dectohex

const char __stdlib_int_to_hex_table[16] =
{
    '0', '1', '2', '3','4','5','6','7','8','9','a','b','c','d','e','f',
};

// lower case
// size of `buf` must be enough for 'ffffffffffffffff' so buf[16]
char __CRT_itoa_internalBuffer[30]; // need to fix itoa(123) = "321"
void
__CRT_dectohex(uint64_t dec, char* buf, int* sz)
{
    if (dec)
    {
        double decInDouble = (double)dec;
        int bufInd = 0;
        while (1)
        {
            decInDouble /= 16.0;

            double ipart = 0.0;
            int i = (int)(modf(decInDouble, &ipart) * 16.0);//(dec % 16) * 16;
            dec /= 16; // for exit from loop

            __CRT_itoa_internalBuffer[bufInd++] = __stdlib_int_to_hex_table[i];

            if (!dec)
                break;
        }
        *sz = bufInd;
        if (bufInd)
        {
            for (int i = 0; i < bufInd; ++i)
            {
                buf[i] = __CRT_itoa_internalBuffer[bufInd - i - 1];
            }
            buf[bufInd] = 0;
        }
    }
    else
    {
        buf[0] = '0';
        buf[1] = 0;
    }
}