LabView VB6 Dll interface causes memory leak


We use a large written in vb6 in our laboratory measurement built house software. Recently, we have added a new detector that is read using a labview vi that is called using a dll directly from vb6. Here, we call the function for example of read_detector which starts the collection of data and in a next step writes the data to a string. When you have finished the called function returns a pointer to the string and its length. VB6, we read this pointer and process data.

This is a memory leak. This could be the case because we presets not memory for labview to write in and so just writing lab data in memory without releasing again? In the current configuration, the function of the dll is just called again and again and each lab view creates a variable outputstring.

If we allocate the memory. How can we achieve for vb6?



Well first of all, a handful of string LabVIEW is a very specific type of data LabVIEW. Not only is only LabVIEW himself knows exactly how to allocate and deallocate, but you must also allocate and deallocate in exactly the same instance of LabVIEW. Given that your LabVIEW DLL is executed for this purpose inside the DLL to run LabVIEW, you must also recover the LabVIEW memory manager functions according to DSDisposeHandle() of the lvrt.dll and call it after you are finished with the strings returned by the DLL function. This is however made very complicated since there are circumstances where not lvrt.dll is actually used by your DLL but another LabVIEW runtime system. And if you happen to have two DLLs created with different versions of LabVIEW LabVIEW, each of them will load and use its enforcement LabVIEW system according to DLL load next. Believe me, you don't want to know how next dll works and how to make this opportunity to work in an application. This is the way for Microsoft to create a monster to manage the hell of the DLL and in this way, rather to create exponential version of hell.

You create just two memory leaks for each call of DLL function at the moment. LabVIEW creates a new handle of string for each of the parameters string at each call, you dereference the pointer, and then copy the contents into a SysString. But you let the chain handles LabVIEW blow created around the stack and the next your function call, you initialize the new variables of the battery which is initialized to null and LabVIEW creates two new handles. Also you need to do is to define an "Integer DSDisposeHandle(LStrHandle As Long)" exported lvrt.dll function and call it once you have copied the data of the handle in your SysString for both of these handles.

But basically calling a DLL LabVIEW from no LabVIEW code and using LabVIEW handles string or array as parameters is several times more trouble that it's worth. The easiest would be fair, allocate a large enough buffer beforehand and pass those to the Configuration of the service DLL LabVIEW function to not accept a handful of string LabVIEW but a pointer to a string C instead. Do you work around the problem on the memory allocated by someone else to worry about your own program of VB and duty he properly release. The disadvantage of this approach is, that you should make sure that the memory allocated buffer is large enough for the maximum possible length that your DLL function may never return.

But another approach might be to leave out LabVIEW total here and call the DAQmx APIs directly from VB.

Tags: NI Software

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