If access to the file is shared with other processes, any of these intrinsics
should be preceded by FLOCK and FUNLOCK intrinsics. This
controls access to the records and reduces contention while a modification
procedure is being performed. All pointer positioning, read intrinsics, and
writing, updating, and deletion procedures should be bounded by the
FLOCK and FUNLOCK intrinsics to guarantee that the proper
record is updated or deleted.
NOTE: File locking keeps the file inaccessible to other users
until the file is unlocked. This could be a potential source of
performance problems. A different file structure may be more suitable
for applications in a shared environment.