如何通过指针读取UTF-8字符？

Question

Suppose I have UTF-8 content stored in memory, how do I read the characters using a pointer? I presume I need to watch for the 8th bit indicating a multi-byte character, but how exactly do I turn the sequence into a valid Unicode character? Also, is wchar_t the proper type to store a single Unicode character?

This is what I have in mind:

   wchar_t readNextChar (char*& p)
   { 
       wchar_t unicodeChar;
       char ch = *p++;

       if ((ch & 128) != 0)
       {
           // This is a multi-byte character, what do I do now?
           // char chNext = *p++; 
           // ... but how do I assemble the Unicode character?   
           ...
       }
       ...
       return unicodeChar;
   }  

Answer 1

You have to decode the UTF-8 bit pattern to its unencoded UTF-32 representation. If you want the actual Unicode codepoint, you have to use a 32-bit data type.

On Windows, wchar_t is NOT large enough, as it is only 16-bit. You have to use an unsigned int or unsigned long instead. Use wchar_t only when dealing with UTF-16 codeunits instead.

On other platforms, wchar_t is usually 32bit. But when writing portable code, you should stay away from wchar_t except where absolutely needed (like std::wstring ).

Try something more like this:

#define IS_IN_RANGE(c, f, l)    (((c) >= (f)) && ((c) <= (l)))

u_long readNextChar (char* &p) 
{  
    // TODO: since UTF-8 is a variable-length
    // encoding, you should pass in the input
    // buffer's actual byte length so that you
    // can determine if a malformed UTF-8
    // sequence would exceed the end of the buffer...

    u_char c1, c2, *ptr = (u_char*) p;
    u_long uc = 0;
    int seqlen;
    // int datalen = ... available length of p ...;    

    /*
    if( datalen < 1 )
    {
        // malformed data, do something !!!
        return (u_long) -1;
    }
    */

    c1 = ptr[0];

    if( (c1 & 0x80) == 0 )
    {
        uc = (u_long) (c1 & 0x7F);
        seqlen = 1;
    }
    else if( (c1 & 0xE0) == 0xC0 )
    {
        uc = (u_long) (c1 & 0x1F);
        seqlen = 2;
    }
    else if( (c1 & 0xF0) == 0xE0 )
    {
        uc = (u_long) (c1 & 0x0F);
        seqlen = 3;
    }
    else if( (c1 & 0xF8) == 0xF0 )
    {
        uc = (u_long) (c1 & 0x07);
        seqlen = 4;
    }
    else
    {
        // malformed data, do something !!!
        return (u_long) -1;
    }

    /*
    if( seqlen > datalen )
    {
        // malformed data, do something !!!
        return (u_long) -1;
    }
    */

    for(int i = 1; i < seqlen; ++i)
    {
        c1 = ptr[i];

        if( (c1 & 0xC0) != 0x80 )
        {
            // malformed data, do something !!!
            return (u_long) -1;
        }
    }

    switch( seqlen )
    {
        case 2:
        {
            c1 = ptr[0];

            if( !IS_IN_RANGE(c1, 0xC2, 0xDF) )
            {
                // malformed data, do something !!!
                return (u_long) -1;
            }

            break;
        }

        case 3:
        {
            c1 = ptr[0];
            c2 = ptr[1];

            switch (c1)
            {
                case 0xE0:
                    if (!IS_IN_RANGE(c2, 0xA0, 0xBF))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;

                case 0xED:
                    if (!IS_IN_RANGE(c2, 0x80, 0x9F))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;

                default:
                    if (!IS_IN_RANGE(c1, 0xE1, 0xEC) && !IS_IN_RANGE(c1, 0xEE, 0xEF))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;
            }

            break;
        }

        case 4:
        {
            c1 = ptr[0];
            c2 = ptr[1];

            switch (c1)
            {
                case 0xF0:
                    if (!IS_IN_RANGE(c2, 0x90, 0xBF))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;

                case 0xF4:
                    if (!IS_IN_RANGE(c2, 0x80, 0x8F))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;

                default:
                    if (!IS_IN_RANGE(c1, 0xF1, 0xF3))
                    {
                        // malformed data, do something !!!
                        return (u_long) -1;
                    }
                    break;                
            }

            break;
        }
}

    for(int i = 1; i < seqlen; ++i)
    {
        uc = ((uc << 6) | (u_long)(ptr[i] & 0x3F));
    }

    p += seqlen;
    return uc; 
}

Answer 2

Here is a quick macro that will count UTF-8 bytes

#define UTF8_CHAR_LEN( byte ) (( 0xE5000000 >> (( byte >> 3 ) & 0x1e )) & 3 ) + 1

This will help you detect the size of the UTF-8 character for easier parsing.

Answer 3

If you need to decode UTF-8 you need do develop an UTF-8 parser. UTF-8 is a variable-length encoding (1 to 4 bytes) so you really have to write a parser that is compliant with the standard : see wikipedia for example.

If you do not want to write your own parser, I suggest to use a library. You will find that in glib for example (I personnaly have used Glib::ustring, the C++ wrapper of glib) but also in any good general purpose library.

Edit:

I think that C++0x will include UTF-8 support too, but I'm no specialist...

my2c

Answer 4

Also, is wchar_t the proper type to store a single Unicode character?

On Linux, yes. On Windows, wchar_t represents a UTF-16 code unit, which isn't necessarily a character.

The upcoming C++0x standard will provide the char16_t and char32_t types designed to represent UTF-16 and UTF-32.

If on a system where char32_t is unavailable and wchar_t is inadequate, use uint32_t to store Unicode characters.

