Re: inodes,major andminor numbers and device files

[Pradeep Padala <ppadala@xxxxxxxxxxxxxx>]

> I read about inode structure that it always point to
> data blocks for that file. but in case of character
> device files created with mknod command doesn't have
> any data block associated with it. so how kernel
> understands by using major and minor number a
> perticular device file.
> and for character device file what is the structure of
> inode ?

Major and minor numbers are encoded and stored in the disk block 
pointers. This is similar to storing the name of a symlink within the 
inode. As you rightly pointed out, there's no disk block associated with 
them.

Relevant code for character devices stored on an ext2fs is below 
(fs/ext2/inode.c)

/* In function ext2_write_inode */
    if (S_ISCHR(inode->i_mode) || S_ISBLK(inode->i_mode)) {
        if (old_valid_dev(inode->i_rdev)) {
            raw_inode->i_block[0] =
                cpu_to_le32(old_encode_dev(inode->i_rdev));
            raw_inode->i_block[1] = 0;
        } else {
            raw_inode->i_block[0] = 0;
            raw_inode->i_block[1] =
                cpu_to_le32(new_encode_dev(inode->i_rdev));
            raw_inode->i_block[2] = 0;
        }
    } else for (n = 0; n < EXT2_N_BLOCKS; n++)
        raw_inode->i_block[n] = ei->i_data[n];

When reading the inode (say with ls -al), following code does the decoding

/* In function ext2_read_inode */
    ...
    } else {
        inode->i_op = &ext2_special_inode_operations;
        if (raw_inode->i_block[0])
            init_special_inode(inode, inode->i_mode,
               old_decode_dev(le32_to_cpu(raw_inode->i_block[0])));
        else
            init_special_inode(inode, inode->i_mode,
               new_decode_dev(le32_to_cpu(raw_inode->i_block[1])));
    }

--
Pradeep Padala
http://ppadala.blogspot.com

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