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FAQ 5.25 How do I randomly update a binary file?

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This is an excerpt from the latest version perlfaq5.pod, which
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reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at .


5.25: How do I randomly update a binary file?

If you're just trying to patch a binary, in many cases something as
simple as this works:

perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

However, if you have fixed sized records, then you might do something
more like this:

$RECSIZE = 220; # size of record, in bytes
$recno = 37; # which record to update
open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!";
seek $fh, $recno * $RECSIZE, 0;
read $fh, $record, $RECSIZE == $RECSIZE or die "can't read record $recno: $!";
# munge the record
seek $fh, -$RECSIZE, 1;
print $fh $record;
close $fh;

Locking and error checking are left as an exercise for the reader. Don't
forget them or you'll be quite sorry.


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are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

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