editing a binary file

[Anton Shterenlikht]

On Fri, Dec 18, 2009 at 09:33:49AM -0700, Warren Block wrote:
> perryh at pluto.rain.com wrote:
> > Greg Larkin <glarkin at freebsd.org> wrote:
> > > ...
> > > > truncate -4 myfile should get rid of the last four bytes.  Maybe
> > > > there's a similar efficient way to truncate the start of a file.
> > >
> > > This should do it:
> > >
> > > dd if=oldfile of=newfile bs=1 skip=4
> > 
> > Or, perhaps marginally more efficient:
> > 
> > dd if=oldfile of=newfile bs=4 skip=1
> 
> It would be nice to avoid the file copy, but maybe there's no way to do 
> that.  The small buffer size for dd will probably make copies of 
> multi-gig files slow.  This might be faster:
> 
> tail -c +5 myfile > outfile
> truncate -4 outfile

yes, quite. On 1.5GHz ia64, on 1GB binary file tail takes about 25 s,
but dd.. I killed after 25 min (!) and it had only done 1/3 of the file.

But even tail is too slow.

So I'll probably have to write a C I/O routine and avoid fortran I/O
alltogether, so I write straight away just my data.

many thanks
anton

-- 
Anton Shterenlikht
Room 2.6, Queen's Building
Mech Eng Dept
Bristol University
University Walk, Bristol BS8 1TR, UK
Tel: +44 (0)117 331 5944
Fax: +44 (0)117 929 4423