Using workspace variables in C++ Mex functions

When you have C or C++ files that you would like to use in your Simulink model, you have to interface them with a mex file. Variables in Matlab workspace can be used inside C++ mex functions. Example m file and mex function demonstrating scalar, vector, matrix, string and structure usage:

	%Tested with Matlab R2019b and Visual Studio 2015
	clc; clear all
	inScalar = 153;
	inVector = [1 2 3];
	inMatrix = [1 2 3; 11 22 33; 111 222 333; 1111 2222 3333];
	inCharArray = 'c:/temp/a/';
	inString = "c:/temp/a/"; %Note double quotes
	inStruct.x = [1 2 3];
	inStruct.y = [11 22; 111 222];
	mex mexWorkspaceDemo.cpp
	mexWorkspaceDemo();

view raw mexDriver.m hosted with ❤ by GitHub

	/* Demonstrate how to use Matlab workspace variables from C++ mex function.
	* Note that you can only set to or get from BASE or GLOBAL workspace, you cannot use caller workspace (workspace of the caller function).
	* For more, see https://www.mathworks.com/help/matlab/matlab_external/set-and-get-variables-in-matlab-workspace.html
	* See mexDriver.m on how to use this mex function.
	* Şamil Korkmaz, Jan 2020
	*/
	#include "mex.hpp"
	#include "mexAdapter.hpp"
	#include <iostream>
	using matlab::mex::ArgumentList;
	using namespace matlab::engine;
	using namespace matlab::data;
	using namespace std;
	class MexFunction : public matlab::mex::Function {
	private:
	ArrayFactory factory;
	shared_ptr<matlab::engine::MATLABEngine> matlabPtr = getEngine();
	void getCharArray(char* name, char* output) {
	CharArray inCharArray = matlabPtr->getVariable(name);
	for(size_t i=0; i < inCharArray.getNumberOfElements(); i++) {
	output[i] = inCharArray[i];
	}
	output[inCharArray.getNumberOfElements()] = 0; //ASCII Null or termination character denoting end of string
	}
	public:
	void operator()(ArgumentList outputs, ArgumentList ins) {
	//inputs from workspace:
	TypedArray<double> inScalarArray = matlabPtr->getVariable(u"inScalar");
	TypedArray<double> inVectorArray = matlabPtr->getVariable(u"inVector");
	TypedArray<double> inMatrixArray = matlabPtr->getVariable(u"inMatrix");
	TypedArray<MATLABString> inStringArray = matlabPtr->getVariable(u"inString");
	char inCharArrayOutput[255];
	getCharArray("inCharArray", inCharArrayOutput);
	StructArray inStructArray = matlabPtr->getVariable(u"inStruct");
	//printf does not print to Command Window, you have to use cout:
	cout <<"\nINPUTS FROM MATLAB WORKSPACE:"<<endl;
	cout << "inScalarArray[0] = " << inScalarArray[0] << endl; //You can only print if inScalarArray is of type TypedArray<double>, you can't if type ia Array, you will get error C2593: 'operator <<' is ambiguous
	cout << "inVectorArray";
	for(size_t i=0; i<inVectorArray.getNumberOfElements(); i++) {
	cout << "[" << i << "] = " << inVectorArray[i] << " ";
	}
	cout << endl;
	cout << "inMatrixArray.getNumberOfElements() = " << inMatrixArray.getNumberOfElements() << endl;
	ArrayDimensions inMatrixDims = inMatrixArray.getDimensions();
	size_t nRows = inMatrixDims[0];
	size_t nCols = inMatrixDims[1];
	for(size_t iRow=0; iRow < nRows; iRow++) {
	for(size_t iCol=0; iCol < nCols; iCol++) {
	cout << "inMatrixArray[" << iRow << "][" << iCol << "] = " << inMatrixArray[iRow][iCol] << "\t ";
	}
	cout << endl;
	}
	cout<<"inCharArray = " << inCharArrayOutput << endl;
	string stringVal = inStringArray[0]; //convert from Matlab string to standard C++ string
	cout << "stringVal = " << stringVal << endl; //You cannot use inStringArray[0] here.
	auto fields = inStructArray.getFieldNames();
	std::vector<std::string> fieldNames(fields.begin(), fields.end());
	//size_t nElements = inStructArray.getNumberOfElements(); cout << "nElements = " << nElements << endl;
	for(size_t i = 0; i<fieldNames.size(); i++) {
	//cout << "fieldNames[" << i << "] = " << fieldNames[i] << endl;
	TypedArray<double> const structField = inStructArray[0][fieldNames[i]];
	ArrayDimensions dims = structField.getDimensions();
	size_t nRows = dims[0];
	size_t nCols = dims[1];
	for(size_t iRow=0; iRow < nRows; iRow++) {
	for(size_t iCol=0; iCol < nCols; iCol++) {
	double value = structField[iRow][iCol];
	cout << "structField[" << iRow << "][" << iCol << "] = " << value << "\t ";
	}
	cout << endl;
	}
	cout << endl;
	}
	//outputs to workspace:
	double b = inScalarArray[0]; //convert to C++ double
	Array outScalarArray = factory.createScalar(b+10);
	double outScalar = outScalarArray[0]; //convert to C++ double
	matlabPtr->setVariable(u"outScalar", outScalarArray, WorkspaceType::GLOBAL);
	Array outVectorArray = factory.createArray<double>({1, 3}, {11, 22, 33});
	matlabPtr->setVariable(u"outVector", outVectorArray, WorkspaceType::GLOBAL);
	Array outMatrixArray = factory.createArray<double>({2, 2}, {1, 2, 11, 22});
	matlabPtr->setVariable(u"outMatrix", outMatrixArray, WorkspaceType::GLOBAL);
	Array outStringArray = factory.createArray<string>({1, 1}, {"samil was here"});
	matlabPtr->setVariable(u"outString", outStringArray, WorkspaceType::GLOBAL);
	cout <<"\nOUTPUTS FROM MATLAB WORKSPACE:"<<endl;
	cout << "outScalar + b = " << outScalar + b << endl; //You cannot use outScalarArray[0] + b, you will get error binary '+': 'matlab::data::ArrayElementRef<false>' does not define this operator
	}
	};

view raw mexWorkspaceDemo.cpp hosted with ❤ by GitHub

When you run mexDriver.m, you will get an output similar to this:

Comparing two models

When you have an mdl/slx model, to compare it with another version of the same model:

1. Open model in Simulink.
2. Go to Analysis menu, click Compare Simulink XML Files...
3. Select the model file you want to compare with the opened one.
4. Simulink will open a window displaying the differences. When you click on a line, it will display both models, highlighting the differences.

File closing bug

Recently, I saw that when I ran a simulink model multiple times in a row from a script with sim() command, the model would throw an error at the 509th time with a message indicating that it could not find a file, i.e. an fopen() operation returned null. The curious thing is that the file exists, otherwise it would not be able to run the previous 508 times.

After one day of debugging, I saw that I forgot to close a file in another unrelated s-function. I guess after a certain number of unclosed file pointers, fopen() commands start to fail. This reminded me of an old heap corruption bug that took me months to find an fix.