Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!apple!voder!kontron!optilink!cramer
From: cramer@optilink.UUCP (Clayton Cramer)
Newsgroups: comp.sys.ibm.pc
Subject: Overlays & Function Pointers
Message-ID: <542@optilink.UUCP>
Date: 4 Oct 88 18:07:07 GMT
Organization: Optilink Corporation, Petaluma, CA
Lines: 22

Consider this a warning.

I was having trouble getting overlays to work with the Microsoft C V5.1
compiler, and after talking to Microsoft Customer Support, and eventually
to someone in the Microsoft Languages Group, my boss pointed out the
source of the problem.  This is an obscure enough problem that even the
engineer in the Languages Group needed it explained in detail to 
understand it.

If you directly call a function which is overlayable, the Microsoft
linker will correctly replace the call with an INT 3FH ... instruction
to invoke the Overlay Manager.  If you call a function indirectly
(i.e. the function pointer is stored in a table, and you call through
that function pointer) the Microsoft linker can't replace that
function pointer with an INT 3FH call, because there isn't a CALL
instruction there -- just an address.  As a consequence, the overlay
manager isn't invoked, and whatever code happens to be in the overlay
area when the indirect call is made will be executed.  (This is almost
never the code you really want).
-- 
Clayton E. Cramer
..!ames!pyramid!kontron!optilin!cramer