Re: Access content of file via inodes

[Jeff Mahoney <jeffm@suse.com>]

Kathy KN (HK) wrote:
> On Apr 6, 2005 1:53 AM, Bryan Henderson <hbryan@us.ibm.com> wrote:
>>>How do I access/read the content of the files via using inodes
>>>or blocks that belong to the inode, at sys_link and vfs_link layer?
>>This is tricky because many interfaces that one would expect to use an
>>inode as a file handle use a dentry instead.  To read the contents of a
>>file via the VFS interface, you need a file pointer (struct file), and the
>>file pointer identifies the file by dentry.  So you need to create a dummy
>>dentry, which you can do with d_alloc_root(), and then create the file
>>pointer with dentry_open(), then read the file with vfs_read().
>>
>>That's for "via inodes."  I don't know what "via blocks" means.
> 
> Bryan,
> 
> Thanks for the description on how to read the contents of a file via
> the VFS interface. I got to try to see if I can write it in codes, and make
> sure that I can read the file via the vfs_read() routine. What I meant by
> via blocks is to gain knowledge of the physical blocks used by the inodes
> and retrieve the content from it directly, by accessing b_data.

The problem with that approach is that some filesystems may store part
of the file outside of a complete block. For example, reiserfs "tails"
will respond with -ENOENT on ->bmap. For files smaller than 16k, they
are quite common.

-Jeff

-- 
Jeff Mahoney
SuSE Labs