将16进制unsigned char数组转换成整数

/**
 * 将unsigned char数组转换成long long数值
 * {0x00 0x00 0x20 0x00}转换之后得到8192
 *
 * @param str 数组
 * @param len str数组长度
 * @returns 0 on error
 */
LONGLONG PNGProcesser::readCharToLL(unsigned char* str, int len)
{
    if (len <= )
    {
        return ;
    }

    int i = ;
    int value = ;
    for (i=;i<len;i++)
    {
        value = value** + (str[i]/)* + str[i]%;
    }

    return value;
}