Date: Sun, 07 Feb 2010 13:27:16 +0200 From: Nikos Vassiliadis <nvass9573@gmx.com> To: Mike Clarke <jmc-freebsd2@milibyte.co.uk> Cc: Polytropon <freebsd@edvax.de>, freebsd-questions@freebsd.org Subject: Re: Detecting cards in USB card reader Message-ID: <4B6EA394.5050604@gmx.com> In-Reply-To: <201002062328.27744.jmc-freebsd2@milibyte.co.uk> References: <201002061511.11639.jmc-freebsd2@milibyte.co.uk> <20100206192540.bb4ce11b.freebsd@edvax.de> <201002062328.27744.jmc-freebsd2@milibyte.co.uk>
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On 2/7/2010 1:28 AM, Mike Clarke wrote:
> curlew:/root# cat /dev/null> /dev/da0
> curlew:/root# ls -l /dev/da0*
> crw-r----- 1 root operator 0, 176 6 Feb 23:15 /dev/da0
> crw-r----- 1 root operator 0, 129 6 Feb 23:18 /dev/da0s1
>
> I can use this to initialise the card reader but I'd feel more
> comfortable with something a bit less dangerous looking.
While it may feel dangerous, is perfectly safe. There is no
way doing an IO operation on a disk-like device using requests
othen than multiplies of the physical block which currently is
512 bytes. Opening the disk for writing and trying to do a
write request, will just force GEOM to re-examine the device.
> lab# echo asd | cat > /dev/da0
> cat: stdout: Invalid argument
failed
> lab# echo > /dev/da0
> lab# echo $?
> 1
failed
> lab# /bin/echo asd > /dev/da0
> /bin/echo: write: Invalid argument
failed
Closer look:
> lab# truss sh -c "echo > /dev/da0"
snip
> open("/dev/da0",O_WRONLY|O_CREAT|O_TRUNC,0666) = 2 (0x2)
> dup2(0x2,0x1,0x1b6,0x1000008,0x2830d040,0x2830235c) = 1 (0x1)
> close(2) = 0 (0x0)
> write(1,"\n",1) ERR#22 'Invalid argument'
failed
FreeBSD lost the ability of doing such transparent
transformations when the support for block devices
went away.
Yes, I know, it feels awkward.
Apparently, you can easily drop the support for block
devices but not the habitual feeling of danger of UNIX
tradition.
HTH, Nikos
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