uint64 sys_open(void)
{
    char path[MAXPATH];
    int fd, omode;
    struct file *f;
    struct inode *ip;
    int n;
    if ((n = argstr(0, path, MAXPATH)) < 0 || argint(1, &omode) < 0)
        return -1;

    begin_op();

    if (omode & O_CREATE)
    {
        ip = create(path, T_FILE, 0, 0);
        if (ip == 0)
        {
            end_op();
            return -1;
        }
    }
    else
    {
        if(omode & O_NOACCESS){
            ip = namei(path, 2);
        }
        else if ((ip = namei(path, 0)) == 0) //file doesn't exist(may be a symlink)
        {
            end_op();
            return -1;
        }
        ilock(ip);

        if (ip->type == T_DIR && omode != O_RDONLY && omode != O_NOACCESS)
        {
            iunlockput(ip);
            end_op();
            return -1;
        }
        if ((ip->type == T_SYMLINK) && !(omode & O_NOACCESS)){  //trace symlink
            while (ip->type == T_SYMLINK) {
                int len = 0;
                readi(ip, 0, (uint64)&len, 0, sizeof(int));
                readi(ip, 0, (uint64)path, sizeof(int), len + 1);
                iunlockput(ip);
                if((ip = namei(path, 0)) == 0){
                    end_op();
                    return -1;
                }
                ilock(ip);
            }
        }

        if(ip->mode == M_READ && ((omode == O_WRONLY) || (omode == O_RDWR))){
            iunlockput(ip);
            end_op();
            return -1;
        }

        if(ip->mode == M_WRITE && ((omode == O_RDONLY) || (omode == O_RDWR))) {
            iunlockput(ip);
            end_op();
            return -1;
        }

        if(ip->mode == 0 && omode != O_NOACCESS){
            iunlockput(ip);
            end_op();
            return -1;
        }

    }

    if (ip->type == T_DEVICE && (ip->major < 0 || ip->major >= NDEV))
    {
        iunlockput(ip);
        end_op();
        return -1;
    }

    if ((f = filealloc()) == 0 || (fd = fdalloc(f)) < 0)
    {
        if (f)
            fileclose(f);
        iunlockput(ip);
        end_op();
        return -1;
    }

    if (ip->type == T_DEVICE)
    {
        f->type = FD_DEVICE;
        f->major = ip->major;
    }
    else
    {
        f->type = FD_INODE;
        f->off = 0;
    }
    f->ip = ip;
    f->readable = !(omode & O_WRONLY);
    f->writable = (omode & O_WRONLY) || (omode & O_RDWR);
    if(omode & O_NOACCESS){
        f->readable = 0;
        f->writable = 0;
    }
    if ((omode & O_TRUNC) && ip->type == T_FILE)
    {
        itrunc(ip);
    }

    iunlock(ip);
    end_op();

    return fd;
}