FAQ 5.24 How do I randomly update a binary file?

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5.24: 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, "+ 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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