lseek()/read() to physical drive returning wrong data.

Corinna Vinschen cygwin@cygwin.com
Mon Feb 5 08:21:00 GMT 2001


On Sat, Feb 03, 2001 at 07:19:15PM +0100, Corinna Vinschen wrote:
> On Sat, Feb 03, 2001 at 12:47:49PM -0500, Robin T. Miller wrote:
> > Hi All,
> > 	I have a Unix test program named 'dt' which runs with Cygwin.
> > This program tests a variety of devices as well as file systems.  The
> > tests which use lseek() to raw mounted disks, are failing with data
> > compare errors.  It appears either lseek() is positioning incorrectly
> > or read() is returning the wrong data after lseek'ing.
> > 
> > 	This random I/O sequence seems to work correctly to a regular
> > file, so maybe the raw device buffering is messing up?  I'm not sure.
> 
> Lseek on raw devices isn't yet fully implemented. The underlying
> *cough* operating system *cough* isn't able to do a bytewise access
> to the raw device but only in steps of 512 bytes. A full lseek
> implementation would need to do it's own buffering to simulate
> bytewise buffering. This is still missing.

I have just checked in a patch to the Cygwin CVS repository which
should allow bytewise lseek on raw disk devices at least for SEEK_SET
and SEEK_CUR.

For some reason my tests using SEEK_END (aka FILE_END) failed on
my HD partition so using SEEK_END in the current implementation 
results in lseek returning -1 and errno set to EINVAL programatically.

If anybody knows if that observation is correct or if I simply
did something wrong I would gladly appreciate any hint.

Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                                mailto:cygwin@cygwin.com
Red Hat, Inc.

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