How to add C++ S-function to Simulink

In our previous examples such as Matlab Level 2 S-Function example: Sine wave (continous output) we’ve shown how to create a simple S-function in C++.

Here’s how to add a C++ S-function to Simulink:

Step 1: Create the source code file of the S-functon

In the directory you have opened in Matlab/Simulink, create a new file named e.g. sine_wave_generator.cpp. See Matlab Level 2 S-Function example: Sine wave (continous output) for an example of the C++ code.

I recommend using Visual Studio Code instead of Matlab as a C++ editor.

Step 2: Compile the S-function

Open a terminal and run

mex sine_wave_generator.cpp

I do not recommend running this in Matlab even though this is possible. I recommend always using a system terminal.

The compilation process works only if the last line of the output is MEX completed successfully.

If you see any other output, you need to debug the issue.

Common issues are:

• How to fix Matlab mex error: You can’t use ‘macro parameter character #’ in vertical mode.
• Missing additional libraries or include paths: How to link additional libraries when compiling Matlab/Simulink S-function via ‘mex’
• If you are using features of modern C++, you might need to select the correct C++ standard: How to specify C++ version when compiling Matlab/Simulink S-function using ‘mex’

Step 3: Create a new Simulink model

For testing, I recommend to create a new Simulink model instead of re-using an old one.

Step 4: Add an S-function block

First start by click the dialog to add a new block:

Now, click User-defined functions and select the S-function block. Do not select the Matlab Level-2 S-function block even if you are using a Level 2 s-function. Level 2 S-functions work just fine with the normal S-function block.

Drag this block into the main Simulink model area, i.e. the big white empty space in the other window which usually is just behind the library browser dialog:

Step 5 : Configure the S-function block

Double-click the S-function block to open the configuration dialog:

In the dialog, edit the Name and enter the name of the S-function.

The name of the S-function is always the name of the .cpp file without the .cpp extension.

In our case, the C++ file is named sine_wave_generator.cpp, hence the name of the S-function is sine_wave_generator.

Press Apply to save the changes and close the dialog.

Step 6: Connect the S-function block in Simulink

In our example, we can connect the S-function block to a Scope block to visualize the output.

This is not mandatory but it’s a good way to verify that the S-function works as expected.

In order to add the Scope block, click on Library browser icon in the navbar again:

Click on Sinks and select the Scope block:

Drag the Scope block into the main Simulink model area:

Close the Library browser dialog.

Now, connect the output of the S-function block to the input of the Scope block:

The connected blocks should now look like this:

Step 7: Run the Simulink model

Click the Run button in the Simulink toolbar to run the simulation:

Step 8: Check the scope

If everything works as expected, you can now double-click the Scope block to see the output of the S-function:

This will open a new window showing the scope output of the S-function: