Re: "dd" collapsed the loop device

[Andreas Dilger <adilger@turbolabs.com>]

On Jan 14, 2002  12:54 -0500, Michael Zhu wrote:
> Hello,everyone,I have a problem when I used the loop
> device. I don't know whether is a loop device bug.

User bug.

> I used the following commands to connect the loop device
> with the floppy disk device.
> 
> losetup -e xor /dev/loop0 /dev/fd0
> mke2fs /dev/loop0
> mount /dev/loop0 /floppy
> 
> Then I copy something to the floppy and read it back.
> Everything is OK. It works perfectly. 

Great.

> The problem was happened when I try to copy something
> directly from the /dev/fd0. I use the following
> demand.
> 
> dd if=test.c of=/dev/fd0
> 
> The output of the upper command is:
> 50+1 records in
> 50+1 records out
> 
> Then I used the "ls /floppy". I found nothing copied
> to the floppy.

Well, this is wrong for several reasons:
1) don't access /dev/fd0 when you use it via loopback, use /dev/loop0
2) don't use "dd" to copy a file, use "cp"
3) don't write into the device, but the filesystem instead:
   cp test.c /floppy

> Then I used "umount /floppy" to umount the floppy disk
> device. After that I used the following command to try
> to mount the floppy disk again.
>
> mount /dev/loop0 /floppy
> 
> It returned an error. Say:
> 
> mount: wrong fs type. bad option. bad superblock on
> /dev/loop0. or too many mounted file systems
> 
> It seemed that the "dd if=test.c of=/dev/fd0"
> corrupted the data on the floppy disk. What is wrong?

Because test.c is not a filesystem, and you have overwritten
the filesystem on /dev/fd0 with junk.  This is not a bug
in the loop driver.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/