# /dev/null: Invalid argument



## Tabs (Jul 2, 2017)

I am writing a program that uses chroot, I have created a /dev/null in my chroot (let's say it's called `/my/chroot/`), with the command `mknod /my/chroot/dev/null c 15 0 root:wheel`

But any time I try and direct output to /dev/null I get this error:
`root@testZFS:~ # chroot /my/chroot/
root@testZFS:/ # echo 1 > /dev/null
/dev/null: Invalid argument.`

Anyone got any ideas? Driving me mad!

P.S - please don't lecture me on using chroot and not jails, I need to use chroot in this instance trust me!


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## ralphbsz (Jul 3, 2017)

Tabs said:


> ... I have created a /dev/null in my chroot (let's say it's called `/my/chroot/`), with the command `mknod /my/chroot/dev/null c 15 0 root:wheel`



Where did you get the device numbers major=15 and minor=0 from?  If you had done an `ls -l` on the newly created device and on /dev/null, the difference would have been obvious.  In FreeBSD 11.0, the correct numbers are major=0 and minor=33 (or 0x21).  The following command works for me: `mknod /tmp/testnull c 0 33`.

Also, don't forget to check the permissions of the newly made device.  It needs to be writeable.  Here is a directory listing that shows workable permissions:

```
# ls -l /dev/null /tmp/testnull
crw-rw-rw-  1 root  wheel  0x21 Jul  2 17:26 /dev/null
crw-rw-rw-  1 root  wheel  0x21 Jul  2 17:25 /tmp/testnull
```


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## Tabs (Jul 3, 2017)

ralphbsz said:


> Where did you get the device numbers major=15 and minor=0 from?  If you had done an `ls -l` on the newly created device and on /dev/null, the difference would have been obvious.  In FreeBSD 11.0, the correct numbers are major=0 and minor=33 (or 0x21).  The following command works for me: `mknod /tmp/testnull c 0 33`.
> 
> Also, don't forget to check the permissions of the newly made device.  It needs to be writeable.  Here is a directory listing that shows workable permissions:
> 
> ...



I had no idea how to get the numbers so I was just copying examples from the net.

Working now - thanks!


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