INTRO(4) | Device Drivers Manual | INTRO(4) |
intro
—
The device abstraction generally provides a common set of system calls layered on top of them, which are dispatched to the corresponding device driver by the upper layers of the kernel. The set of system calls available for devices is chosen from open(2), close(2), read(2), write(2), ioctl(2), select(2), and mmap(2). Not all drivers implement all system calls, for example, calling mmap(2) on terminal devices is likely to be not useful at all.
Note that this could lead to an inconsistent state, where either
there are device nodes that do not have a configured driver associated with
them, or there may be drivers that have successfully probed for their
devices, but cannot be accessed since the corresponding device node is still
missing. In the first case, any attempt to reference the device through the
device node will result in an error, returned by the upper layers of the
kernel, usually ENXIO
. In the second case, the
device node needs to be created before the driver and its device will be
usable.
Some devices come in two flavors: block and
character devices, or to use better terms, buffered and
unbuffered (raw) devices. The traditional names are reflected by the letters
‘b
’ and
‘c
’ as the file type identification in
the output of ‘ls -l
’. Buffered
devices are being accessed through the buffer cache of the operating system,
and they are solely intended to layer a file system on top of them. They are
normally implemented for disks and disk-like devices only and, for
historical reasons, for tape devices.
Raw devices are available for all drivers, including those that
also implement a buffered device. For the latter group of devices, the
differentiation is conventionally done by prepending the letter
‘r
’ to the path name of the device
node, for example /dev/rsd0[cd] denotes the raw
device for the first SCSI disk, while /dev/sd0[cd]
is the corresponding device node for the buffered device.
Unbuffered devices should be used for all actions that are not related to file system operations, even if the device in question is a disk device. This includes making backups of entire disk partitions, or to raw floppy disks (i.e., those used like tapes).
Access restrictions to device nodes are usually subject to the regular file permissions of the device node entry, instead of being enforced directly by the drivers in the kernel.
December 18, 2017 | NetBSD 9.4 |