16- len = 6 fc 92 8d 85 99 b8 bit Unicode to a file. One solution to this problem is to encode characters such that 7-bit ASCII is represented as itself, that is, a The character a is encoded as itself, while the character \377 is single byte, while other character values are encoded using mul- encoded as two bytes 0xc3 and 0xbf. The constant tiple bytes. L’\x12345678’, internally represented as a four-byte long, is encoded using six bytes. But if an encoding is applied, then it no longer makes sense to mix byte and wide-file operations. This is especially true given Here’s another example of encoding and decoding: that an encoding may be state dependent, and dipping into a #include <stdio.h> byte stream in the middle of a multiple-byte encoding of a wide #include <stdlib.h> character has no meaning. #include <wchar.h> Encodings int main() Let’s look a little deeper into the encoding issue, with another { example. This demo converts wide character values into char buf[MB_CUR_MAX]; wchar_t wc1 = L'\x12345678'; sequences of encoded bytes: wchar_t wc2; #include <stdio.h> int n, nn; #include <stdlib.h> // convert a wide character to a multibyte #include <wchar.h> // character int main() n = wctomb(buf, wc1); { printf("%d\n", mblen(buf, n)); char buf[MB_CUR_MAX]; int n; // reverse the process // convert a single-byte character to a multibyte nn = mbtowc(&wc2, buf, n); // character // check result n = wctomb(buf, L'a'); if (wc1 == wc2 && n == nn) printf("len = %d\n", n); printf("equal\n"); for (int i = 0; i < n; i++) else printf("%hhx ", buf[i]); printf("unequal\n"); printf("\n"); } // convert another single-byte character The wctomb() function encodes a wide character into a stream n = wctomb(buf, L'\377'); of bytes, and mbtowc() reverses the process. printf("len = %d\n", n); for (int i = 0; i < n; i++) Restartable Functions printf("%hhx ", buf[i]); Consider the second part of the last example. The processing of printf("\n"); the first part of the example – a wide character encoded into a // convert a wide character buffer of one or more bytes – was reversed, by taking the buffer n = wctomb(buf, L'\x12345678'); and converting it back into a wide character. printf("len = %d\n", n); In a real-world example, things might not be quite as simple. for (int i = 0; i < n; i++) For instance, you might have an application where bytes are printf("%hhx ", buf[i]); coming in across a network one at a time, and several of the printf("\n"); bytes put together represent a wide character.

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