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Call External C Code from Model and Generated Code

Call existing, external functions from a simulation or from the generated code by using the Legacy Code Tool.

Learn how to:

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

  • Call a C function from the code that you generate from a model.

For information about the example model and other examples in this series, see Generate C Code from a Control Algorithm for an Embedded System.

Replacement Process

Open the example model, rtwdemo_PCG_Eval_P4.

For many Model-Based Design applications, in addition to Simulink® models, a design includes a set of existing C functions that have been tested and validated. You can integrate these functions into a Simulink® model and generate code that uses the functions.

In this example, you create a custom Simulink® block that calls an existing C function. You then include the block in a model and test the overall system through model simulation in Simulink®.

In the example model, you can replace the Lookup blocks (lookup tables) in the PI controllers with calls to an existing C function. The function is defined in the files SimpleTable.c and SimpleTable.h.

View SimpleTable.c.

View SimpleTable.h.

Create Block That Calls C Function

To specify a call to an existing C function, use an S-Function block. You can automate the creation of the S-Function block by using the Legacy Code Tool. In the tool, you first specify an interface for your existing C function. The tool then uses that interface to create an S-Function block.

Use the Legacy Code Tool to create an S-Function block for the existing C function in SimpleTable.c.

1. Create a structure to contain the definition of the function interface.

def = legacy_code('initialize')

You can use the structure def to define the function interface to the existing C code.

2. Populate the fields of the structure.

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)

The generated S-Function block calls the C function in SimpleTable.c. You can now use and reuse this S-Function block in models.

6. Create the TLC file.

legacy_code('sfcn_tlc_generate',def)

This command creates a TLC file, which is the component of an S-Function that specifies how to generate code for the block.

Validate External Code Through Simulation

When you integrate existing C code in a Simulink® model, first validate the generated S-Function block.

To validate the replacement of the Lookup blocks, compare the simulation results produced by the Lookup blocks with the results produced by the new S-Function block.

1. Open the validation model.

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

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

  • The lookup table outputs the absolute value of the input.

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

2. Run the validation model.

The figure shows the validation results. The existing C code and the Simulink® table block produce the same output values.

Validate C Code as Part of Simulink® Model

After validating the existing C function code as a standalone component, you can validate the S-function in the model. To complete the validation, use the test harness model.

1. Open the test harness.

2. Run the test harness.

The simulation results match the golden values.

Call C Function from the Generated Code

The code generator uses the TLC file to process the S-Function block like any other block. The code generator can implement expression folding with the S-Function block, an operation that combines multiple computations into a single output calculation.

1. Build the full model.

2. Examine the generated code in PI_Control_Reusable.c.

The generated code now calls the SimpleTable C function.

The figures show the generated code before and after the C code integration. Before the integration, the code calls a generated lookup routine. After the integration, the generated code calls the C function SimpleTable.

For the next example in this series, see Integrate Generated Code with Custom Code in External Environment.

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