Error reported dd'ing close of end of block device with skip

Corinna Vinschen
Wed Dec 7 18:36:00 GMT 2005

On Dec  7 11:55, Loh, Joe wrote:
> Is there a way in Cygwin to do a read of a block device using "C" that
> does not do a read-ahead?  We needed to develop an application that will
> issue the exact transfer size to the target device as requested.
> Looking at the strace, it appears that read() less than 61440-bytes gets
> translated to reading 61440 bytes into buffer and then a subset of that
> read is returned to the caller.

There's a non-portable (only Cygwin) way to set the buffer size after
opening a device:

  #include <cygwin/rdevio.h>

  struct rdop rd;

  fd = open ("dev/sda", ...);

  rd.rd_op = RDSETBLK;
  rd.rd_parm = 0;	/* Unbuffered reading */
  rd.rd_parm = 1;	/* Unbuffered reading */
  rd.rd_parm = n;	/* Buffered reading with buffer size n */

  ioctl (fd, RDIOCDOP, &rd);

Note that the ioctl fails if the buffer already contains data, so ioctl
should be called before the first read.  Also note that in unbuffered
mode the usual Windows blocking rule applies, the length given to read
must be a multiple of 512.

Sigh, it seems that I introduced a bug into this ioctl also not long ago.
I fixed this in CVS.  Recent code will probably not work very well when
setting it to unbuffered mode.  Please wait for the next developers


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat, Inc.

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