OPC DA client ItemMgt Incorrect function error - c++

Question

I have created a simple OPC DA client for the C++ COM API in Qt5.
The client connects to the remote server, gets an OPCServer pointer, creates a new OPC group with an ItemMgt interface, and fails when I try to add items to the group.
The error message is: Incorrect function.
As far as I can see, the IUnknown:: QueryInterface works for this pItemMgt, but the ValidateItems, CreateEnumerator and AddItems calls results in the same Incorrect function error. The OPC server is a QMS220Simulator (Quadera).
Any idea what could be the problem?
This is my first attempt to write a DCOM client, so many, many thing could be wrong with this code.
The qms220.h file contains the CLSID for the QMS220Simulator.
The shortest code to reproduce the problem is this:
#include "opcda.h"
#include "qms220.h"
#include <QApplication>
#include <QDebug>
#include <comdef.h>
static void showStatus(const QString &message,HRESULT code);
IOPCServer *pOPCServer = nullptr;
IOPCItemMgt *pItemMgt = nullptr;
OPCHANDLE serverGroupHandle;
bool initializeCOM()
{
HRESULT hr = CoInitializeEx(nullptr,COINIT_APARTMENTTHREADED);
if (FAILED(hr)) {
showStatus("COM initialization failed!",hr);
return false;
}
hr = CoInitializeSecurity(
NULL, //security descriptor
-1, //COM authentication
NULL, //authentication services
NULL, //reserved
RPC_C_AUTHN_LEVEL_DEFAULT, //default authentication
RPC_C_IMP_LEVEL_IMPERSONATE, //default impersonation
NULL, //authentication info
EOAC_NONE, //additional capabilities
NULL //reserved
);
if (hr == RPC_E_TOO_LATE) {
showStatus("RPC initalization is too late, ignoring...",hr);
} else {
if (FAILED(hr)) {
showStatus("CoInitializeSecurity",hr);
return false;
}
}
return true;
}
void deinitializeCOM()
{
CoUninitialize();
}
static const int INTERFACE_COUNT = 1;
bool connectToServer(const QString &address)
{
_bstr_t serverName = address.toStdString().c_str();
COSERVERINFO cs;
memset(&cs,0,sizeof(cs));
cs.pwszName = serverName;
MULTI_QI qi[INTERFACE_COUNT];
memset(qi,0,sizeof(qi));
qi[0].pIID = &IID_IOPCServer;
HRESULT hr = CoCreateInstanceEx(
CLSID_QMG220SIMDA,
NULL,
CLSCTX_SERVER,
&cs,
INTERFACE_COUNT,
qi
);
if (FAILED(hr)) {
showStatus("CoCreateInstanceEx",hr);
return false;
}
pOPCServer = (IOPCServer*)(qi[0].pItf);
return true;
}
void disconnectFromServer()
{
if (pOPCServer != nullptr) {
pOPCServer->Release();
pOPCServer = nullptr;
}
}
void showOPCStatus(const QString &message,HRESULT hr)
{
if (pOPCServer != nullptr) {
LPWSTR buffer = nullptr;
HRESULT hr2 = pOPCServer->GetErrorString(hr,LOCALE_SYSTEM_DEFAULT,&buffer);
if (hr2 != S_OK) {
qDebug() << message << QString(": HRESULT: 0x%1").arg(hr,8,16,QChar('0'));
} else {
qDebug() << message << QString(": ") << QString::fromWCharArray(buffer);
CoTaskMemFree(buffer);
}
} else {
qDebug() << message << QString(": HRESULT: 0x%1").arg(hr,8,16,QChar('0'));
}
}
static const LPCWSTR MIDGROUPNAME = L"mid";
bool createMIDGroup()
{
if (pOPCServer == nullptr) return false;
OPCHANDLE clientGroupHandle = 1;
DWORD revisedUpdateRate;
HRESULT hr = pOPCServer->AddGroup(
MIDGROUPNAME,
FALSE, //active
0, // requestedUpdateRate
clientGroupHandle,
NULL, //timebias
NULL, //percentDeadBand,
LOCALE_SYSTEM_DEFAULT, //lcid
&serverGroupHandle,
&revisedUpdateRate,
IID_IOPCItemMgt,
(LPUNKNOWN *)(&pItemMgt)
);
showOPCStatus("OPCServer::AddGroup",hr);
if (hr != S_OK) return false;
qDebug() << "The server group handle is: " << QString("0x%1").arg(serverGroupHandle,4,16);
qDebug() << "The revised update rate is: " << revisedUpdateRate;
#define ITEM_ID L"Hardware.Modules.Analyser.SI220.SimulationMode"
QString accessPath("");
QString itemId("Hardware.Modules.Analyser.SI220.SimulationMode");
wchar_t accessPathBuffer[1024];
wchar_t itemIdBuffer[1024];
accessPath.toWCharArray(accessPathBuffer);
itemId.toWCharArray(itemIdBuffer);
static const int ITEM_COUNT = 1;
OPCITEMDEF ItemArray[ITEM_COUNT] =
{{
/*szAccessPath*/ accessPathBuffer,
/*szItemID*/ itemIdBuffer,
/*bActive*/ FALSE,
/*hClient*/ 1,
/*dwBlobSize*/ 0,
/*pBlob*/ NULL,
/*vtRequestedDataType*/ VT_UI1,
/*wReserved*/0
}};
OPCITEMRESULT *itemResults = nullptr;
HRESULT *errors = nullptr;
hr = pItemMgt->AddItems(ITEM_COUNT,ItemArray,&itemResults,&errors);
bool failed = false;
if (hr != S_OK) {
failed = true;
}
showOPCStatus("createMidGroup/AddItems ",hr);
for(DWORD k=0;k<ITEM_COUNT;k++) {
showOPCStatus(QString("createMidGroup/AddItems[%1]").arg(k),errors[k]);
if (errors[k] != S_OK) {
failed = true;
}
CoTaskMemFree(itemResults[k].pBlob);
}
CoTaskMemFree(itemResults);
CoTaskMemFree(errors);
return !failed;
}
void removeMIDGroup()
{
if (pOPCServer != nullptr) {
if (pItemMgt != nullptr) {
pItemMgt->Release();
pItemMgt = nullptr;
}
HRESULT hr = pOPCServer->RemoveGroup(serverGroupHandle,false);
if (hr != S_OK) {
showStatus("deleteMIDGroup",hr);
}
}
}
int main(int argc, char *argv[])
{
Q_UNUSED(argc)
Q_UNUSED(argv)
if (!initializeCOM()) return -1;
if (connectToServer(QString("192.168.12.106"))) {
if (createMIDGroup()) {
removeMIDGroup();
}
disconnectFromServer();
}
deinitializeCOM();
return 0;
}
static void showStatus(const QString &message,HRESULT code)
{
_com_error error(code);
qDebug() << message + QString(": " ) + QString::fromWCharArray(error.ErrorMessage());
}

Answer 1

So, according to the Qt documentation: https://doc.qt.io/qt-5/qstring.html#toWCharArray
The toWCharArray creates a NOT NULL CHAR TERMINATED unicode string.
So a bit better usage would be:
int size = itemId.toWCharArray(itemIdBuffer);
itemIdBuffer[size] = L'\0';
And the same for accessPathBuffer.
Probably it's not a really good idea to send an unterminated group name to the OPC server.
The good thing is, that the CreateGroupEnumerator sends back the same unterminated group names as received.
So the problem is not COM nor OPC related, it's just not reading the fine documentation.