FAQ 5.25 How do I randomly update a binary file?

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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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