Re: [LIP] doubt regarding /dev/zero

On Sunday 04 July 2004 4:48 pm, Vardhman Jain wrote:
> Hi,
> 	I came accross following code for allocation swap memory.
> I am really confused about the /dev/zero /dev/null files now. I previously
> assumed that they are some kind of sink files but this example changes my
> thought can some one tell me about these files /dev/zero etc.

/dev/null is a bit-sink. You can write anything to it, and it will happily eat 
it up. If you read from it, you will immediately get EOF.

$ wc -c /dev/null
0 /dev/null

/dev/zero is supposed to be used for reading only. You can read endlessly from 
it and will never get an EOF. Every octet that it returns is the '\0' byte. 

$ dd if=/dev/zero of=dump bs=1 count=1
1+0 records in
1+0 records out
$ od -a dump
0000000 nul
0000001


>
> # dd if=/dev/zero of=/swap bs=1024 count=65535
> # mkswap /swap
> # swapon /swap
>

This is ok, it just returns enough null bytes to rewrite every octet of 
the /swap partition with '\0'. Using /dev/null would have been a mistake 
here, because dd wouldnt have been able to read anything from it. It would 
have immediately got an eof.

$ dd if=/dev/null of=dump bs=1 count=1
0+0 records in
0+0 records out




- Sandip

-- 
Sandip Bhattacharya
sandip (at) puroga.com
Puroga Technologies Pvt. Ltd.
Work: http://www.puroga.com        Home: http://www.sandipb.net

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