After the Specify path on diagram parameter is checked, a terminal named path in appears on the Call Library Function. This terminal accepts the path to the DLL that you want to call. When that Call Library Function is executed, the DLL specified in the reference is loaded into memory. See the figure below.
The LabVIEW 2017 Help: Call Library Function Dialog Box/ also states that the path in input can be used to unload a previously referenced library from memory. To do so, wire an empty or invalid path to the path in input and LabVIEW will unload any library previously referenced by the node from memory. In other words, you must wire an empty path constant or null path constant to the path in input of the same Call Library Function node that initially called your DLL. The picture below shows an example. The first call loads the DLL and the boolean unloads the DLL as well as stops this code. The false case, not shown, has timing but no code.
If an empty or invalid path is wired to a different Call Library Function node than the one that initially calledyour DLL it will not unload the DLL from memory. The picture below shows an example of what not to do.
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