Real-Time Workshop®

Introduction

S-Function Target Overview

Using the S-function target, you can build an S-function component and use it as an S-Function block in another model. The S-function code format used by the S-function target generates code that conforms to the Simulink C MEX S-function application programming interface (API). Applications of this format include

• Conversion of a model to a component. You can generate an S-Function block for a model, m1. Then, you can place the generated S-Function block in another model, m2. Regenerating code for m2 does not require regenerating code for m1.

• Conversion of a subsystem to a component. By extracting a subsystem to a separate model and generating an S-Function block from that model, you can create a reusable component from the subsystem. See Creating an S-Function Block from a Subsystem for an example of this procedure.

• Speeding up simulation. In many cases, an S-function generated from a model performs more efficiently than the original model.

• Code reuse. You can incorporate multiple instances of one model inside another without replicating the code for each instance. Each instance will continue to maintain its own unique data.

The S-function target generates noninlined S-functions. Within the same release, you can generate an executable from a model that contains generated S-functions by using the generic real-time or real-time malloc targets. This is not supported when incorporating a generated S-function from one release into a model that you build with a different release.

You can place a generated S-Function block into another model from which you can generate another S-function format. This allows any level of nested S-functions.

You should avoid nesting S-functions in a model or subsystem having the same name as the S-function (possibly several levels apart). In such situations, the S-function can be called recursively. The Real-Time Workshop software currently does not detect such loops in S-function dependency, which can result in aborting or hanging your MATLAB session.

To prevent this from happening, you should be sure to name the subsystem or model to be generated as an S-function target uniquely, to avoid duplicating any existing MEX filenames on the MATLAB path.

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Intellectual Property Protection for S-Function in Simulation

In addition to the technical applications of the S-function target listed above, you can use the S-function target to protect your designs and algorithms. By generating an S-function from a proprietary model or algorithm, you can share the model's functionality for simulation use without providing the source code. You need only provide the binary MEX-file object to users.

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Required Files for S-Function Deployment

To deploy your generated S-Function block for inclusion in other models for simulation, you need only provide the binary MEX-file object that was generated in the current working directory when the S-Function block was created:

where subsys is the subsystem name and mexext is a platform-dependent MEX-file extension (see mexext). For example, SourceSubsys_sf.mexw32.

To deploy your generated S-Function block for inclusion in other models for code generation, you must provide all of the files that were generated in the current working directory when the S-Function block was created:

• subsys_sf.c or .cpp, where subsys is the subsystem name (for example, SourceSubsys_sf.c)

• subsys_sf.h

• subsys_sf.mexext, where mexext is a platform-dependent MEX-file extension (see mexext)

• Subdirectory subsys_sfcn_rtw and its contents

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Sample Time Propagation in Generated S-Functions

A generated S-Function block can inherit its sample time from the model in which it is placed if certain criteria are met. Six conditions that govern sample time propagation for S-functions and for the S-function code format are described in Inheriting Sample Times in the Simulink documentation. These conditions also apply to sample times propagated to Model blocks, and are further discussed in Inherited Sample Time for Referenced Models.

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Choice of Solver Type

If the model containing the subsystem from which you generate an S-function uses a variable-step solver, the generated S-function contains zero-crossing functions and will work properly only in models that use variable-step solvers.

If the model containing the subsystem from which you generate an S-function uses a fixed-step solver, the generated S-function contains no zero-crossing functions and the generated S-function will work properly in models that use variable-step or fixed-step solvers.

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S-Function Target

Creating an S-Function Block from a Subsystem