On Dec 7 19:35, Corinna Vinschen wrote:
> On Dec 7 11:55, Loh, Joe wrote:
> > QUESTION:
> >
> > 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
> snapshot.
I just added another requirement, that rd.rd_parm must be either 0, 1,
or a multiple of 512. Other sizes for buffers don't make much sense.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat, Inc.
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