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 tell the C++ compiler to use C calling 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.
   * 
   */
  
  #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.c 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.c 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