Question

I have a C++ DLL which I call from Matlab code using calllib.
I have no trouble calling a C++ function which has input parameters only or a function which returns mxArray.

Now I have trouble calling a function which has several output parameters. Let’s say, I need a C++ equivalent of this Matlab function which returns a matrix and an integer.

function [matrix, status] = foo()
status = 42;
matrix = ones(3,2);
end

Whatever I tried, it makes Matlab crash, for example:

DLL_API void foo(mxArray* iop_matrix, int* op_status)
  {
  mxSetM(iop_matrix, 3);
  mxSetN(iop_matrix, 2);
  *op_status = 42;
  }

However I could easily get it to work when I need only one output parameter

DLL_API mxArray* foo(void)
  {
  return mxCreateNumericMatrix(3, 2, mxDOUBLE_CLASS, mxREAL);
  }

What is the correct implementation of such function in C++?

Answer 1

The Matlab function’s 1st output parameter should be declared as the C function’s output parameter. The Matlab function’s 2nd output parameter should be declared as the C function’s 1st input parameter which has mxArray ** type and so on.

A C++ function with several output parameters should look like

mxArray *foo(mxArray **matrix);

If the function contains input parameters in this case the function is declared as

mxArray *foo(mxArray **matrix, mxArray *_1stInpParam, mxArray *_2ndInpParam);