Date: Wed, 26 Apr 1995 13:26:09 -0400
From: kagel AT quasar DOT bloomberg DOT com
To: B DOT S DOT Runnacles AT soton DOT ac DOT uk
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Unable to free memory..
Reply-To: kagel AT ts1 DOT bloomberg DOT com

   Date: Wed, 26 Apr 1995 15:03:58 +0100 (BST)
   From: "B.S.Runnacles" <B DOT S DOT Runnacles AT soton DOT ac DOT uk>

   Dear all....

   I have this bit of (schematic) code that frees the memory used in a linked 
   list:

   LinkedList::~LinkedList(void)
   {
   listnode *Root,*Current; // a data structure with next pointer and data
   _go32_dpmi_meminfo info1,info2;
   for (Current = Root; Current != NULL; Current = Current->next) {
	   _go32_dpmi_get_free_memory_information(&info1);
	   delete Current;
	   _go32_dpmi_get_free_memory_information(&info2);
	   cout << info2.available_memory - info1.available_memory << endl;
	   }
   }

   But the result says that there is no difference in the free memory before and 
   after the delete call.  Has anyone got any ideas as to why?  I may be doing
   something fundementally wrong here, any hints would be nice.  I think the
   memory isn't being freed because after a few hundred iterations the code
   falls over complaining of a full swap file.

   Ben Runnacles
   Image, Speech and Intelligent Systems Group
   University of Southampton, UK.


OK, here's what is going on:

   1) Memory freed is only freed to malloc's free block data structure not back
to the OS or to go32 so _go32_dpmi_get_free_memory_information does not know
about it.

   2) Other allocations intermixed with the ones you have freed are fragmenting
memory so that it cannot be reused.  Thus after "a few hundred iterations" the
code fails.  This looks like the most likely scenario.

-- 
Art S. Kagel, kagel AT ts1 DOT bloomberg DOT com