device file ?

[sam111]

I am wondering how you can create your own device file.
I have read that mknod is the command to do this.
However how do you get the major and minor numbers for a particular device ? (like for a thumb drive , HDD , webcam ,...etc?)

And how can I create a device file that is like /dev/zero but instead of sending/receiving zeros I can send/receive 1's or any other words/numbers I want?

By reading the man pages for it, it seems their are 2 different types of device files block and character. So everything that doesn't spit back characters I am assuming is block.
Confused though what /dev/dsp or /dev/null are interms of block or character?

Any help with creating devices file would be great.
I am assuming if you deleted your /dev folder you could recreate it using this command. Correct me if I am wrong

[hazel]

I am wondering how you can create your own device file.
I have read that mknod is the command to do this.
However how do you get the major and minor numbers for a particular device ? (like for a thumb drive , HDD , webcam ,...etc?)

Look in the kernel documentation. There's a file called devices.txt that lists all the node numbers.

By reading the man pages for it, it seems their are 2 different types of device files block and character. So everything that doesn't spit back characters I am assuming is block.
Confused though what /dev/dsp or /dev/null are interms of block or character?

/dev/null is definitely char and I think /dev/dsp is too. Block devices are disks and disk equivalents like pen-drives and flash-cards.

I am assuming if you deleted your /dev folder you could recreate it using this command. Correct me if I am wrong

I think if you deleted /dev, your system would crash! But modern 2.6 kernels use a dynamic /dev directory created by udev at boot, so you would have /dev back again when you rebooted.

[sam111]

Ok, they both are character devices looking at the device.txt. (for /dev/dsp , dev/null ,...etc)

So I have tried creating my own device that is equivalent to /dev/zero but it doesn't seem to work or let me do anything with it even when I sudo.

I created it by this command
sudo mknod zeroit c 5 5

the 5 5 is the major and minor numbers for the zero device file and the c is for character unbuffered special file.
when I do cat zeroit > tmp | hexedit or dd if=zeroit of=/dev/sdb count=1 ...etc it doesn't seem to work
But when I use /dev/zero instead everything works fine?

ls -ltr gives
crw-r--r-- 1 root root 5, 5 2011-10-18 17:06 zeroit
So make mknod created it but I just cann't do anything with it?

Question 2
Is their away to create with mknod a device file that spites back all ones instead of all zeros?

[hazel]

My guess is that this doesn't work because you can't have two different devices in the same family with the same nodes.

To create a device that spewed out 1's rather than 0's, you would first need to write a suitable kernel module. Just creating a device file wouldn't do the trick because device files are only an interface with the kernel. I don't know if that would even be possible; you'd have to post a question in the kernel forum.