Hi Jonathan,

assuming that you are talking about a 32 bit process here, you only have
(on XP) 2 GB of virtual memory accessible in user mode. Even if you have
more physical RAM and plenty of paging file space, your process will not
be able to use more than 2GB of VM.

Your modules, stacks and heaps are sprinkled throughout these 2 GB of
VM, so you have to assume that the 2 GB are already quite a bit
fragmented. Given these constraints, it is pretty obvious that
allocating 500 MB of contiguous VM is very likely to fail.

So first you should ask yourself if you really need such large blocks of
contiguous virtual memory or if you can split them into chains of
multiple, much smaller sized blocks (of which maybe you do not even need
all in memory at once).

Finally, if you cannot avoid using large contiguous blocks of VM,
consider reserving a fair amount of VM early in the lifecycle of your
process and commit parts of it as you need them.

--Johannes