Re: Trying to use MemoryMappings but having a simple problem...

From: Rob Kennedy (me3_at_privacy.net)
Date: 01/05/05 

Date: Wed, 05 Jan 2005 10:30:39 -0600

jklimek@gmail.com wrote:
> 1) Will each process that attachs to the DLL and then gets a
> MappingHandle (to the shared memory aka file mapping) have the same
> handle? (eg. will IntToStr(MappingHandle)) produce the same integer?

Probably not. That is, the handles _might_ have the same value, but you
should consider it nothing more than a coincidence when it happens. By
the way, the same applies to the hook handle.

To pass a handle to another process, you need to use the DuplicateHandle
API function. It allows you to generate a handle that will be valid in
another process. (It won't be valid in the process that creates it.)

But you don't really need to do that for mapping handles since each
interested process can simply open the file mapping by name. Be sure to
read the documentation carefully about choosing unique names accessible
to everything that's supposed to have access to it. The rules differ by
OS version.

> 2) Same as the above, but will each pointer (eg. HookInformation =
> MapViewOfFile()) have the same address?

Nope.

You can _ask_ for the file to be mapped to a particular address, but
there's no guarantee that that's the address you'll get. It all depends
on what addresses are already reserved.

When you want to send another program an address in the memory-mapped
file, you should instead send just the offset into the file -- pass the
address releative to the file's base address returned from
MapViewOfFile. The other process can then add that offset to the address
MapViewOfFile returns in _that_ process. Memory-mapped files allow you
to share memory, but not address space. Each process's address space is
entirely its own. 

-- 
Rob

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