Writing S-Functions

Source File Format

The format of the C++ source for an S-function is nearly identical to that of the source for an S-function written in C. The main difference is that you must use tell the C++ compiler to use C call conventions when compiling the callback methods. This is necessary because the Simulink simulation engine assumes that callback methods obey C calling conventions.

To tell the compiler to use C calling conventions when compiling the callback methods, wrap the C++ source for the S-function callback methods in an extern "C" statement. The C++ version of the sfun_counter S-function example (matlabroot/simulink/src/sfun_counter_cpp.cpp) illustrates usage of the extern "C" directive to ensure that the compiler generates Simulink-compatible callback methods.

/*  File    : sfun_counter_cpp.cpp
 *  Abstract:
 *
 *      Example of an C++ S-function which stores an C++ object in
 *      the pointers vector PWork.
 *
 *  Copyright 1990-2000 The MathWorks, Inc.
 *  $Revision: 1.1 $
 */

#include "iostream.h"

class  counter {
    double  x;
public:
    counter() {
        x = 0.0;
    }
    double output(void) {
        x = x + 1.0;
        return x; 
    }
};

#ifdef __cplusplus
extern "C" { // use the C fcn-call standard for all functions  
#endif       // defined within this scope                     

#define S_FUNCTION_LEVEL 2
#define S_FUNCTION_NAME  sfun_counter_cpp

/*
 * Need to include simstruc.h for the definition of the SimStruct and
 * its associated macro definitions.
 */
#include "simstruc.h"

/*====================*
 * S-function methods *
 *====================*/

/* Function: mdlInitializeSizes ===============================================
 * Abstract:
 *    The sizes information is used by Simulink to determine the S-function
 *    block's characteristics (number of inputs, outputs, states, etc.).
 */
static void mdlInitializeSizes(SimStruct *S)
{
    /* See sfuntmpl.doc for more details on the macros below */

    ssSetNumSFcnParams(S, 1);  /* Number of expected parameters */
    if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
        /* Return if number of expected != number of actual parameters */
        return;
    }

    ssSetNumContStates(S, 0);
    ssSetNumDiscStates(S, 0);

    if (!ssSetNumInputPorts(S, 0)) return;
    
    if (!ssSetNumOutputPorts(S, 1)) return;
    ssSetOutputPortWidth(S, 0, 1);

    ssSetNumSampleTimes(S, 1);
    ssSetNumRWork(S, 0);
    ssSetNumIWork(S, 0);
    ssSetNumPWork(S, 1); // reserve element in the pointers vector
    ssSetNumModes(S, 0); // to store a C++ object
    ssSetNumNonsampledZCs(S, 0);

    ssSetOptions(S, 0);
}




/* Function: mdlInitializeSampleTimes =========================================
 * Abstract:
 *    This function is used to specify the sample time(s) for your
 *    S-function. You must register the same number of sample times as
 *    specified in ssSetNumSampleTimes.
 */
static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, mxGetScalar(ssGetSFcnParam(S, 0)));
    ssSetOffsetTime(S, 0, 0.0);

}

#define MDL_START  /* Change to #undef to remove function */
#if defined(MDL_START) 
  /* Function: mdlStart =======================================================
   * Abstract:
   *    This function is called once at start of model execution. If you
   *    have states that should be initialized once, this is the place
   *    to do it.
   */
  static void mdlStart(SimStruct *S)
  {
      ssGetPWork(S)[0] = (void *) new counter; // store new C++ object in the
  }                                            // pointers vector
#endif /*  MDL_START */

/* Function: mdlOutputs =======================================================
 * Abstract:
 *    In this function, you compute the outputs of your S-function
 *    block. Generally outputs are placed in the output vector, ssGetY(S).
 */
static void mdlOutputs(SimStruct *S, int_T tid)
{
    counter *c = (counter *) ssGetPWork(S)[0];   // retrieve C++ object from
    real_T  *y = ssGetOutputPortRealSignal(S,0); // the pointers vector and use
    y[0] = c->output();                          // member functions of the
}                                                // object

/* Function: mdlTerminate =====================================================
 * Abstract:
 *    In this function, you should perform any actions that are necessary
 *    at the termination of a simulation.  For example, if memory was
 *    allocated in mdlStart, this is the place to free it.
 */
static void mdlTerminate(SimStruct *S)
{
    counter *c = (counter *) ssGetPWork(S)[0]; // retrieve and destroy C++
    delete c;                                  // object in the termination
}                                              // function
/*======================================================*
 * See sfuntmpl.doc for the optional S-function methods *
 *======================================================*/

/*=============================*
 * Required S-function trailer *
 *=============================*/

#ifdef  MATLAB_MEX_FILE    /* Is this file being compiled as a MEX-file? */
#include "simulink.c"      /* MEX-file interface mechanism */
#else
#include "cg_sfun.h"       /* Code generation registration function */
#endif

#ifdef __cplusplus
} // end of extern "C" scope
#endif

Creating C++ S-Functions

Making C++ Objects Persistent