Main Content

Call External C Functions

About This Example

Learning Objectives

  • Evaluate a C function as part of a model simulation.

  • Call an external C function from generated code.


  • Ability to open and modify Simulink® models and subsystems.

  • Ability to set model configuration parameters.

  • Ability to read C code.

  • An installed, supported C compiler.

Required Files

  • rtwdemo_throttlecntrl_extfunccall model file

  • rtwdemo_ValidateLegacyCodeVrsSim model file

  • /toolbox/rtw/rtwdemos/EmbeddedCoderOverview/stage_4_files/SimpleTable.c

  • /toolbox/rtw/rtwdemos/EmbeddedCoderOverview/stage_4_files/SimpleTable.h

Include External C Functions in a Model

Simulink models are one part of Model-Based Design. For many applications, a design also includes a set of pre-existing C functions created, tested (verified), and validated outside of a MATLAB® and Simulink environment. You can integrate these functions easily into a model and the generated code. You can use external C code in the generated code to access hardware devices and external data files during rapid simulation runs.

This example shows you how to create a custom block that calls an external C function. When the block is part of the model, you can take advantage of the simulation environment to test the system further.

Create a Block That Calls a C Function

To specify a call to an external C function, use an S-Function block. You can automate the process of creating the S-Function block by using the Simulink Legacy Code Tool. Using this tool, specify an interface for your external C function. The tool then uses that interface to automate creation of an S-Function block.

  1. Make copies of the files SimpleTable.c and SimpleTable.h, located in the folder matlabroot/toolbox/rtw/rtwdemos/EmbeddedCoderOverview/stage_4_files (open). Put the copies in your working folder.

  2. Create an S-Function block that calls the specified function at each time step during simulation:

    1. In the MATLAB Command Window, create a function interface definition structure:


      The data structure def defines the function interface to the external C code.

      def = 
                        SFunctionName: ''
          InitializeConditionsFcnSpec: ''
                        OutputFcnSpec: ''
                         StartFcnSpec: ''
                     TerminateFcnSpec: ''
                          HeaderFiles: {}
                          SourceFiles: {}
                         HostLibFiles: {}
                       TargetLibFiles: {}
                             IncPaths: {}
                             SrcPaths: {}
                             LibPaths: {}
                           SampleTime: 'inherited'
                              Options: [1x1 struct]
    2. Populate the function interface definition structure by entering the following commands:

      def.OutputFcnSpec=['double y1 = SimpleTable(double u1,',...
         'double p1[], double p2[], int16 p3)'];
      def.HeaderFiles = {'SimpleTable.h'};
      def.SourceFiles = {'SimpleTable.c'};
      def.SFunctionName = 'SimpTableWrap';
    3. Create the S-function:

      legacy_code('sfcn_cmex_generate', def)
    4. Compile the S-function:

      legacy_code('compile', def)
    5. Create the S-Function block:

      legacy_code('slblock_generate', def)

      A new model window opens that contains the SimpTableWrap block.


      Creating the S-Function block is a one-time task. Once the block exists, you can reuse it in multiple models.

  3. Save the model to your working folder as: s_func_simptablewrap.

  4. Create a Target Language Compiler (TLC) file for the S-Function block:

    legacy_code('sfcn_tlc_generate', def)

    The TLC file is the component of an S-function that specifies how the code generator produces the code for a block.

For more information on using the Legacy Code Tool, see:

Validate External Code in the Simulink Environment

When you integrate external C code with a Simulink model, before using the code, validate the functionality of the external C function code as a standalone component.

  1. Open the model rtwdemo_ValidateLegacyCodeVrsSim. This model validates the S-function block that you created.

    • The Sine Wave block produces output values from [-2 : 2].

    • The input range of the lookup table is from [-1 : 1].

    • The output from the lookup table is the absolute value of the input.

    • The lookup table output clips the output at the input limits.

  2. Simulate the model.

  3. View the validation results by opening the Validation subsystem and, in that subsystem, clicking the Scope block.

    The following figure shows the validation results. The external C code and the Simulink Lookup table block provide the same output values.

  4. Close the validation model.

Validate C Code as Part of a Model

After you validate the functionality of the external C function code as a standalone component, validate the S-function in the model. Use the test harness model to complete the validation.


The following procedure requires a Stateflow® license.

  1. Open rtwdemo_throttlecntrl_extfunccall and save a copy to throttlecntrl_extfunccall in a writable folder on your MATLAB path.

  2. Examine the PI_ctrl_1 and PI_ctrl_2 subsystems.

    1. Lookup blocks have been replaced with the block you created using the Legacy Code Tool.

    2. Examine the block parameter settings for SimpTableWrap and SimpTableWrap1.

    3. Close the Block Parameter dialog boxes and the PI subsystem windows.

  3. Open the test harness model, right-click the Unit_Under_Test Model block, and select Block Parameters (ModelReference).

  4. Set Model name to throttlecntrl_extfunccall. Click OK.

  5. Update the test harness model diagram.

  6. Simulate the test harness.

    The simulation results match the expected golden values.

  7. Save and close throttlecntrl_extfunccall and throttlecntrl_testharness.

Call a C Function from Generated Code

The code generator uses a TLC file to process the S-Function block. Calls to C code embedded in an S-Function block:

  • Can use data objects.

  • Are subject to expression folding, an operation that combines multiple computations into a single output calculation.

  1. Open throttlecntrl_extfunccall.

  2. Generate code for the model.

  3. Examine the generated code in the filethrottlecntrl_extfunccall.c.

  4. Close throttlecntrl_extfunccall and throttlecntrl_testharness.

Key Points

  • You can easily integrate external functions into a model and generated code by using the Legacy Code Tool.

  • Validate the functionality of external C function code which you integrate into a model as a standalone component.

  • After you validate the functionality of external C function code as a standalone component, validate the S-function in the model.

Related Topics