On Sat, Feb 19, 2005 at 01:23:10AM +0100, [EMAIL PROTECTED] wrote:
>
> On Fri, 18 Feb 2005 15:23:44 EST, Bill Davidsen said:
>
> > I'll try to build a truth table for this, I'm now working with some
> > non-iso data sets, so I'm a bit more interested. I would expect read()
> > to only try to read one sector, so I'll just do a quick and dirty to get
> > the size from the command line, seek and read.
> >
> > I haven't had a problem using dd to date, as long as I know how long the
> > data set was, but I'll try to have results tonight.
>
> The problem is that often you don't know exactly how long the data set is
> (think "backup burned to CD/RW") - there's a *lot* of code that does stuff
> like
>
> while (actual=read(fd,buffer,65536) > 0) {
> ...
> }
>
> with the realistic expectation that the last read might return less than
> 64k,
> in which case 'actual' will tell us how much was read. Instead, we just
> get
> an error on the read.
>
> Note that 'dd' does this - that's why you get messages like '12343+1 blocks
> read'.
> We *really* want to get to a point where 'dd' will work *without* having to
> tell it a 'bs=' and 'count=' to get the size right....
> )
my point was that even specifying the exact byte count was not sufficient.
e.g
strace sdd if=/dev/zero bs=32k of=/dev/null ivsize=41234 gives:
......
munmap(0x46a000, 4096) = 0
_llseek(3, 0, [0], SEEK_SET) = 0
_llseek(4, 0, [0], SEEK_SET) = 0
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 32768) =
32768
write(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 32768) =
32768
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 8466) =
8466
write(2, "sdd: Done with input volume # 1."..., 33sdd: Done with input volume #
1.
) = 33
write(2, "Do you want to continue with inp"..., 53Do you want to continue with
input volume # 2 (y/n): ) = 53
that is, it requests exactly ivsize bytes (for iso cd's this can be got from
isosize).
even this was not good enough....
Karl
--
Karl Kiniger mailto:[EMAIL PROTECTED]
GE Medical Systems Kretztechnik GmbH & Co OHG
Tiefenbach 15 Tel: (++43) 7682-3800-710
A-4871 Zipf Austria Fax: (++43) 7682-3800-47
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