User`s guide

3 Writing S-Functions As C-MEX files

3-2

Introduction

A C MEX-file that definesan S-Function block must provide information about

the model to Simulink during the simulation. As the simulation proceeds,

Simulink, the ODE solver, and the MEX-file interact to perform specific tasks.

These tasks incl ude d efining initial conditions and block characteristics, and

computing derivatives, discrete states, and outputs.

C MEX-file S-functions have the same structure and perform the same

functions as M-file S-functions. In addition, C MEX S-functions provide you

with more functio nality than M-file S- functions. Simulink includes a template

file for writing C MEX S-functions, called

sfuntmpl.c and a more complete

version called

sfuntmpl.doc (both located in simulink/src).

The general format of a C MEX S-function is shown below:

#define S_FUNCTION_NAME your_sfunction_name_here

#define S_FUNCTION_LEVEL 2

#include "simstruc.h"

static void mdlInitializeSizes(SimStruct *S)

{

}

static void mdlTerminate(SimStruct *S)

{

}

#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

mdlInitializeSizes

is the first routine Simulink calls when interacting with

the S-function. There are several other

mdl* routines within an S-function. All

S-functionsmust conform to this format. This means that differentS -functions

can implemented using S-function routines of the same name but different