Mail Archives: djgpp/1998/04/23/00:16:11
On Mon, 20 Apr 1998 22:33:59 +0900 in comp.os.msdos.djgpp SangGyu Nam
<halite AT medison DOT co DOT kr> wrote:
: I'm making an image handling program using DPMI.
...
: Whenever an image displayed, I traced the free amount of
: free memory by using DPMI function 0x0500.
: The function returns correct value when I allocate memory
: but after free it, I could not get valid information.
: Only the amount before free() was returned.
...
: In other words, free() does not return memory to DPMI
: automatically. So if I want to get correct information,
...
: How can I get the realtime free amount of DPMI memory?
I think you're chasing pigeons. When `free' is called, the block is
then available for `malloc' to allocate -- so the memory is not lost.
That block can only be used for a whole allocation, though; it can't
put part of an allocation in that block, and the rest somewhere else.
If you allocate a large block, and no previously freed blocks are big
enough for the allocation, it will have to come from the pooled DPMI
memory. So you should restrict your allocations to that amount
anyway.
There isn't really a way to get an accurate figure for how much memory
you'll be able to use. The DPMI server can change its mind about how
much it will give you at any time; if you ask it how much free
physical memory there is, and then allocate it all (and it lets you),
then ask again, it might choose to offer you some more memory. Don't
forget that most DPMI servers also provide virtual memory, so the
chances of physically running out are quite small really, unless you
go overboard.
However, different DPMI servers respond to different requests in
different ways. Just because they say there's X megabytes of free
memory, that doesn't mean you can allocate it all. Sometimes they
won't let you allocate Y bytes but they will let you allocate two
blocks of Y/2 bytes.
The only reliable way to find out whether you'll be allowed to create
some complex structure is to actually try to create it, and watch out
for failed mallocs. If you organise the allocation function properly
you should be able to catch this and exit the function cleanly,
freeing up all the blocks you've already allocated which can't be used
now that later blocks could not be allocated.
You could of course try to allocate a large block, then subdivide that
for further allocations; that should always work, if you can allocate
the block. Note what I said above, though, about some DPMI servers
not letting you allocate very large chunks all at once.
--
george DOT foot AT merton DOT oxford DOT ac DOT uk
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