Comments: Authenticated sender is <mert0407 AT sable DOT ox DOT ac DOT uk>
From: "George Foot" <george DOT foot AT merton DOT ox DOT ac DOT uk>
To: bitland AT aol DOT com (Bitland)
Date: Fri, 25 Sep 1998 14:08:46 +0000
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Subject: Re: malloc/new once again
Reply-to: mert0407 AT sable DOT ox DOT ac DOT uk
CC: djgpp AT delorie DOT com
Message-Id: <E0zMXeV-00036t-00@orange.easynet.co.uk>
Precedence: bulk

On 25 Sep 98 at 7:59, Bitland wrote:

> I got version 2.8.1 and still have malloc-problems. Allocating memory in an
> absolutely simple program and freeing it in the next moment, displaying the
> memory with  __dpmi_get_free_memory_information(&dfmi); gives confusing
> memory values and then the memory slowly gets low.

When you free blocks they won't be added to the free DPMI 
memory information, but they will be available for future 
mallocs to use.

> Here's the main loop (simulating making allegro bitmaps), with MAXSIZE 1024,
> CDEPTH 32 and MAPS so, that you cannot allocate more than you have, but be sure
> to get closely to the limit, then the effect comes faster.
> 
>   do
>   {
>     for (i=0;i<MAPS;i++)
>     {
>       hm[i] = malloc((CDEPTH>>3)*(random()%MAXSIZE * (random()%MAXSIZE)));
>       if (!hm[i])     
>         textout(screen,font,"there it is !",200,20,COLWHITE);   /* via allegro
> */
>     }
>     checkmem("allocated");
> 
>     for (i=0;i<MAPS;i++)
>       if (hm[i])
>         free(hm[i]);
>     checkmem("freed    ");
> 
>   } while (!keypressed());
> 
> After this, you SOMETIMES cannot get a large memory block !
> And 'there it is' shouldn't appear at all, what if allocated with 'new' without
> a newhandler ? The program crashes...

I can understand this; your block sizes are random.  Initially 
then you get MAPS blocks of various sizes, then free them all.  
Now the allocatable memory is fragmented into whatever block 
sizes you used.  This means that if you malloc again with 
smaller block sizes then they'll fit in the old blocks, but 
when you use a larger block it won't fit, and a new chunk of 
DPMI memory must be allocated.  You could be wasting a huge 
amount of memory with these allocations which are effectively 
slowly increasing in size.  Allocate the largest possible size 
initially, and either leave it allocated (generally unused 
memory won't be paged in anyway) or free it if you must.

I don't think your example is very typical of a game program 
(which I presume is what you're writing) anyway.  I imagine it 
took quite a few iterations of the loop before the problem 
occured.  In any case, running your system close to its memory 
limit is not a good idea; set MAPS lower and you probably won't 
have the problem, even if you leave it running for a very long 
time.

-- 
george DOT foot AT merton DOT oxford DOT ac DOT uk