linux-kernel - Re: dd if=/dev/random of=data.bin bs=512 count=1

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Message-ID: <4A189BDA.5050002@tmr.com>
Date:	Sat, 23 May 2009 20:59:06 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Jon <jg@...k.org>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: dd if=/dev/random of=data.bin bs=512 count=1

Jon wrote:
> Hello KOSAKI,
> 
> Thank you for your reply.
> 
> KOSAKI Motohiro wrote:
>>> Hello
>>>
>>> Issuing this command, I expected to get a file 4096 bytes in size.
>>> Could someone let me know if I have missed something..? Or is there a
>>> problem with using /dev/random this way?
>>>
>>> if I have a count=10 it hangs forever (several hours).
>>>
>>> Changing to use /dev/urandom and it generates the file ok.
>>
>> you already have the answer. /dev/random might blocked, /dev/urandom 
>> doesn't.
> 
> Is this a "feature" of /dev/random ? ..it does not seem very useful to 
> have it able to block for so long.
> 
Of course. /dev/random generates better random numbers (in the sense of less 
predictable), while /dev/urandom gives you some pseudo-random bits, which mat or 
may not be appropriate for your task. For timing disk speed /dev/zero is faster 
and less overhead.

> Please include my email address in replies,  as I'm not a member of this 
> list.
> 
> Best regards, Jon


-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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