FAQ 5.22: How do I randomly update a binary file?

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5.22: 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(FH, "+<somewhere") || die "can't update somewhere: $!";
        seek(FH, $recno * $RECSIZE, 0);
        read(FH, $record, $RECSIZE) == $RECSIZE || 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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