On 10/3/05, Gus Wirth <[EMAIL PROTECTED]> wrote:
> At 23:37 09/30/2005 -0700, Carl Lowenstein wrote:
> >I never heard of CPM image format, neither has Google. But I have
> >learned tonight that the ACARD controller in our CD/DVD duplicator
> >will export files from the duplicator's internal hard drive to a drive
> >on the host computer, using its USB connection. By actual trial, I
> >have found that this exported file is the same as the original .ISO
> >file but has 2456 bytes prepended to it. The file name defaults to
> >*.CPM. The text at the beginning of this is "CPM Image V1."
> >
> >Oh, well, I can strip it off using dd(1) as we all have discussed at
> >great length.
>
> I wonder if there is a way to resize the file "in situ" , i.e. trim the
> head without having to make a duplicate.
>
> I had originally asked this question of carl at the installfest pertaining
> to trimming the tail of a file, but he couldn't think of a utility that did
> that. I know I could do something to reset the size and then release the
> tail blocks to the free pool, but I'm willing to bet someone has already
> done that.
>
> Anyone know of any utilities to trim files in place?
>
Search Google groups "comp.*" with key words "truncate" "file" "in place".
One learns that there is a system call "ftruncate" (man 2 ftruncate)
that can trim the tail end of a file. After much discussion, the
conclusion is that it isn't easy, and is not portable across different
flavors of Unix. Nobody seems to have actually done it in a
releasable-to-others form. And trimming the head end of a file is
more difficult than trimming the tail end, because you need to pack
everything forward to fill the "hole" at the beginning.
Unless you are trimming multi-gigabyte files on a regular basis, it is
probably best to swallow the overhead of reading and re-writing the
file, and just use dd(1). Besides, then you aren't as vulnerable to
power failures in the middle of a long operation.
carl
--
carl lowenstein marine physical lab u.c. san diego
[EMAIL PROTECTED]
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