GNU bug report logs - #9734
[solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file of 133120 bytes

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: "Clark J. Wang" <dearvoid <at> gmail.com>
Subject: bug#9734: closed (Re: bug#9734: [solaris] `dd if=/dev/urandom
 of=file bs=1024k count=1' gets a file of 133120 bytes)
Date: Wed, 12 Oct 2011 14:15:03 +0000

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Your bug report

#9734: [solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file of 133120 bytes

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 9734 <at> debbugs.gnu.org.

-- 
9734: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=9734
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Eric Blake <eblake <at> redhat.com>
To: "Clark J. Wang" <dearvoid <at> gmail.com>
Cc: 9734-done <at> debbugs.gnu.org
Subject: Re: bug#9734: [solaris] `dd if=/dev/urandom of=file bs=1024k count=1'
	gets a file of 133120 bytes
Date: Wed, 12 Oct 2011 08:14:15 -0600

tag 9734 notabug
thanks

On 10/12/2011 02:22 AM, Clark J. Wang wrote:
> I'm not sure if it's a bug but it's not reasonable to me. On Solaris 11
> (SunOS 5.11 snv_174, i86pc):
>
> $ uname -a
> SunOS sollab-242.cn.oracle.com 5.11 snv_174 i86pc i386 i86pc
> $ pkg list gnu-coreutils
> NAME (PUBLISHER)                                  VERSION
> IFO
> file/gnu-coreutils                                8.5-0.174.0.0.0.0.504
> i--
> $ /usr/gnu/bin/dd if=/dev/urandom of=file bs=1024k count=1
> 0+1 records in

Notice that this means you read a partial record - read() tried to read 
1024k bytes, but the read ended short at only 133120 bytes.

> 0+1 records out

And because you didn't request dd to group multiple short reads before 
doing a full write, you got a single (short) record written.

> I'm new to Solaris but I've never seen this problem whe I use Linux so it
> really suprises me.

Solaris and Linux kernels differ on when you will get short reads, and 
magic files like /dev/urandom are more likely to display the issue than 
regular files.  That said, Linux also has the "problem" of short reads; 
it's especially noticeable when passing the output of dd to a pipe.

You probably wanted to use this GNU extension:

dd if=/dev/urandom of=file bs=1024k count=1 iconv=fullblock

where the iconv flag requests that dd pile together multiple read()s 
until it has a full block, so that you no longer have a partial block 
output.

>
> I found this in the man page of /dev/urandom on Solaris: "The limitation per
> read for /dev/random is 1040 bytes. The limit for /dev/urandom is (128 *
> 1040 = 133120)." That seems to be the reason but I think dd should handle
> that and check the return value of the read() system call and make sure
> 1024k bytes have really been read from /dev/urandom.

Only if the iconv=fullblock flag is specified, since it is a violation 
of POSIX to do more than one read() without an explicit flag requesting 
multiple reads per block.

-- 
Eric Blake   eblake <at> redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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From: "Clark J. Wang" <dearvoid <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: [solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file
	of 133120 bytes
Date: Wed, 12 Oct 2011 16:22:12 +0800

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I'm not sure if it's a bug but it's not reasonable to me. On Solaris 11
(SunOS 5.11 snv_174, i86pc):

$ uname -a
SunOS sollab-242.cn.oracle.com 5.11 snv_174 i86pc i386 i86pc
$ pkg list gnu-coreutils
NAME (PUBLISHER)                                  VERSION
IFO
file/gnu-coreutils                                8.5-0.174.0.0.0.0.504
i--
$ /usr/gnu/bin/dd if=/dev/urandom of=file bs=1024k count=1
0+1 records in
0+1 records out
133120 bytes (133 kB) copied, 0.00290536 s, 45.8 MB/s
$ ls -l file
-rw-r--r-- 1 root root 133120 2011-10-12 16:12 file
$

I'm new to Solaris but I've never seen this problem whe I use Linux so it
really suprises me.

I found this in the man page of /dev/urandom on Solaris: "The limitation per
read for /dev/random is 1040 bytes. The limit for /dev/urandom is (128 *
1040 = 133120)." That seems to be the reason but I think dd should handle
that and check the return value of the read() system call and make sure
1024k bytes have really been read from /dev/urandom.

Any idea?

Thanks.

-Clark

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