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Write Fully Inlined S-Functions with mdlRTW Routine

You can inline more complex S-functions by using the S-function mdlRTW routine. The mdlRTW routine provides the code generation process with more information about how the S-function is to be inlined by creating a parameter record of a non-tunable parameter for use with a TLC file. The mdlRTW routine places information in the model.rtw file. The mdlRTW function is described in the text file matlabroot/toolbox/simulink/blocks/src/sfuntmpl_doc.c.

To use the mdlRTW function, take steps to create a direct-index lookup S-function. Lookup tables are collections of ordered data points of a function. Typically, these tables use some interpolation scheme to approximate values of the associated function between known data points. To incorporate the example lookup table algorithm into a Simulink® model, the first step is to write an S-function that executes the algorithm in mdlOutputs. To produce the most efficient code, the next step is to create a corresponding TLC file to eliminate computational overhead and improve the speed of the lookup computations.

The Simulink product provides support for general-purpose lookup 1-D, 2-D, and n-D algorithms. You can use these algorithms as they are or create a custom lookup table S-function to fit your requirements. You can create a 1-D lookup S-function, sfun_directlook.c, and its corresponding inlined sfun_directlook.tlc file (see Target Language Compiler for more details). You can:

  • Error check of S-function parameters.

  • Cache information for the S-function that does not change during model execution.

  • Use the mdlRTW function to customize the code generator to produce the optimal code for a given set of block parameters.

  • Create a TLC file for an S-function that either fully inlines the lookup table code or calls a wrapper function for the lookup table algorithm.

S-Function RTWdata

RTWdata is a property of blocks, which can be used by the Target Language Compiler when inlining an S-function. RTWdata is a structure of character vectors that you can attach to a block. RTWdata is saved with the model and placed in the model.rtw file when you generate code. For example, this set of MATLAB® commands:

mydata.field1 = 'information for field1';
mydata.field2 = 'information for field2';

produces this result:

ans = 
    field1: 'information for field1'
    field2: 'information for field2'

The information for the associated S-Function block inside the model.rtw file is:

Block {
  Type                    "S-Function"
  RTWdata {
    field1                  "information for field1"
    field2                  "information for field2"


RTWdata is saved in the model file for S-functions that are not linked to a library. However, RTWdata is not persistent for S-Function blocks that are linked to a library.

Direct-Index Lookup Table Algorithm

The 1-D lookup table block provided in the Simulink library uses interpolation or extrapolation when computing outputs. In this example, you create a lookup table that directly indexes the output vector (y-data vector) based on the current input (x-data) point.

This direct 1-D lookup example computes an approximate solution p(x)to a partially known function f(x) at x=x0, given data point pairs (x,y) in the form of an x-data vector and a y-data vector. For a given data pair (for example, the i'th pair), y_i = f(x_i). It is assumed that the x-data values are monotonically increasing. If x0 is outside the range of the x-data vector, the first or last point is returned.

The parameters to the S-function are:

XData, YData, XEvenlySpaced

XData and YData are double vectors of equal length representing the values of the unknown function. XDataEvenlySpaced is a scalar, 0.0 for false and 1.0 for true. If the XData vector is evenly spaced, XDataEvenlySpaced is 1.0 and more efficient code is generated.

The graph shows how the parameters XData=[1:6]and YData=[1,2,7,4,5,9] are handled. For example, if the input (x-value) to the S-Function block is 3, the output (y-value) is 7.

Direct-Index Lookup Table Example

Improve the lookup table by inlining a direct-index S-function with a TLC file. This direct-index lookup table S-function does not require a TLC file. The example uses a TLC file for the direct-index lookup table S-function to reduce the code size and increase efficiency of the generated code.

Implementation of the direct-index algorithm with an inlined TLC file requires the S-function main module, sfun_directlook.c and a corresponding lookup_index.c module. The lookup_index.c module contains the GetDirectLookupIndex function that is used to locate the index in the XData for the current x input value when the XData is unevenly spaced. The GetDirectLookupIndex routine is called from the S-function and the generated code. The example uses the wrapper concept for sharing C/C++ code between Simulink MEX-files and the generated code.

If the XData is evenly spaced, then both the S-function main module and the generated code contain the lookup algorithm to compute the y-value of a given x-value because the algorithm is short.

The inlined TLC file is sfun_directlook.tlc, which is used to either perform a wrapper call or embed the optimal C/C++ code for the S-function. (See the example in mdlRTW Usage).

Error Handling

In sfun_directlook.tlc, the mdlCheckParameters routine verifies that:

  • The new parameter settings are valid.

  • XData and YData are vectors of the same length containing real, finite numbers.

  • XDataEvenlySpaced is a scalar.

  • The XData vector is a monotonically increasing vector and evenly spaced.

The mdlInitializeSizes function explicitly calls mdlCheckParameters after it verifies the number of parameters passed to the S-function. After the Simulink engine calls mdlInitializeSizes, it then calls mdlCheckParameters whenever you change the parameters or reevaluate them.

