Date: Tue, 1 Jun 1999 09:06:05 -0700 (PDT) From: Julian Elischer <julian@whistle.com> To: Wes Peters <wes@softweyr.com> Cc: Zhihui Zhang <zzhang@cs.binghamton.edu>, freebsd-hackers@FreeBSD.ORG Subject: Re: Accessing special device files Message-ID: <Pine.BSF.3.95.990601090301.12924A-100000@current1.whistle.com> In-Reply-To: <3753F33E.74DAA18C@softweyr.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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.... julian To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.BSF.3.95.990601090301.12924A-100000>