Accessing workspace from C-Mex s-function

In Matlab Simulink you can access workspace variables from a C-Mex (C++/C) s-function as follows:

	/*Demo for getting workspace variables from C-Mex s-function*/
	#define S_FUNCTION_NAME getInputs
	#define S_FUNCTION_LEVEL 2
	#include "simstruc.h"
	static void mdlInitializeSizes( SimStruct *S ) {
	printf("\nHello from getInputs.cpp, mdlInitializeSizes()\n");
	if ( !ssSetNumOutputPorts( S, 2 ) ) return;
	ssSetOutputPortWidth( S, 0, 1 ); // mass_kg
	ssSetOutputPortWidth( S, 1, 3 ); // position_m
	}
	void setOutputPortReal(SimStruct *S, int iPort, double value) {
	double *out = (double *)ssGetOutputPortRealSignal(S, iPort);
	out[0] = value;
	}
	void setOutputPortRealVector3(SimStruct *S, int iPort, double* values) {
	double *out = (double *)ssGetOutputPortRealSignal(S, iPort);
	for (int i = 0; i<3; i++) {
	out[i] = values[i];
	}
	}
	double* getVectorFromWorkspace(char* varName) {
	const mxArray* varPtr = mexGetVariablePtr("base", varName);
	double* vals = mxGetPr(varPtr);
	return vals;
	}
	double getDoubleFromWorkspace(char* varName) {
	double* vals = getVectorFromWorkspace(varName);
	return vals[0];
	}
	#define MDL_INITIALIZE_CONDITIONS
	static void mdlInitializeConditions( SimStruct *S ) {//Invoked at the beginning of simulation.
	printf("\nHello from getInputs.cpp, mdlInitializeConditions()\n");
	setOutputPortReal(S, 0, getDoubleFromWorkspace("mass_kg"));
	setOutputPortRealVector3(S, 1, getVectorFromWorkspace("position_m"));
	}
	static void mdlOutputs( SimStruct *S, int_T tid ){ //Invoked at each simulation time step (not just major time steps)
	printf("\nHello from getInputs.cpp, mdlOutputs()\n");
	}
	/*Obligatory Simulink code*/
	static void mdlInitializeSampleTimes(SimStruct *S) {}
	#define MDL_START
	#if defined(MDL_START)
	static void mdlStart(SimStruct *S) {}
	#endif /* MDL_START */
	#define MDL_UPDATE
	static void mdlUpdate( SimStruct *S, int_T tid ) {}
	static void mdlTerminate( SimStruct *S ) {
	UNUSED_ARG(S); /* unused input argument */
	}
	#ifdef MATLAB_MEX_FILE
	#include "simulink.c"
	#else
	#include "cg_sfun.h"
	#endif

view raw getInputs.cpp hosted with ❤ by GitHub

To write to workspace from C-Mex, you can use
#include "matrix.h"
...
mxArray* pa = mxCreateDoubleScalar(12.34);
mexPutVariable("base", "myVarName", pa);

Note that you cannot use myVarName in a simulink block unless you call it before or during InitFcn.