User Data Caching

In sfun_directlook.tlc, the mdlStart routine shows how to cache information that does not change during the simulation or while the generated code is executing. The example caches the value of the XDataEvenlySpaced parameter in UserData, a field of the SimStruct. The following line in mdlInitializeSizes instructs the Simulink engine to disallow changes to XDataEvenlySpaced.

ssSetSFcnParamTunable(S, iParam, SS_PRM_NOT_TUNABLE);

During execution, mdlOutputs accesses the value of XDataEvenlySpaced from UserData rather than calling the mxGetPr MATLAB API function.

mdlRTW Usage

The code generator calls the mdlRTW routine while generating the model.rtw file. To produce optimal code for your Simulink model, you can add information to the model.rtw file about the mode in which your S-Function block is operating.

The example adds parameter settings to the model.rtw file. The parameter settings do not change during execution. In this case, the XDataEvenlySpaced S-function parameter cannot change during execution (ssSetSFcnParamTunable was specified as false (0) for it in mdlInitializeSizes). The parameter setting (XSpacing) uses the function ssWriteRTWParamSettings.

Because xData and yData are registered as run-time parameters in mdlSetWorkWidths, the code generator writes to the model.rtw file automatically.

Before examining the S-function and the inlined TLC file, consider the generated code for this model.

The model uses evenly spaced XData in the top S-Function block and unevenly spaced XData in the bottom S-Function block. When creating this model, specify the following commands for each S-Function block.


The build process uses the module lookup_index.c when creating the executable.

When generating code for this model, the code generator uses the S-function mdlRTW method to generate a model.rtw file with the value EvenlySpaced for the XSpacing parameter for the top S-Function block and the value UnEvenlySpaced for the XSpacing parameter for the bottom S-Function block. The TLC-file uses the value of XSpacing to determine what algorithm to include in the generated code. The generated code contains the lookup algorithm when the XData is evenly spaced, but calls the GetDirectLookupIndex routine when the XData is unevenly spaced. The generated model.c or model.cpp code for the lookup table example model is similar to this code:

 * sfun_directlook_ex.c
 * Code generation for Simulink model 
 * "sfun_directlook_ex.slx".

#include "sfun_directlook_ex.h"
#include "sfun_directlook_ex_private.h"

/* External outputs (root outports fed by signals with auto storage) */
ExtY_sfun_directlook_ex_T sfun_directlook_ex_Y;

/* Real-time model */
RT_MODEL_sfun_directlook_ex_T sfun_directlook_ex_M_;
RT_MODEL_sfun_directlook_ex_T *const sfun_directlook_ex_M =

/* Model output function */
void sfun_directlook_ex_output(void)
  /* local block i/o variables */
  real_T rtb_SFunction;
  real_T rtb_SFunction1;

  /* Sin: '<Root>/Sine Wave' */
  rtb_SFunction1 = sin(sfun_directlook_ex_M->Timing.t[0]);
/* Code that is inlined for the top S-function block in the 
   * sfun_directlook_ex model
  /* S-Function (sfun_directlook): '<Root>/S-Function' */
    const real_T *xData = sfun_directlook_ex_ConstP.SFunction_XData;
    const real_T *yData = sfun_directlook_ex_ConstP.SFunction_YData;
    real_T spacing = xData[1] - xData[0];
    if (rtb_SFunction1 <= xData[0] ) {
      rtb_SFunction = yData[0];
    } else if (rtb_SFunction1 >= yData[20] ) {
      rtb_SFunction = yData[20];
    } else {
      int_T idx = (int_T)( ( rtb_SFunction1 - xData[0] ) / spacing );
      rtb_SFunction = yData[idx];

  /* Outport: '<Root>/Out1' */
  sfun_directlook_ex_Y.Out1 = rtb_SFunction;

/* Code that is inlined for the bottom S-function block in the 
   * sfun_directlook_ex model
  /* S-Function (sfun_directlook): '<Root>/S-Function1' */
    const real_T *xData = sfun_directlook_ex_ConstP.SFunction1_XData;
    const real_T *yData = sfun_directlook_ex_ConstP.SFunction1_YData;
    int_T idx;
    idx = GetDirectLookupIndex(xData, 5, rtb_SFunction1);
    rtb_SFunction1 = yData[idx];

  /* Outport: '<Root>/Out2' */
  sfun_directlook_ex_Y.Out2 = rtb_SFunction1;

/* Model update function */
void sfun_directlook_ex_update(void)
  /* signal main to stop simulation */
  {                                    /* Sample time: [0.0s, 0.0s] */
    if ((rtmGetTFinal(sfun_directlook_ex_M)!=-1) &&
          > sfun_directlook_ex_M->Timing.t[0] * (DBL_EPSILON))) {
      rtmSetErrorStatus(sfun_directlook_ex_M, "Simulation finished");

  /* Update absolute time for base rate */
  /* The "clockTick0" counts the number of times the code of this task has
   * been executed. The absolute time is the multiplication of "clockTick0"
   * and "Timing.stepSize0". Size of "clockTick0" ensures timer will not
   * overflow during the application lifespan selected.
   * Timer of this task consists of two 32 bit unsigned integers.
   * The two integers represent the low bits Timing.clockTick0 and the high bits
   * Timing.clockTickH0. When the low bit overflows to 0, the high bits increment.
  if (!(++sfun_directlook_ex_M->Timing.clockTick0)) {

  sfun_directlook_ex_M->Timing.t[0] = sfun_directlook_ex_M->Timing.clockTick0 *
    sfun_directlook_ex_M->Timing.stepSize0 +
    sfun_directlook_ex_M->Timing.clockTickH0 *
    sfun_directlook_ex_M->Timing.stepSize0 * 4294967296.0;

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