On Feb 12, 2013, at 10:02 PM, Philip Guenther <guent...@gmail.com> wrote:
> On Tue, Feb 12, 2013 at 5:34 PM, jordon <open...@sirjorj.com> wrote:
>> Today I was looking into some of the more simple devices to see how they
>> are implemented. I figured the basic text ones (zero, random, null,
>> etc) would be a nice place to start. I went to /usr/src/sys/dev to look
>> for them, but I couldn't find them. Where is the source of these basic
>> devices? Are they actual drivers like network drivers or are they some
>> completely different devices?
>
> Devices != drivers. Indeed, network drivers generally do not present
> devices in /dev/. That was one of the (early) criticisms of the BSD
> networking model: it created a new namespace for interface names.
>
> Anyway, if you're looking for the code behind a device file (a block
> device or character device file in the filesystem), you start by
> looking up its major number in the arrays in the file
> /usr/src/sys/arch/${ARCH}/${ARCH}/conf.c. For example, /dev/zero is a
> character device with major 2, and
> /usr/src/sys/arch/amd64/amd64/conf.c has this:
>
> struct cdevsw cdevsw[] =
> {
> cdev_cn_init(1,cn), /* 0: virtual console */
> cdev_ctty_init(1,ctty), /* 1: controlling terminal */
> cdev_mm_init(1,mm), /* 2: /dev/{null,mem,kmem,...} */
> ...
>
> cdev_mm_init() is a macro in <sys/conf.h>, and if you stare there and
> work out the cpp expansion, you'll eventually look for functions
> mmopen(), mmclose(), mmrw(), etc. For amd64, those are implemented in
> /usr/src/sys/arch/amd64/amd64/mem.c, where the code figures out which
> of the minor devices is involved in a given call by examining the
> 'dev' argument to the function.
>
>
> Philip Guenther
>
An excellent explanation. Thank you for clarifying this.
I have been studying Linux driver dev lately for work (hey, it's a step up from
.net/c# dev!) and I am comparing it with OpenBSD. This answer clears up a
question I discovered today.
Thanks again.
Jordon
