Re: Application design for shared data repository programmed in VC on Win32

"Carl Daniel [VC++ MVP]" <>
Wed, 20 Sep 2006 15:13:33 -0700
"M Taha Masood" <> wrote in message

Hello folks,

This might be slightly offtopic but not very much so ,since the
development is to be done in VC for Win32 environment.

I have a requirement whereby there is an in-memory data repository
which should be housed in a certain binary module .

The access to this in-memory data repository is only through an API
exposed by the same binary module.The operations allowable are the
creation of new record , deletion of record , update of record, and
record search.

Moreoever this binary module should provide this API to any application
on the same machine and simultaneous access ( protected by some
synchronization primitive for data integrity ofcourse ) is also

Now i have a couple of ideas on how to implement this on Win32 ,but
would also like to hear about any advice or suggestions that you might

One of the ideas i am thinking of is along these lines. The binary in
which the data repository is housed ,should be a DLL , with exported
functions that provide the API for data access.

Any application which wants to use the data repository ,would load the
DLL in its address space. There should be exactly one copy of the data
( let us say for the sake of discussion for the time being , in a
statically allocated array of structures ) for as many applications
that have loaded the DLL. The data structure would be a global in the

-The memory is typically owned by the any process that loads the dll ,
so how do we make it sure that this data structure has memory which is
common to as many processes that have loaded the DLL.

How does this type of design sound , i would really appreciate any
comments and suggestions on this. Or any related best practices.

A couple of suggestions -

First, you can simply created a shared data segment in the DLL. I wouldn't
recommend this approach as you have to deal in the minutae of PE executable
memory layout, and there are potential issues that can make your shared
segement unreachable from some processes.

The more flexible technique is to use a memory mapped file to store your
shared memory. Note, however, that the address of your shared segment is
not guaranteed to be the same in all processes - you need to use
window-relative addresses for any "pointers" in your shared structure, and
ensure that the necessary offsets are applied/removed when manipulating
these structures.

Since you want access from multiple processes, you need to use cross-process
synchronization techniques, such as a Mutex for synchronization. A
CriticalSection can only synchronize access from within a single process.

Given all of the complications with maintaining a shared memory structure,
you should seriously consider a different design unless your performance
requirements demand this level of complexity. Most likely they don't. In
the vast majority of cases, having a single process that manages the shared
data and communicates through named pipes, sockets, or some other IPC
mechanism is a cleaner, easier and more maintainable solution.


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