Pointer to LabVIEW Array

Question

LabVIEW memory management question:
Question Part 1: When I write a C .dll that takes a LabVIEW array,
LabVIEW passes a pointer into LabVIEW's memory to the array to C. 
Can C return the pointer to me (e.g., not the array I passed in, but
it's memory location)?
i.e.: void GetNativeMemoryPointer(float *pointer, float *array);
where *array is my input array, and GetNativeMemoryPointer returns the pointer to this array on *pointer.
Question Part 2: What issues will I run in to if I pass this returned
pointer to another .dll call.  What if this .dll call is in
.NET? 
Concerns: How does LabVIEW hold this array in memory?  When I call
GetNativeMemoryPointer, does LabVIEW make a copy and send this a
pointer to the copy to the dll, or is the pointer to the
original?  Will LabVIEW ever "move" the original, e.g., will my
pointer become invalid after some time?
Joe Gerhardstein
Averna Test Systems
Certified LabVIEW Architect
http://www.averna.com

Answer 1

Part 1: I must confess that I don't understand your question. Pointers and arrays are very closely related in C so I'm not following the sequence. So I don't know if the following applies, but here is a possible answer...
The pointer to a LV array that is passed to a call library node is valid ONLY during that call. Once the call to the DLL completes, you should consider that pointer value invalid. You can, of course, make a copy of the array and return a pointer to your copy - but LV won't know what it is. It definately won't show up in LV as an array. However, you can pass that pointer to the copy around to other DLLs as much as you want.
* Don't forget to free the copy. If you make a copy of the array, you own it - therefore you must free the memory of the copy at some point *
Part 2: Easier to answer.
* If you want LV to convert your pointer to a LV data type (such as a LV array): Take a look at page 2-4 in the "Using External Code in LabVIEW". In almost all cases we do not support pointers. The only exceptions are the ones shown in the Call Library Node configuration. For arrays, notice that if you want to return an array to LV, we have our own defined data types for this - read up on the CIN section for more information.
* If you want to return a pointer than LV just messages to other DLLs: No problem. Put the parameter to the routine as a pointer to integer (or a return value of type int). Thus your pointer is an integer as far as LV is concerned. When you pass it to another DLL, we'll pass the exact same value to the DLL - thus the pointer is never modified or the data touched.
Did this help?
Message Edited by Lycangeek on 08-10-2005 02:12 PM
Brian Tyler
http://detritus.blogs.com/lycangeek