Writing S-Functions

S-Function Source File Requirements

This section describes requirements that every S-function source file must meet to compile correctly. The S-function templates meet these requirements.

Statements Required at the Top of S-Functions

For S-functions to operate properly, each source module of your S-function that accesses the SimStruct must contain the following sequence of defines and include

• #define S_FUNCTION_NAME your_sfunction_name_here
  #define S_FUNCTION_LEVEL 2
  #include "simstruc.h"
  

where your_sfunction_name_here is the name of your S-function (i.e., what you enter in the Simulink S-Function block dialog). These statements give you access to the SimStruct data structure that contains pointers to the data used by the simulation. The included code also defines the macros used to store and retrieve data in the SimStruct, described in detail in Converting Level 1 C MEX S-Functions to Level 2. In addition, the code specifies that you are using the level 2 format of S-functions.

Note    All S-functions from Simulink 1.3 through 2.1 are considered to be level 1 S-functions. They are compatible with Simulink 3.0, but we recommend that you write new S-functions in the level 2 format.

The following headers are included by matlabroot/simulink/include/simstruc.h when compiling as a MEX-file.

Table 3-1: Header Files Included by simstruc.h When Compiling as a MEX-File

Header File	Description
matlabroot/extern/include/tmwtypes.h	General data types, e.g., real_T
matlabroot/extern/include/mex.h	MATLAB MEX-file API routines
matlabroot/extern/include/matrix.h	MATLAB MEX-file API routines

When compiling your S-function for use with the Real-Time Workshop, simstruc.h includes the following.

Table 3-2: Header Files Included by simstruc.h When Used
by the Real-Time Workshop

Header File	Description
matlabroot/extern/include/tmwtypes.h	General types, e.g., real_T
matlabroot/rtw/c/libsrc/rt_matrx.h	Macros for MATLAB API routines

Statements Required at the Bottom of S-Functions

Include this trailer code at the end of your C MEX S-function main module only.

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

These statements select the appropriate code for your particular application:

• simulink.c is included if the file is being compiled into a MEX-file.
• cg_sfun.h is included if the file is being used in conjunction with the Real-Time Workshop to produce a stand-alone or real-time executable.

Note    This trailer code must not be in the body of any S-function routine.

Templates for C S-Functions

The SimStruct