Use the reference number controls located on the Refnum and Classic Refnum palettes to work with files, directories, devices, and network connections. Use the control refnum to pass front panel object information to subVIs.
A reference number, or refnum, is a unique identifier for an object, such as a file, device, or network connection. When you open a file, device, or network connection, LabVIEW creates a refnum associated with that file, device, or network connection. All operations you perform on open files, devices, or network connections use the refnums to identify each object. Use a refnum control to pass a refnum into or out of a VI. For example, use a refnum control to modify the contents of the file that a refnum is referencing without closing and reopening the file.
Because a refnum is a temporary pointer to an open object, it is valid only for the period during which the object is open. If you close the object, LabVIEW disassociates the refnum from the object, and the refnum becomes obsolete. If you open the object again, LabVIEW creates a new refnum that is different from the first refnum. LabVIEW allocates memory for an object that is associated with a refnum. Close the refnum to release the object from memory.
LabVIEW remembers information associated with each refnum, such as the current location for reading from or writing to the object and the degree of user access, so you can perform concurrent but independent operations on a single object. If a VI opens an object multiple times, each open operation returns a different refnum. LabVIEW automatically closes refnums for you when a VI finishes running, but it is a good programming practice to close refnums as soon as you are finished with them to most efficiently use memory and other resources. Close refnums in the opposite order that you opened them. For example, if you obtain a refnum to object A and invoke a method on object A to obtain a refnum to object B, close the refnum to object B first and then close the refnum to object A.
If you open a refnum inside a For Loop or While Loop, close that refnum for each iteration of the loop because LabVIEW repeatedly allocates memory for the refnum and does not free the memory until the VI finishes running.