Date: Tue, 6 Feb 1996 07:21:30 +0200 (IST)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: John Beale <beale AT leland DOT Stanford DOT EDU>
Cc: djgpp AT delorie DOT com
Subject: Re: malloc/free ends up fragmenting dynamic memory?


On 5 Feb 1996, John Beale wrote:

> I am guessing that the many malloc/free operations fragments the memory
> store analgous to hard disk filesystem fragmentation with many saves and
> deletes, such that a contiguous memory segment for a large malloc() can no
> longer be found. Is there a malloc_unfragment() command or equivalent?  How
> do other people handle this problem?

This is a problem with many implementations of `malloc', the one from 
DJGPP library included.  The usual method of solving such problems in 
programs that allocate and free many small arrays, is to allocate a large 
buffer of memory and use parts of it as needed instead of calling 
`malloc' and `free'.

One piece of knowledge that might help you: the DJGPP `malloc' always
allocates blocks whose sizes are an integer power of 2; if you ask for
32KB, you get 64KB (because of the 4-byte overhead of `malloc' itself). 
So you might make your problems easier if you never get too close to sizes
of the form 2^n for any integer n.

Another possibility, in the case that your allocations only last as long 
as the function which allocated them, is to use `alloca' instead (which 
allocates the memory from the stack).  This way, you don't have the 
fragmentation problem, and also are spared the need to call `free'.