Is there a C library to convert Unicode code points to UTF-8?

Question

I have to go through some text and write the UTF-8 output according to the character patterns. I thought it’ll be easy if I can work with the code points and get it converted to UTF-8. I have been reading about Unicode and UTF-8, but couldn’t find a good solution. Any help will be appreciated.

Answer 1

Converting Unicode code points to UTF-8 is so trivial that making the call to a library probably takes more code than just doing it yourself:

if (c<0x80) *b++=c;
else if (c<0x800) *b++=192+c/64, *b++=128+c%64;
else if (c-0xd800u<0x800) goto error;
else if (c<0x10000) *b++=224+c/4096, *b++=128+c/64%64, *b++=128+c%64;
else if (c<0x110000) *b++=240+c/262144, *b++=128+c/4096%64, *b++=128+c/64%64, *b++=128+c%64;
else goto error;

Also, doing it yourself means you can tune the api to the type of work you need (character-at-a-time? Or long strings?) You can remove the error cases if you know your input is a valid Unicode scalar value.

The other direction is a good bit harder to get correct. I recommend a finite automaton approach rather than the typical bit-arithmetic loops that sometimes decode invalid sequences as aliases for real characters (which is very dangerous and can lead to security problems).

Even if you do end up going with a library, I think you should either try writing it yourself first or at least seriously study the UTF-8 specification before going further. A lot of bad design can come from treating UTF-8 as a black box when the whole point is that it's not a black box but was created to have very powerful properties, and too many programmers new to UTF-8 fail to see this until they've worked with it a lot themselves.

Answer 2

iconv could be used I figure.

#include <iconv.h>

iconv_t cd;
char out[7];
wchar_t in = CODE_POINT_VALUE;
size_t inlen = sizeof(in), outlen = sizeof(out);

cd = iconv_open("utf-8", "wchar_t");
iconv(cd, (char **)&in, &inl, &out, &outlen);
iconv_close(cd);

But I fear that wchar_t might not represent Unicode code points, but arbitrary values.. EDIT: I guess you can do it by simply using a Unicode source:

uint16_t in = UNICODE_POINT_VALUE;
cd = iconv_open("utf-8", "ucs-2");