README
showflaw-1.2

Wed Sep 15 15:32:54 EDT 1999
David Harris <dharris@drh.net>

This archive contains the example program and explanation for
showing database corruption using the following locking method
which is proffered in the DB_File documentation:
    
    $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
        || die "dbcreat /tmp/foo.db $!";
    $fd = $db->fd;
    open(DB_FH, "+<&=$fd") || die "dup $!"; 
    flock (DB_FH, LOCK_SH) || die "flock: $!";

The basic problem is that the first 4k of the database file are
read and cached on the tie line -- so if the process is blocking
in the flock call while another process modifies that area of the
database file, database corruption can result.

Files in this archive...

README       -- This file

showflaw.pl  -- This program actually creates corruption on my system

doc.showflaw -- Some text explaining how the showflaw.pl program works.
                However the real documentation is still the source code.

doc.strace   -- The output of my strace testing that shows the read call
                before the flock call actually happening.

example/     -- This directory has example output from running showflaw.pl
                on my system

write.pl     -- Super simple test program for manipulating the database file
                that I used in developing showflaw.pl