Knowledge check 8.1

vam

Why is video adapter is not listed at /dev?

$ ls -l /dev/ati

total 0

crw-rw-rw- 1 root root 250, 0 янв. 29 16:05 card0

crw-rw-rw- 1 root root 250, 1 янв. 29 21:55 card1

dbuday

Because network and video adapter are not listed under /dev as special files.

vam

is it true? what about my output of
$ ls -l /dev/ati

there are two files representing ati video cards.

and note that in material preceding the knowledge check video adapters was not mentioned.

linux supports 3 classes of devices: character, block and network.
character and block usually present in /dev/
does this mean that video adapters goes to network class?

saqman2060

vam wrote:

Why is video adapter is not listed at /dev?

$ ls -l /dev/ati
total 0
crw-rw-rw- 1 root root 250, 0 янв. 29 16:05 card0
crw-rw-rw- 1 root root 250, 1 янв. 29 21:55 card1

It seems like your video adaptor is listed.

/dev/ati

This appears to be a directory of an ati controller that points to two video cards or connections.

What I was able to find out, /dev is a directory that holds files of devices on a linux system. The devices it holds are block, character and special device files. Block devices are devices that store data, character devices are devices that transmit data.Then there are special devices which I am not sure what those are. /dev is where linux stores files of all devices it detects on the system. If this is not the case, where else would linux keep device files? Perhaps some place we haven't heard yet.

This is my opinion, the files in /dev are not files that can be easily read with a simple text editor. In fact, these device files are actually virtual device links to the actual devices themselves. They are handles for the kernel to access the devices.

The drivers of these devices give names and attributes to the kernel. In which case, no driver, or supported driver, no access to the device. lspci might show the device but not /dev. There is a script that you can run to make the kernel detect every device in your system and locate them in /dev. If it exists of course "MAKEDEV".

/dev will not give you the actual name of the device in human readable forms. These names are supplied by the driver for the kernel. It is just a matter of knowing what you are reading. Check out these sources,

dev filesystem

accessing the video

coop

Thanks Istimsak for shedding some light. Let me explain further.

If you look at your disk when the system is not running, /dev is an empty directory. It is an in-memory pseudofilesystem (a ram fs) mounted at boot and populated by udev as devices are found. (read the udev chapter). The entries in /dev are indeed *not* normal files; they are character nodes; do ls -l and you will see block devices have a "b" in the first
character (like disk partitions such as /dev/sda1) and character devices have a "c" (like /dev/lp0 for first printer).

Network devices do not have device nodes.

Many device nodes do not correspond to hardware but are "software devices", an abstraction
useful to the kernel, such as /dev/null or /dev/fuse.

You have two character devices under /dev/ati; /dev/ati/card0 and /dev/ati/card1. Some video cards create character devices, some don't. They would be used for streaming bytes to the card. I'm not familiar with the ati cards and it will also depend on whether you are using an in-kernel device driver or a proprietary closed source one from ati. On the two systems I am looking at right now, both have Intel graphics chips on board and neither seems to have a character device associated with them.

By the way, unless you know what you are doing, avoid doing any I/O directly to anything
under /dev; you are quite likely to do something like corrupt your hard disk or fry something else. For example, when you read/write a file the filesystem will eventually do I/O with /dev/sda1 for example, but if you do it directly you will trash the filesystem.

vam

Thank you all for your comments.

I want to point out that the question in knowledge check 8.1 is not quite correct to make the course better.
It gives checklist of hardware devices and one must pick those that not listed in /dev as special files.
In material preceding the knowledge check video adapters are not mentioned. And video adapter is listed in /dev on my system.

luisviveropena

Hi vam,

Tipically you won't have any video device under /dev. In the case of an Ati adapter, it seems to put some entries under /dev, as you are pointing us to. So, we may consider to check what other video devices goes there. But we can safely say that generally there won't be any video device under /dev.

You can see a good explanation here:

http://unix.stackexchange.com/questions/23199/why-are-network-interfaces-not-in-dev-like-other-devices

The paragraph starting at 'On many devices'...

Regards,
Luis.

saqman2060

Here is a good article that explains how Linux detects and names devices connected to it.
https://www.linux.com/news/hardware/peripherals/180950-udev

There are other areas and methods in a Linux systems that will aid tracking down devices connected to your system. We know the first place to check is /dev. Linux does store devices it successfully detects in that directory. However, that is not the only place to located your devices.