Answer 5

This is my solution, in pure ANSI-C, including a unit test for the corner cases.

Beware that int must be at least 32 bits wide. Otherwise you have to change the definition of codepoint .

#include <assert.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

typedef unsigned char byte;
typedef unsigned int codepoint;

/**
 * Reads the next UTF-8-encoded character from the byte array ranging
 * from {@code *pstart} up to, but not including, {@code end}. If the
 * conversion succeeds, the {@code *pstart} iterator is advanced,
 * the codepoint is stored into {@code *pcp}, and the function returns
 * 0. Otherwise the conversion fails, {@code errno} is set to
 * {@code EILSEQ} and the function returns -1.
 */
int
from_utf8(const byte **pstart, const byte *end, codepoint *pcp) {
        size_t len, i;
        codepoint cp, min;
        const byte *buf;

        buf = *pstart;
        if (buf == end)
                goto error;

        if (buf[0] < 0x80) {
                len = 1;
                min = 0;
                cp = buf[0];
        } else if (buf[0] < 0xC0) {
                goto error;
        } else if (buf[0] < 0xE0) {
                len = 2;
                min = 1 << 7;
                cp = buf[0] & 0x1F;
        } else if (buf[0] < 0xF0) {
                len = 3;
                min = 1 << (5 + 6);
                cp = buf[0] & 0x0F;
        } else if (buf[0] < 0xF8) {
                len = 4;
                min = 1 << (4 + 6 + 6);
                cp = buf[0] & 0x07;
        } else {
                goto error;
        }

        if (buf + len > end)
                goto error;

        for (i = 1; i < len; i++) {
                if ((buf[i] & 0xC0) != 0x80)
                        goto error;
                cp = (cp << 6) | (buf[i] & 0x3F);
        }

        if (cp < min)
                goto error;

        if (0xD800 <= cp && cp <= 0xDFFF)
                goto error;

        if (0x110000 <= cp)
                goto error;

        *pstart += len;
        *pcp = cp;
        return 0;

error:
        errno = EILSEQ;
        return -1;
}

static void
assert_valid(const byte **buf, const byte *end, codepoint expected) {
        codepoint cp;

        if (from_utf8(buf, end, &cp) == -1) {
                fprintf(stderr, "invalid unicode sequence for codepoint %u\n", expected);
                exit(EXIT_FAILURE);
        }

        if (cp != expected) {
                fprintf(stderr, "expected %u, got %u\n", expected, cp);
                exit(EXIT_FAILURE);
        }
}

static void
assert_invalid(const char *name, const byte **buf, const byte *end) {
        const byte *p;
        codepoint cp;

        p = *buf + 1;
        if (from_utf8(&p, end, &cp) == 0) {
                fprintf(stderr, "unicode sequence \"%s\" unexpectedly converts to %#x.\n", name, cp);
                exit(EXIT_FAILURE);
        }
        *buf += (*buf)[0] + 1;
}

static const byte valid[] = {
        0x00, /* first ASCII */
        0x7F, /* last ASCII */
        0xC2, 0x80, /* first two-byte */
        0xDF, 0xBF, /* last two-byte */
        0xE0, 0xA0, 0x80, /* first three-byte */
        0xED, 0x9F, 0xBF, /* last before surrogates */
        0xEE, 0x80, 0x80, /* first after surrogates */
        0xEF, 0xBF, 0xBF, /* last three-byte */
        0xF0, 0x90, 0x80, 0x80, /* first four-byte */
        0xF4, 0x8F, 0xBF, 0xBF /* last codepoint */
};

static const byte invalid[] = {
        1, 0x80,
        1, 0xC0,
        1, 0xC1,
        2, 0xC0, 0x80,
        2, 0xC2, 0x00,
        2, 0xC2, 0x7F,
        2, 0xC2, 0xC0,
        3, 0xE0, 0x80, 0x80,
        3, 0xE0, 0x9F, 0xBF,
        3, 0xED, 0xA0, 0x80,
        3, 0xED, 0xBF, 0xBF,
        4, 0xF0, 0x80, 0x80, 0x80,
        4, 0xF0, 0x8F, 0xBF, 0xBF,
        4, 0xF4, 0x90, 0x80, 0x80
};

int
main() {
        const byte *p, *end;

        p = valid;
        end = valid + sizeof valid;
        assert_valid(&p, end, 0x000000);
        assert_valid(&p, end, 0x00007F);
        assert_valid(&p, end, 0x000080);
        assert_valid(&p, end, 0x0007FF);
        assert_valid(&p, end, 0x000800);
        assert_valid(&p, end, 0x00D7FF);
        assert_valid(&p, end, 0x00E000);
        assert_valid(&p, end, 0x00FFFF);
        assert_valid(&p, end, 0x010000);
        assert_valid(&p, end, 0x10FFFF);

        p = invalid;
        end = invalid + sizeof invalid;
        assert_invalid("80", &p, end);
        assert_invalid("C0", &p, end);
        assert_invalid("C1", &p, end);
        assert_invalid("C0 80", &p, end);
        assert_invalid("C2 00", &p, end);
        assert_invalid("C2 7F", &p, end);
        assert_invalid("C2 C0", &p, end);
        assert_invalid("E0 80 80", &p, end);
        assert_invalid("E0 9F BF", &p, end);
        assert_invalid("ED A0 80", &p, end);
        assert_invalid("ED BF BF", &p, end);
        assert_invalid("F0 80 80 80", &p, end);
        assert_invalid("F0 8F BF BF", &p, end);
        assert_invalid("F4 90 80 80", &p, end);

        return 0;
}