Hello all, in my last post, I got the C callback working in LabVIEW. LabVIEW calls a function, which in turn calls PostLVUserEvent() to send data to an event structure in LabVIEW. However, this is not, what I want to do in my finished application. I want to register Event callbacks in my C dll, which execute when certain things happen (up to here, I got it working) and then use PostLVUserEvent() from those callbacks to tell LabVIEW something has happened. As the title says, I'm wondering how to pass the LVUserEventRef from the function, where I set up the callback to the callbacks. 1) store it in a global. I don't know if that's possible within a dll, but it's probably not a good idea to begin with 2) using the context parameter that the setcallback function provides. This could look something like this: void connlost(void *context, char *cause) { PostLVUserEvent(*context, <data>); } int init(LVUserEventRef *Ref) { setcallback(*Ref, cbfunction); } The function is defined like this: int MQTTClient_setCallbacks(MQTTClient handle, void* context, MQTTClient_connectionLost* cl, MQTTClient_messageArrived* ma, MQTTClient_deliverycomplete* dc) Documentation here. Is it possible to somehow pass the reference that way? Does a 3rd way exist, that I haven't thought of? Thanks in advance for any pointers on how to sort these pointers!

A good callback design usually allows to register a user data value (normally an opaque pointer value) together with the callback function. The function then is called with this user data value as one of its parameters. Looking at your code I suspect (but can't be sure) that the context parameter of the callback function is actually this user data value. And the first value to setCallback() where you pass in NULL might be the according parameter to pass in this context. So if you really just care about the user event refnum you could pass it in there (casting it to the void* pointer) and inside the callback routine you cast it back to a user event refnum and use it. Usually you tend to need more than just the refnum. In that case you have to allocate a memory area for the structure that will hold all the settings you want to pass into the callback function when registering the callback, and then deallocate that memory pointer when deregistering the callback.

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How to pass the LV Event refnum to a C callback function

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You can debug a DLL that is called from LabVIEW using Visual C. You just need to make a debug DLL build and configure the project to use the LabVIEW.exe program to start the debug session. Only drawback is that you need to exit LabVIEW whenever you want to fix some errors.

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How to pass the LV Event refnum to a C callback function

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