Date: Tue, 01 Jun 1999 12:49:39 -0600 From: Wes Peters <wes@softweyr.com> To: Julian Elischer <julian@whistle.com> Cc: Zhihui Zhang <zzhang@cs.binghamton.edu>, freebsd-hackers@FreeBSD.ORG Subject: Re: Accessing special device files Message-ID: <37542B43.FD1CFFF3@softweyr.com> References: <Pine.BSF.3.95.990601090301.12924A-100000@current1.whistle.com>
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Julian Elischer wrote:
>
> Bzzzt!
>
> On Tue, 1 Jun 1999, Wes Peters wrote:
>
> > Zhihui Zhang wrote:
> > >
> > > I write a small program to read/write each FreeBSD partition via special
> > > device file names, e.g. /dev/wd0s2e, /dev/rwd0s2e, etc. I have two
> > > questions about doing this:
> > >
> > > (1) If I try to read() on these files, the buffer size must be given in
> > > multiples of 512 (sector size). Otherwise, I will get an EINVAL error.
> > > Why is this the case? Does the same thing happen to the write() system
> > > call?
> > >
> > > (2) I use lseek() on these device files, it returns the correct offset for
> > > me. But actually it does not work. I read in a recent posting saying that
> > > you can't expect lseek(fd, 0, SEEK_END) to work unless the file descriptor
> > > is associated with a regular file because file size information is not
> > > available at that level. Does this apply to all kinds of lseek(), include
> > > SEEK_SET and SEEK_CUR? Or maybe the offset must also given in a multiple
> > > of 512 for some reason. If I give lseek(fd, 8193, SEEK_SET), it will
> > > actually do lseek(fd, 8192, SEEK_SET)?
> >
> > They're block devices, they can only be read, written, and seeked in
> > terms of their native block size. The block size is typically 512
> > bytes for disks, other types of media (like CD-ROM) may vary greatly
> > from this.
> >
> > If you want to read, write, or seek to arbitrary ranges, use the "raw"
> > devices, i.e. rwd0s2e. These are character devices that will allow
> > you to read, write, and seek to arbitrary byte locations on the device;
> > the kernel handles the buffering and blocking for you.
>
> I know this is confusing but you are 100% backwards..
>
> the RAW device can only be accesses in units supported by the hardware..
> i.e. usually 512 byte chunks..
> however the Buffered device (e.g. wd0s1a) may be accessed on any boundary
> as it buffers the data, (at least for reads).
> it accesses the device in 512 byte chunks but this is hidden from you..
> this is why it's a buffered device....
???
dd verifies the behavior you report:
root@homer# dd if=/dev/rwd0s2b of=/dev/null bs=1
dd: /dev/rwd0s2b: Invalid argument
...
root@homer# dd if=/dev/rwd0s2b of=/dev/null bs=512
^C18805+0 records in
...
wes@homer$ ls -l /dev/*wd0s2a
crw-r----- 1 root operator 3, 0x00030000 Apr 1 11:10 /dev/rwd0s2a
brw-r----- 1 root operator 0, 0x00030000 Apr 1 11:10 /dev/wd0s2a
The rwd device is clearly a character-special device, the wd device a
block special. Character devices can always be read byte-at-a-time,
by definition. When did the semantics of this change?
--
"Where am I, and what am I doing in this handbasket?"
Wes Peters Softweyr LLC
http://www.softweyr.com/~softweyr wes@softweyr.com
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