C++ Call to DLL made in LabVIEW Fails the second time it is called

Question

I have an application where a C++ executive calls a LabVIEW DLL (a LabVIEW application built into a DLL).
The executive calls and operates the DLL fine the first time (NT, the applications task manager shows LabVIEW runtime runs, then stops when the DLL is done. However, when the executive calls the DLL a second time, it hangs, and the runtime engine never shows up in the applications task manager as running.
However, this only occurs when something like IMAQ, DAQ, or another custom DLL call is occurring. We tried it with a simple DLL that only uses a call to the sound VIs, and it seems to run fine, although as the program called is very short, and the task manager slow, we never see the runtime engin
e in the task manager. We are going to put a popup dialog in to ensure that the DLL is being called reliably, but we believe it is.
We are unable to answer why when we call something that uses IMAQ, that the DLL doesn't run right the second time. We are unsure of the reasons, but believe it to be related to thread creation and destruction in the executive.
If anyone out there has any insight into this, we would be happy to see it.
We will be putting in a formal request for support from NI, but wanted to poll the audience as well, as we know from first hand experience that LabVIEW users often times have more experience with this sort of thing than developers.
Thanks a bunch

Answer 1

> Thanks for the information Greg. There is no documentation on this
> issue, except for the email that we got from NI Support. This is
> quite a new realm for us and NI alike. We would like to know what
> your experiences have been on this subject. If you have a bit of
> information, perhaps we should suggest a topic. As I said, this is a
> new frontier for LabVIEW, and there are a lot of things that NI
> doesn't know, and I'm sure a few of the programmers out there have
> figured out.
>
I often answer emails from home, but I work for NI on the LV development
team. So anything I know about, NI knows about. The info I was
describing about the DLL execution system is not well documented since
we were hoping that it would work we
ll with the way Windows Apps are
typically written. I suspect that we will need to write a Tech Note
to cover the nitty gritty. This sort of info gets lost in manuals,
and it is subject to change as we learn how people are expecting to
use it.
Anyway, my post was to explain why your use of it didn't work.
In general, we believe that LV DLLs are thread savvy, reentrant
VIs can be called from multiple threads simultaneously, execution can
continue in the background after the DLL call has returned, and the UI
is live provided the calling app processes messages in that thread.
If you have other issues or questions, just ask.
Greg McKaskle