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
comes with the standard Perl distribution. These postings aim to
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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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