utf16_to_utf8 function, untested by me, but I hope it works well, comes from opusfile with some modifications.

utf16_to_utf8.c

#include <stdlib.h>
#include <string.h>
#include <wchar.h>


static char* utf16_to_utf8 (const wchar_t *src)
{
    size_t len = wcslen(src), si, di;

    char *dst = (char*)malloc(sizeof(*dst)*(3*len+1));

    if (dst == NULL)
		return dst;

    for (di = si = 0; si < len; si++)
    {
        unsigned c0 = src[si];

        if (c0 < 0x80)
        {
            /*Can be represented by a 1-byte sequence.*/
            dst[di++] = (char)c0;
            continue;
        }
        else if (c0 < 0x800)
        {
            /*Can be represented by a 2-byte sequence.*/
            dst[di++] = (char)(0xC0|c0>>6);
            dst[di++] = (char)(0x80|c0&0x3F);
            continue;
        }
        else if (c0 >= 0xD800 && c0 < 0xDC00)
        {
        	/*This is safe, because c0 was not 0 and src is NUL-terminated.*/
            unsigned c1 = src[si+1];

            if (c1 >= 0xDC00 && c1 < 0xE000)
            {
            	/*Surrogate pair.*/
                unsigned w = ((c0&0x3FF)<<10|c1&0x3FF)+0x10000;

                /*Can be represented by a 4-byte sequence.*/
                dst[di++] = (char)(0xF0|w>>18);
                dst[di++] = (char)(0x80|w>>12&0x3F);
                dst[di++] = (char)(0x80|w>>6&0x3F);
                dst[di++] = (char)(0x80|w&0x3F);
                si++;
                continue;
            }
        }

        /*Anything else is either a valid 3-byte sequence, an invalid surrogate
           pair, or 'not a character'.
          In the latter two cases, we just encode the value as a 3-byte
           sequence anyway (producing technically invalid UTF-8).
          Later error handling will detect the problem, with a better
           chance of giving a useful error message.*/

        dst[di++] = (char)(0xE0|c0>>12);
        dst[di++] = (char)(0x80|c0>>6&0x3F);
        dst[di++] = (char)(0x80|c0&0x3F);
    }

    dst[di++] = '\0';
    return dst;
}