to use mandatory locking on a linux system, the filesystem has to be well prepared.

# /etc/fstab: static file system information.
#
# <file system> <mount point>   <type>  <options>               <dump>  <pass>
/dev/hda1       /               ext3    errors=remount-ro,mand  0       1
/dev/hda2       none            swap    sw                      0       0
proc            /proc           proc    defaults                0       0
/dev/fd0        /floppy         auto    user,noauto             0       0
/dev/cdrom      /cdrom          iso9660 ro,user,noauto          0       0 

For example here the ext3 partition has been prepared for mandatory locking. Otherwise no dio_function will work on the system.