Array of doubles filled by DLL contains weird values

Question

Hello everyone
I am working on a DLL invocation which needs to resize and fill an array of doubles named 'values' (2nd to last parameter). I have defined the signature as follows:
_declspec (dllexport) uint8_t readDoubleArray(uint8_t handle, LStrHandle lPath, int64_t startTime,
int64_t endTime, DblArrHdl *values, LongArrHdl *timebase);
Where DblArrHdl is
typedef struct {
int32_t dimSize;
double doubles[1];
} DblArr;
typedef DblArr **DblArrHdl;
In the invocation I read doubles from a database, and resize the double array as follows:
int size = doubleArray.size();
if (NumericArrayResize(0x0A, 1, (UHandle*)values, size) != noErr) {
setLastError(MG_ERR_LABVIEW_ERROR, "Failed to allocate memory");
return FALSE;
 I set the size, and copy the values:
(**values)->dimSize = size;
for (int i = 0; i < size; i++) {
(**values)->doubles[i] = doubleArray[i];
When I use the DbgPrintF to validate the values it gives me a value around 120000 which is correct. However, when returned by labview the array contains strange data, like 5.38564E-315. Here is print out of the 'values' array after it has been through the DLL:
Here is how I wired it, ignore the clustering, it occurs in the array of values which exits the DLL call.
I have no idea how this can be. I followed the examples on the site. I also tried the normal DSSetHandleSize, but it didn't change anything. Any idea what I did wrong?
Thanks in advance,
Vincent
Message Edited by DaVince on 10-22-2008 10:10 AM
Solved!
Go to Solution.
Attachments:
doubleArrayWeird.png ‏7 KB
wireUpDoubles.png ‏18 KB

Answer 1

You're right, this not directly described...  
First you should read how LabVIEW stores data in memory,
then read about structure packing pragma,
and finally take a look to C/C++ preprocessor reference,
then you will get feeling how to solve this problem...
Otherwise own DLLs used in LabVIEW code pretty rarely, and Double precision also not very often...
The same trouble you may get with clusters, but here behaviour described:
"...The memory layout of a cluster depends on the platform you are running. LabVIEW may add padding between cluster elements so that those elements are aligned to certain address boundaries. Address boundaries relate to the concept of natural alignment. A piece of data is aligned on its natural boundary if the address at which it begins is a multiple of the size of the data. For example, a 4-byte integer is aligned naturally if it begins at an address which is a multiple of four. Most compilers have an upper limit on this boundary at which the restriction is relaxed. For instance, an 8-byte integer may be aligned on a 4-byte boundary. For the platforms on which LabVIEW runs, the alignment constraints are as follows:
(Windows) Data is aligned only to 1-byte boundaries.
(Mac OS and Linux) Data is aligned naturally up to 4-byte boundaries.
(VxWorks PowerPC) Data is aligned naturally up to 8-byte boundaries.
On all platforms, clusters adopt the alignment of their most restrictively aligned element..."
best regards and happy wiring...
Andrey.
Message Edited by Andrey Dmitriev on 10-23-2008 03:38 PM