Writing S-Functions

Building S-Functions Automatically

The S-Function Builder is a Simulink block that builds an S-function from specifications and C code that you supply. The S-Function Builder also serves as a wrapper for the generated S-function in models that use the S-function. This section explains how to use the S-Function Builder to build simple C MEX S-functions.

To build an S-function with the S-Function Builder:

1. Set the MATLAB current directory to the directory in which you want to create the S-function.
2. Create a new Simulink model.
3. Copy an instance of the S-Function Builder block from the Simulink User-Defined Functions library into the new model.

4. Double-click the block to open the S-Function Builder dialog box.

   

5. Enter the name of the S-function in the S-function name field.
6. If the S-function has parameters, enter default values for the parameters in the S-function parameters field.
7. Use the specification and code entry panes on the S-Function Builder dialog box to enter information and custom source code required to tailor the generated S-function to your application (see S-Function Builder Dialog Box).
8. If you have not already done so, configure the MATLAB mex command to work on your system.

1. To configure the mex command, type mex -setup at the MATLAB command line.

9. Click Build on the dialog box to start the build process.

1. Simulink builds a MEX file that implements the specified S-function and saves the file in the current directory (see "How the S-Function Builder Builds an S-Function" on page 3-7).

10. Save the model containing the S-Function Builder block.

Deploying the Generated S-Function

To use the generated S-function in another model, first check to ensure that the directory containing the generated S-function is on the MATLAB path. Then copy the S-Function Builder block from the model used to create the S-function into the target model and set its parameters, if necessary, to the values required by the target model.

How the S-Function Builder Builds an S-Function

The S-Function Builder builds an S-function as follows. First, it generates the following source files in the current directory:

• sfun.c

• where sfun is the name of the S-function that you specified in the S-function name field of the S-Function Builder's dialog box. This file contains the C source code representation of the standard portions of the generated S-function.

• sfun_wrapper.c

• This file contains the custom code that you entered in the S-Function Builder dialog box.

• sfun.tlc

• This file permits Simulink to run the generated S-function in accelerated mode and RTW to include this S-function in the code it generates.

After generating the S-function source code, the S-Function Builder uses the MATLAB mex command to build the MEX file representation of the S-function from the generated source code and any external custom source code and libraries that you specified.

Writing S-Functions in C

S-Function Builder Dialog Box