Date: Sat, 19 Dec 2009 21:59:07 +0000 From: Anton Shterenlikht <mexas@bristol.ac.uk> To: Warren Block <wblock@wonkity.com> Cc: glarkin@freebsd.org, mexas@bristol.ac.uk, perryh@pluto.rain.com, freebsd-questions@freebsd.org Subject: Re: editing a binary file Message-ID: <20091219215907.GA10349@mech-cluster241.men.bris.ac.uk> In-Reply-To: <alpine.BSF.2.00.0912180904180.11293@lightning.wonkity.com> References: <alpine.BSF.2.00.0912180904180.11293@lightning.wonkity.com>
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On Fri, Dec 18, 2009 at 09:33:49AM -0700, Warren Block wrote: > perryh@pluto.rain.com wrote: > > Greg Larkin <glarkin@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
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