Calling LabVIEW DLL from C with 2d array strings

Question

I am trying to write a DLL in C that calls a LabVIEW DLL that has a
number of functions that each return a 2D array of strings which are the formatted
measurements, with column 1 being the measurement name and column 2
being the measurement. I wanted to use the 2D array of strings since
this would give a uniform interface for returning measurements. Ive
cobbled together bits and pieces of messages and example code I've
found here into something that almost works, and was hoping someone
with a little more experience in interfacing C with LabVIEW DLLs would
be able to help me.
Here's a little
snippet from the header file generated when building the LabVIEW DLL:
typedef struct {
long dimSizes[2];
LStrHandle String[1];
} TD1;
typedef TD1 **TD1Hdl;
long __cdecl TestVI(char serialNumber[],  TD1Hdl *formattedMeasurements, char ErrorDescription[], long len);
The "TestVI" VI generates a 2D array of strings. The cell contents and number of rows are random to
simulate different measurement names and measurements.
Here's a snippet for the wrapper class:
class CArgoTestsLabVIEW 
public:
CArgoTestsLabVIEW();
virtual ~CArgoTestsLabVIEW();
// test modules called from parent DLL
BOOL GetResult_LV(CString resultName, double *result);
long TestVI_LV(CString serialNumber);
private:
TD1Hdl m_LVStringArray;
CMapStringToString m_resultMap;
InitMeasurementsArray();
PopulateResultMap();
The "m_LVStringArray" is intended to hold the 2D array of strings that
the test modules spit out. The "m_resultMap" is just for accessing the
measurements from the parent DLL via the "GetResult_LV" function.
The implementation of TestVI_LV is:
long CArgoTestsLabVIEW::TestVI_LV(CString serialNumber)
long rc = 0;
char errStr[BUFFERLEN];
strcpy(errStr,"");
rc = InitMeasurementsArray();
if (rc == 0) {
rc = TestVI(serialNumber.GetBuffer(serialNumber.GetLength()), &m_LVStringArray, errStr, BUFFERLEN);
PopulateResultMap();
return rc;
The accessor functions InitMeasurementsArray and PopulateResultMap are:
long CArgoTestsLabVIEW::InitMeasurementsArray()
MgErr err;
if (m_LVStringArray)
DSDisposeHandle(m_LVStringArray);
m_LVStringArray = (TD1Hdl)DSNewHandle(sizeof(TD1));
// code to check for NULL
// Set number of strings in array to 0
(*m_LVStringArray)->dimSizes[0] = 0;
(*m_LVStringArray)->dimSizes[1] = 0;
// Set total size of array structure. For now it is only as big as the long variable.
err = DSSetHandleSize((UHandle)m_LVStringArray, sizeof(long));
// code to check for error
return 0;
void CArgoTestsLabVIEW::PopulateResultMap()
int numberOfResults;
int i;
int charCnt;
numberOfResults = (*m_LVStringArray)->dimSizes[0];
if (numberOfResults == 0) {
return;
m_resultMap.RemoveAll();
for (i = 0; i < numberOfResults; i++) {
charCnt = (*((*m_LVStringArray)->String[i * 2]))->cnt;
(*((*m_LVStringArray)->String[i * 2]))->str[charCnt] = '\0';
charCnt = (*((*m_LVStringArray)->String[i * 2 + 1]))->cnt;
(*((*m_LVStringArray)->String[i * 2 + 1]))->str[charCnt] = '\0';
m_resultMap.SetAt((LPCTSTR) (*((*m_LVStringArray)->String[i * 2]))->str, (LPCTSTR) (*((*m_LVStringArray)->String[i * 2 + 1]))->str);
The problem I have is that I eventually get an access violation from
the run-time engine if I try to call the test VI in a loop. I'm
obviously having a memory management problem, but it escapes me as to
where the problem is. Anybody have any ideas?

Answer 1

smercurio_fc wrote:
The problem I have is that I eventually get an access violation from the run-time engine if I try to call the test VI in a loop. I'm obviously having a memory management problem, but it escapes me as to where the problem is. Anybody have any ideas?
(*((*m_LVStringArray)->String[i * 2]))->str[charCnt] = '\0';
Hi smercurio,
      Have you solved this?  I don't work with C much anymore, but it looks, here, as if the memory at str[charCnt] isn't yours to address/change.
Is index "charCnt" one char past the end of the String-memory allocated by LabVIEW? 
just an idea!
Hmmm, maybe page 9 is a bit far back to fish in the unanswered posts.
Message Edited by Dynamik on 03-03-2006 02:03 AM
When they give imbeciles handicap-parking, I won't have so far to walk!