RE: Possible dd bug - no close of "if=/of=" fds

[HARDY, Steven]

Thanks very much - your tip led me to the problem which is that
/dev/mydevice's driver returns -EIO from it's release() function, and this
return value is not returned to the user via close() - reimplementing this
as a flush() function solved the problem, since the -EIO is passed up
through close() to dd.

Apologies for blaming dd, I've certainly learnt a few things looking through
it's code!

Many thanks, 
Steve

-----Original Message-----
From: Jim Meyering [mailto:address@hidden 
Sent: Wednesday, December 07, 2005 3:36 PM
To: HARDY, Steven
Cc: 'address@hidden'
Subject: Re: Possible dd bug - no close of "if=/of=" fds


"HARDY, Steven" <address@hidden> wrote:
> I'm experiencing some problems trying to use "dd" to load firmware 
> onto a device, and I guess this could be a potential bug?
>
> - dd if=myfirmware.bin of=/dev/mydevice
>
> The problem is that /dev/mydevice returns -EIO if the load has been 
> unsuccessful, but dd always returns 0 whether this occurs or not.
>
> I was expecting dd to explicitly close the if=/of= file descriptors 
> and return any errors, but looking at the code it seems to close only 
> stdin/stdout, and does not check for successful close of the input and 
> output files??
>
> I am using coreutils-5.9.3 - any help or suggestions much appreciated.

If you're using Linux, try using strace on it:

  strace -o log dd if=myfirmware.bin of=/dev/mydevice

You'll see that dd reopens stdin on the if=...-specified name, and reopens
stdout on the of=...-specified name. So when it closes STDIN_FILENO, it's
actually closing a file handle that is open on your input file.

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