Xref: utzoo comp.lang.pascal:4069 comp.sys.ibm.pc.misc:1554
Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!dkuug!freja.diku.dk!terra
From: terra@diku.dk (Morten Welinder)
Newsgroups: comp.lang.pascal,comp.sys.ibm.pc.misc
Subject: TP overlays are not safe!
Keywords: tp, overlays, bug
Message-ID: <1990Sep10.101723.11142@diku.dk>
Date: 10 Sep 90 10:17:23 GMT
Organization: Department Of Computer Science, University Of Copenhagen
Lines: 47

Turbo Pascal 5.5 has a problem with the O+ switch; it doesn't work.



Setup: The following code is placed in an overlaid unit (#1):

  writeln(   pos(           { pos has 2 (>1) arguments, 1st is a string }
                 '.',       { 1st argument is string CONSTANT }
                 myfunc));  { calls other modules (#2,...). }


Problem:
  The expression '.' generates a reference to the code segment; CS and some
  offset is pushed. Then myfunc is called and (a pointer to) the result is 
  pushed. Finally pos is called.

  Since the second expression may cause unit #1 to be disposed of by the 
  overlay manager, there is 
  simply no way to ensure that unit #1 is reloaded at the same address before 
  the pos-function is evaluated. If it is reloaded elsewhere, the first 
  argument is LOST.

  The problem may arise with as little as three units. I will post source
  code when I can get my fingers at it.

  The problem only shows if little memory is available for the overlay buffer.
  Debugging consumes a large amount of memory.

Solution:

1. Yell at Borland

2. Take special care when using string constant as non last argument in an
   overlaid unit. Rearange the expression:
      s:=myfunc;
      writeln(pos('.',s));

3a.Make the overlay buffer large enough to hold all active units. (Then
   #1 will not be disposed of.)

3b.Do not change the size of the buffer. (The #1 will be loaded at the same 
   address.)



Morten Welinder
terra@freja.diku.dk