| Re: Can DLLs Multi-task? [message #32400 is a reply to message #32394] |
Thu, 03 October 2002 15:32   |
Peter Mason
Messages: 145
Registered: June 1996
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Senior Member |
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Here's my "string and glue" approach to this sort of problem. I haven't
done image-capturing like this in IDL, but I have done other threaded
programs.
The temperature-monitoring calls are probably okay as they are (fast?) but
the image-capturing call needs some work. From the sound of it, it is a
synchronous call. (Gianluca is basically asking about ways around
synchronous calls.) Some wrapper functions are required to make it run in
a separate thread so that it is asynchronous as far as your IDL program is
concerned. Briefly (assuming you'll be capturing many images): A call to
create a C-side thread, a call to trigger an image capture (which is then
done in the separate thread), possibly a status-polling call, possibly one
or two retrieve-capture calls, and finally a call to end the thread and
clean up once you're done with all your image capturing.
If the image-capturing call can be passed an array (allocated on the IDL
side with a statement like img=BYTARR(x,y)) into which to store the image
then life is relatively simple. All you need is some way for the C routine
to let the calling IDL program know when the image is ready. The simplest
approach is to write a little C routine that just reports whether or not the
image-capturing thread is busy (or something like that). (This is off the
main IDL thread, not the image-capture thread.) You'd then poll this off a
timer event on the IDL side. A more sophisticated approach is to issue an
event from the C image-capture thread to IDL when the image is ready, along
the lines that Rick has described. (I haven't used
IDL_WidgetStubIssueEvent() myself, but I have other, less elegant ways of
achieving this sort of thing.)
If the image-capturing insists on its own address for the image then things
could be a bit more complicated. Three possibilities come to mind. If
the image is always a fixed size you could allocate it up front in a "heap"
variable on the IDL side, pass this to the image-capture trigger call, and
just make the image-capture call copy the image across when it's ready.
The next approach is to have an "image size" C routine that you call from
IDL when the image is ready, allocate an array for the image in IDL, then
call an "image retrieve" C routine that simply copies the image to this
array. The slickest approach requires that you build a DLM rather than
just use CALL_EXTERNAL. Here, the "image-retrieve" C routine returns an
IDL array of the captured image. This is very easy to do with RSI's
IDL_ImportArray() C function.
The mechanics of threading on Win32 are relatively straightforward. The
hard parts are knowing what to thread (not a problem here) and retaining a
clear understanding of which thread might be accessing what memory when,
especially on multiprocessor computers (often a problem; nasty little
teeth, too). There are various ways to prevent memory-access conflicts
(e.g., Win32 critical sections, events, mutexes), but simply taking care
with your "logic flow" goes a long way towards avoiding them.
Cheers
Peter Mason
"David Fanning" <david@dfanning.com> wrote in message
news:MPG.1805ef78a52763df9899d1@news.frii.com...
> Folks,
>
> Here is a question from an IDL user who has no newsgroup
> access. He asked me if I would pass this question on
> to you:
>
> ;*************************************************
> I am developing an IDL program which have to call some external C
> routines (actually some DLL, because I work on Windows environment).
> I am wondering if it is possible in some ways to call more than one DLL
> at the same time, I.e. in multitasking, so telling to IDL not to wait
> for the DLL return.
>
> For example I need to launch a DLL to continuously monitor some
> temperature sensors, but in the meanwhile I have to run another DLL to
> read an image from a CCD device. I would need to have the temperature
> variables continuously updated by the first DLL so that I can read and
> display their values while getting the image from the CCD device.
> Is all that possible with IDL?
> (I have IDL 5.5)
>
>
> Thank you very very much for your valuable help,
>
> Gianluca Li Causi
> licausi@coma.mporzio.astro.it
> ;*************************************************
>
> Cheers,
>
> David
>
> --
> David W. Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Phone: 970-221-0438, E-mail: david@dfanning.com
> Coyote's Guide to IDL Programming: http://www.dfanning.com/
> Toll-Free IDL Book Orders: 1-888-461-0155
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