Пример использования библиотеки iconv. Компилируется в cygwin командой: gcc charset_convert.c -liconv -o charset_convert.exe

charset_convert.c

#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <iconv.h>

char *charset_convert(const char *string, char *fromcode, char *tocode)
{
    iconv_t cd;
    size_t in, out, len, err;
    char *dest, *outp, *inp = (char *) string;

    cd = iconv_open(tocode, fromcode);
    if (cd < 0)
        return NULL;

    in = strlen(string);
    /* Chinese characters in GB18030 are commonly 2 bytes, in UTF-8 they are
     * 3 bytes or more, it quite rare to find a character exceeds such range,
     * plus we may have some non-Chinese characters which only need 1 byte,
     * so the size allocated here should be sufficient for most cases */
    out = len = in * 3 / 2 + 1;
    outp = dest = (char *) malloc(len);

again:
    err = iconv(cd, &inp, &in, &outp, &out);
    /* If the pre-allocated output buffer is not large enough, we have to
     * enlarge it by realloc(), then update related pointers and counters */
    if (err == (size_t)(-1) && errno == E2BIG)
    {
        size_t used = outp - dest;
        len *= 2; /* double the size may not be the most economic option though */
        char *newdest = (char *) realloc(dest, len);
        if (! newdest)
            goto out;

        dest = newdest;
        outp = dest + used;
        out = len - used - 1;
        goto again;
    }

out:
    /* Make sure we have the trailing '\0' */
    if (outp)
        *outp = '\0';
    iconv_close(cd);
    return dest;
}

int main(void)
{
    printf(charset_convert("Строка в CP1251", "CP1251", "UTF-8"));
    return 0;
}