How to receive WM_USER messages from other applicatio​ns in LabVIEW

Question

I'm trying to receive WM_User messages from LV and from non-LV applications. Using the VIs from NIs Windows Message Queue.llb
it seems to work with standard WM_messages only (e.g. 0x0200 WM_MOUSEMOVE). What I'm looking for is a way to receive
programatically sent messages (by SendMessageA from user32.dll) in LabVIEW.
Are there any ideas or hints?
Greetings
Steffen

Answer 1

Odd_Modem wrote:
I got it to work with
PostMessageA, message 0x8000. It works with occurrence drive or polling
drive. I used U32 for all the function inputs. I used the original dll
and LabVIEW 7.1.1. This, at least, shows that it works between two
windows in the same process.
Stupid me! Of course!
SendMessage waits for the message to be handled and the return value of
the message be provided by the target window before returning.
Apparently Windows has some protection built into SendMessage to avoid
deadlocks by checking if the target window message loop runs in the
same thread as the one calling SendMessage and in that case reverts
possibly to PostMessage for known WM_xxx messages and just simply
returns for unknown messages. If it wouldn't do that you would end up
blocking in SendMessage for indefinite time as it waits for the message
to be handled which never will happen since the thread handling the
message loop is blocking on SendMessage.
PostMessage always places the message in the windows message queue and
returns immediately avoiding possible deadlocks in this way.
Rolf Kalbermatter
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions