MEX: C++ function doesn't find DLL when called from Matlab using mexFunction

I have to call functions of a 32-Bit-DLL from Matlab to control a positioner system for Antenna Measurements. Since I can’t import the DLL functions properly using the “loadlibrary” command (some forum contributions suggest because 32-Bit DLLs won’t work with 64-Bit Matlab), I want to write a C++ controller class that loads the DLL once at instantiation and then through its methods acts as an interface to Matlab. For that purpose I modified the class wrapper that can be found on Mathworks File Exchange platform ( https://de.mathworks.com/matlabcentral/fileexchange/38964-example-matlab-class-wrapper-for-a-c++-class ). The C++ class I've implemented loads the 32-Bit DLL, using the LoadLibrary() command. Problem:

• I tested the functionality of this C++ controller class by writing a short console application in Microsoft Visual Studio 2017, and it work's fine - in particular, the DLL is loaded properly.
• When I add the class wrapper, compile it with MEX (compiler: Microsoft Windows SDK 7.1 (C++)) and call the class method that loads the DLL, the program principally works fine, too (issues no error) - but it doesn't find the DLL.

My C++ class PositionerController has a loadCanApiDll function, which calls LoadLibrary to load the DLL:

// Load CanApi DLL
bool PositionerController::loadCanApiDll(LPCTSTR dllPath)
{
  hCanApi = LoadLibrary(dllPath);
  if (hCanApi != NULL)
  {
    return TRUE;
  }
  else
  {
    return FALSE;
  }
}

In the mexFunction, the handle held in the corresponding Matlab class is used to find the C++ object. Then, the loadCanApiDll function is called from the mexFunction on the previously found PositionerController instance, posCtrl:

// Get class instance pointer
PositionerController *posCtrl = convertMat2Ptr<PositionerController>(prhs[1]);

// Load DLL
if (!strcmp("LoadDLL", cmd)) {
  // Get dllFolder 
  dllPath = mxArrayToString(prhs[2]);
  dllPath = "C:\\Users\\(User Name)\\Documents\\MATLAB\\Ansteuerung C++\\CanApi.dll";
  // Call loadCanApiDll
  if (posCtrl->loadCanApiDll(dllPath)) {
  // success --> return 1 to Matlab
  plhs[0] = mxCreateDoubleScalar(1);
  return;
  }
  else {
  // failed --> return 0 to Matlab
  plhs[0] = mxCreateDoubleScalar(0);
  return;
  }
}

The dllPath is hard-encoded for testing purposes to make sure that it's no Matlab/ C++ data conversion issue. In Matlab, I have a PositionerController.m class and a very basic script just creating an instance of the C++ class and calling its various methods:

%%Create instance of PositionerController matlab class
posCtrl = PositionerController();

%%Call LoadDll fct.
returnVal = posCtrl.LoadDll(dllPath);

%%Call mctl_Initialize fct.
returnVal = posCtrl.mctl_Initialize(iniFilePath);

%%Call mctl_Reset fct.
returnVal = posCtrl.mctl_Reset();

%%Call mctl_Reference fct.
returnVal = posCtrl.mctl_Reference(dwAxis);

The mexFunction and Class definition C++ file both compile without an error. From the return values I also can infer that the mexFunction and the loadCanApiDll class method principally both work fine - except the fact, that LoadLibrary() doesn't find the DLL.

Does anybody have an idea, why the LoadLibrary() C++ command works when it is used in a test program written and compiled using Microsoft Visual Studio 2017, but fails to do so when it is compiled and called from within Matlab using Microsoft Windows SDK 7.1 (C++) and MEX?

[Nicolai Kern]

Just tried to use a arbitrary 64-Bit DLL, and the program found the DLL handle. Are the Matlab compilers in general unable to deal with 32-Bit DLLs? And if so, is there any reasonable workaround?