How pass VISA resource string from C++ as uintptr_t* to a LabVIEW generated DLL ?

Question

My coworker build a DLL from LabVIEW that I am trying to call from C++.
void__cdeclMeasureDCvolts(uintptr_t*VISAResourceName, int32_tChannelNumber,LVBoolean*MonitorResult,double*MeasuredVoltage, TD1*errorOut);
Does "extcode.h" define macros or functions for string conversions?  Or should I request my coworker rebuild the VI's to make arguments more C/C++ friendly?
Thanks in advance for any tips or direction.  Examples are * much * appreciated.
-Ed
TestObjectMeasurement* testObjectMeasurement = m_measurementMap[name];
TestObjectInstrument* testObjectInstrument = m_instrumentMap[selectedInstrumentName];
int slot = 1;
int bank = 1;
int channel = 1;
int channelNumber = 1000 * slot + channel;
char * visaResourceName = testObjectInstrument->visaResourceName().toLatin1().data();
LVBoolean monitorResult = LVBooleanFalse;
double measuredVoltage = -1.0;
TD1 errorOut = {0};
try
// void __cdecl MeasureDCvolts(uintptr_t *VISAResourceName,
// int32_t ChannelNumber, LVBoolean *MonitorResult, double *MeasuredVoltage,
// TD1 *errorOut);
MeasureDCvolts(reinterpret_cast<uintptr_t*>(visaResourceName), channelNumber, &monitorResult, &measuredVoltage, &errorOut);
catch(...)
qDebug() << "Exception thrown by: MeasureDCvolts";
QString errorSource = QString::fromUtf8(reinterpret_cast<const char*>(LHStrBuf(errorOut.source)), LHStrLen(errorOut.source));
qDebug() << errorSource;
LStrHandle errorTextHandle = {0};
bool foundErrorText = NIGetOneErrorCode(errorOut.code, &errorTextHandle);
if(foundErrorText)
QString errorText = QString::fromUtf8(reinterpret_cast<const char*>(LHStrBuf(errorTextHandle)), LHStrLen(errorTextHandle));
MessageHelper::messageError(errorText, errorSource);

Answer 1

Why is it not a solution? It was exactly about what you wanted to do and the solution in the last post is how it can be made to work. There is NO way to ddirectly create a LabVIEW native VISA resource in C code that is not called from within LabVIEW, unless you consider hacking the LabVIEW system and using undocumented functions a solution. But that has very good chances of breaking with future LabVIEW versions.
Your only other option is to export another function from the DLL that accepts a String and uses VISA Open to explicitedly open the VISA session in your LabVIEW DLL and returning that VISA session to your C code. This is functionally equivalent to passing a String to your current function but avoids the lookup and potential recreation of the VISA refnum on each call. Something I would only feel bothered if you intend to call this function many thousend times each second, which for VISA communication seems very unlikely.
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions