medusa# mount | egrep '^/dev'
/devices on /devices read/write/setuid/devices/rstchown/dev=9640000 on Fri Jan 19 16:33:07 2018
/dev on /dev read/write/setuid/devices/rstchown/dev=9680000 on Fri Jan 19 16:33:07 2018
/dev/fd on fd read/write/setuid/devices/rstchown/dev=9940001 on Fri Jan 19 16:33:22 2018
medusa# ls -l /dev/rdsk/c1t*d0
lrwxrwxrwx   1 root     root          65 Jan  2  2015 /dev/rdsk/c1t0d0 -> ../../devices/pci@0,0/pci8086,340b@4/pci1028,1f10@0/sd@0,0:wd,raw
lrwxrwxrwx   1 root     root          65 Jan  2  2015 /dev/rdsk/c1t1d0 -> ../../devices/pci@0,0/pci8086,340b@4/pci1028,1f10@0/sd@1,0:wd,raw
medusa# uname -a
SunOS medusa 5.11 11.3 i86pc i386 i86pc


On 2/6/2018 9:06 PM, Dan Cross wrote:
On Tue, Feb 6, 2018 at 8:48 PM, Dave Horsfall <dave@horsfall.org> wrote:
On Wed, 7 Feb 2018, Greg 'groggy' Lehey wrote:
V3 and earlier still *called* them special files, but it seems they were essentially just magic inode numbers (there was no physical file on disk, just any directory entry with the given inode would be the special file).

Isn't that still the case?

 Wasn't that "devfs" (which Penguin/OS calls "udev")?  I've never grokked that concept.

No. devfs was (is?) a pseudo-filesystem where only special files corresponding to the devices enumerated by the kernel during autoconfiguration are present. The contents are synthesized at boot time and the filesystem is mounted at some canonical location (like /dev), but is otherwise ephemeral. This is in contrast to the older /dev, which is usually just a directory on the root filesystem, wherein one created a number of device files that may (or may not) correspond to an actual hardware device in the system (remember the old dance of, "cd /dev && ./MAKEDEV foo" when you added a "foo" onto your system?). The inodes and directory entries for those files actually exist in the disk-resident filesystem structures (though of course data blocks aren't allocated to those files and the inode doesn't refer to any data blocks).

My understanding is that udev is an elaboration on devfs on Linux that includes a rules engine that supports things like assigning set names to specific devices, setting permissions, group/user ownership and the like. For example, one can configure a rule so that when USB UART device with serial number 0xdeadbeef gets attached to the system, it appears as /dev/console_for_foo, owned by group "fooadmin" and mod 660. Presumably whoever configured that knows that that serial device is physically connected to the serial console for "foo".

        - Dan C.