FAQ 5.25 How do I randomly update a binary file?

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 11, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
    comes with the standard Perl distribution. These postings aim to
    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 http://faq.perl.org .

    --------------------------------------------------------------------

    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.



    --------------------------------------------------------------------

    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    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.

    If you'd like to help maintain the perlfaq, see the details in
    perlfaq.pod.
     
    PerlFAQ Server, Jan 11, 2011
    #1
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