Re: Stay resident dll

Hector Santos <>
Wed, 23 Jun 2010 18:10:16 -0400
In principle, each thread that wishes to have its own GUI needs a
message queue processor to pump messages to the controls.

Every EXE has a main thread and every GUI EXE has a message queue
associated with the main thread to draw things.

So if you have secondary threads to do background processing, in my
design experience there were two safe ways to pass "data" that is
going to be drawn in the main thread GUI window:

1) Using SendMessage/PostMessage:

The secondary thread calls SendMessage() (send/wait) or PostMessage()
(send/no wait) to the Main thread Message Queue. Here you have to
create some "buffer" to pass via the Send/PostMessage().

If you call SendMessage() which is a SEND/WAIT for completion, you can
  pass a pointer to the local buffer and use it in the main thread
because it will still be "alive" when the main thread gets it.

If you call PostMessage() which is a SEND/NO WAIT, then here you are
queuing the message in main thread message queue to be processed when
the main thread message pump gets to it at some delayed time. So
here, you need to pass a new allocated buffer pointer that the
eventual main thread processing will release.

2) Using a common shared Data Structure.

Here you prepare a common (GLOBAL) data structure filled with fields
and information that:

    1) The secondary thread fills (WRITE)

    2) The main thread via a TIMER or a "WAKE UP" event will
       READ data structure to update "refresh" the screen.

Here you need to synchronize using locks (semaphores, mutexes, etc)
the WRITE/READ of the shared common resource (data structure) to
prevent potential clobbering of data.

For example, lets say you want to data points in the main form window.

       Counter1: ________ << member controls m_Counter1
       Counter2: ________ << member controls m_Counter2

That is updated per threads:

typedef struct _tagCommonData {
     int data1;
     int data2;

The easiest lock and guaranteed to be CORRECT and SAFE is a Critical
Section but it also the least efficient (speed wise) because its a
mutual exclusive READ or WRITE only operation, never two threads can
access it at the same time which is possible with a concept called
ReaderWriter Locks. ReaderWriter are easy if you get the proper class
from some library.

So you create a global data and critical section:

COMMONDATA globalData = {0};
CCriticalSection csLock();

Now in your secondary threads, your lock and write to the structure:



Now in your main program, you add a OnTimer() lets say for evert 100

       m_Counter1 = data1;
       m_Counter2 = data2;

You can add some efficiency ideas by only updating if dirty:

       if (m_Counter1 != globalData.data1)
           m_Counter1 = globalData.data1;

       if (m_Counter2 != globalData.data2)
           m_Counter2 = globalData.data2;

Instead of a timer, you can consider using a signal like a custom
registered WM_MY_REFRESH message that will call a OnMyRefresh()
message handler in your MyMainWindow to do the screen updating. This
makes it more dynamic if there aren't tons of updating going on.

A Reader/Writer lock allows you to add logic like a Writer Thread puts
a lock on ALL allowing only One writer at a time, but you can have
Multiple Readers as long as they are no active writers. So this an
more efficient than a Critical Section.

So the child threads will enable a Writer Lock:


and the main thread will use a Reader Lock:

       if (m_Counter1 != globalData.data1)
           m_Counter1 = globalData.data1;

       if (m_Counter2 != globalData.data2)
           m_Counter2 = globalData.data2;

The above reader/writer lock example is pseudo code, I don't see a MFC
based Reader/Writer class but there are many in the public domain.

Finally, if you want the child threads to have their own GUI, then the
same and only requirement is that the thread has own message pump.
The same "Inter Thread" communications apply here as with dealing with
the main thread only.


JRGlide wrote:

Thank you. You are right that MATLAB loads a DLL and then calls another DLL
which displays the data. In case it matters, the viewing program is an
application that I converted into a DLL, so it has its own CWinApp, menus,
CView, etc. In fact, you may have been the one who helped me develop it
several years ago.

My question about your approach is that I alway read on this forum that the
GUI should always be in the application and never in a thread - that only
processing should be done is a separate thread. So why is this different?

"David Ching" wrote:

It sounds like MATLAB loads your DLL and calls an exported function which
shows the 3D window. And when user closes the window, only then does the
exported function return to MABLAB. So MATLAB waits for your exported
function to return before allowing any more interaction with the main MATALB

If so, why don't you alter your exported function to create a new thread
that shows the 3D window and returns immediately? Then MATLAB and the 3D
window function simultaneously. The next time MATLAB calls your exported
function, you check if the thread/3D window already appear and pass the new
graph data to the window, if so.

-- David

"JRGlide" <> wrote in message

I currently have a MATLAB mex function that allows the user to view &
manipulate 3D point cloud data directly from MATLAB. A mex function is
nothing more than a standard dll with a specific MATLAB entry point. The
viewer was written in MFC using OpenGL.

My problem is that since it is a dll the user must close the viewer before
returning to MATLAB. In other words, the user calls the viewer from
looks at the data, closes the viewer and then returns to MATLAB.

I would like to find a way to keep the viewer up permanently so that it
works independently from MATLAB so they can both be run at once. What I
envision is that MATLAB would call another mex function (or dll) with the
data for display. This function would check to see if the viewer is
and activate it if necessary. It would then pass the data to the viewer
using some sort of handshake and then return to MATLAB without the viewer
disappearing at it does now. If the user changed the data it would make
same call and the dll would update the data in the viewer.

My question is that I???m not sure how to go about doing this:

1. I suspect that to do this, the viewer would need to be a regular
executable with some sort of interface and not a dll, but maybe I assume

2. It would first have to know if the viewer is loaded or not and the

3. I???m not sure what the handshake would be between the mex function and
the viewer. The data could potentially be hundreds of megs in size. Is
there a way I can do that through shared memory or some sort of global
pool? I???m trying to keep from writing the data to disk and read it back
again. Having said that, if I have to limit the data size to say, less
50 meg, I can live with that.

4. As far as closing the viewer, I guess that would be up to the user
they are finished, just like any other application.

Thank you for your help.


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