int readi (struct inode *ip, char *dst, uint off, uint n)
{
    uint tot, m;
    struct buf *bp;

    if (ip->type == T_DEV) {
        if (ip->major < 0 || ip->major >= NDEV || !devsw[ip->major].read) {
            return -1;
        }

        return devsw[ip->major].read(ip, dst, n);
    }

    if (off > ip->size || off + n < off) {
        return -1;
    }

    if (off + n > ip->size) {
        n = ip->size - off;
    }

    for (tot = 0; tot < n; tot += m, off += m, dst += m) {
        bp = bread(ip->dev, bmap(ip, off / BSIZE));
        m = min(n - tot, BSIZE - off%BSIZE);
        memmove(dst, bp->data + off % BSIZE, m);
        brelse(bp);
    }

    return n;
}