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LabVIEW
Need Example Code to Interface with a USB HID compliant Device
If it gets unstable with mcHID that would point to one of two possible reasons:
1) You made some mistake in configuring the Call Library Node. The more trivial mistakes are datatype mismatches of function parameters. The most common mistake however is to not initialize output buffer parameters (to a big enough size) so the DLL corrupts memory when trying to return data in them.
2) The DLL itself does unruly things somewhere that fell through the cracks during testing but make themselves apparent in LabVIEW, since LabVIEW is typically a bigger application so an unruly memory access easily can destroy something in memory that sooner or later lets LabVIEW trip over its feet.
@MichaGue_01 wrote:
Hi Rolf,
that sounds interesting.
>> The most common mistake however is to not initialize output buffer parameters (to a big enough size) so the DLL corrupts memory when trying to return data in them.
How do I initialize buffer for the dll?
Well assume you have a function like this:
int read_data(HANDLE handle, char *buf, int len);
The casual LabVIEW developer is usually tempted to just declare that function in the Call Library Node and pass it an empty array of U8 elements as second parameter since in LabVIEW you don't usually worry about the correct array size preallocation at all. But for C this is totally bad and will not work.
Instead you need to preallocate an array big enough for the function to fill in its data. In this case the caller would tell the function how big the array is with the third parameter. So use an Initialize Array node to allocate an array with len elements and pass this to the second parameter, passing the len number in the third.
Alternatively you can also just pass in a useful len value and configure the second parameter in the Call Library Node to have a minimum size equal or bigger to the len parameter. But I prefer the explicit Initialize Array allocation as it is directly clear to the reviewer that the array allocation has been taken care off and how, without the need to open the Call Library Node configuration dialog.
However your mcHID.dll seems to expect to communicate with the host application through its main window message loop. That is not so trivial to do in LabVIEW although there are ways to do that with an example called Windows Message Queue Library. But its implementation in an application is rather complicated and it doesn't work for every possible message type. I'm not sure how tolerant mcHID is about being passed an invalid NULL host window.
HIDAPI might be easier to interface as long as you do not want to access the device enumeration itself. The enumeration requires handling of structures with memory pointers, which is most easily done in C and which would make me create a C wrapper.
