Place External C/C++ Code in Generated Code

You can customize code that the code generator produces for a model by placing external code into the specific location of the generated code. For example, you can:

Place external code at the start and end of the generated code for the root model.
Place declaration, body, and exit code in generated function code for blocks in the root model or nonvirtual subsystems.

The functions that you can augment with external code depends on the functions that the code generator produces for blocks that are in the model. For example, if a model or atomic subsystem includes blocks that have states, you can specify code for a disable function. Likewise, if you need the code for a block to save data, free memory, or reset target hardware, specify code for a terminate function. For more information, see Block Target File Methods.

Choose an Integration Approach

Within the Simulink^® modeling environment, two approaches are available for placing external C/C++ code into the generated code:

Adding custom code blocks.
Setting configuration parameters on the Code Generation > Custom Code pane.

Custom code blocks provide you more flexibility to control the positioning of the external code into the generated code. For example, you can control whether you want to insert the external code precisely at the bottom or end of the generated code or at the declaration, body, and exit of the generated function code. Precisely controlling the positing of the external code is necessary for:

Profiling of a system function
Disabling and enabling interrupts
Tracing comments that are required by build environments to correctly integrate system code.

The following table compares the two approaches. Choose the approach that aligns best with your integration requirements.

Requirement	Blocks	Model Configuration Parameters
Include a representation of your external code in the modeling canvas.	✓
Place code in functions generated for root models.	✓	✓
Place code in functions generated for atomic subsystems.	✓
Save code placement in a model configuration set.		✓
Place code at the top and bottom of the header and source files generated for a model.	✓
Place code near the top of the header and source files generated for a model.		✓
Place code inside of the `SystemInitialize` and `SystemTerminate` functions that the code generator creates.		✓
Place code within declaration, execution, and exit sections of the `SystemInitialize` and `SystemTerminate` functions that the code generator creates.	✓
Place code within declaration, execution, and exit sections of the `SystemStart`, `SystemEnable`, `SystemDisable`, `SystemOutputs`, `SystemUpdate`, or `SystemDerivatives` functions that the code generator creates.	✓
Add preprocessor macro definitions to generated code.		✓
Use the custom code settings that are specified for the simulation target.		✓
Configure a library model to use custom code settings of the parent model to which the library is linked.		✓

For more information about how to apply each approach, see Integrate External Code by Using Custom Code Blocks and Integrate External Code by Using Model Configuration Parameters.

Integrate External Code by Using Custom Code Blocks

Custom Code Block Library
Add Custom Code Blocks to the Modeling Canvas
Place External Code in Generated start function

Custom Code Block Library

The Custom Code block library contains blocks that you can use to place external C or C++ code into specific locations and functions of the generated code. The library consists of 10 blocks that add your code to the model header (model.h) and source (model.c or model.cpp) files that the code generator produces.

The Model Header and Model Source blocks can add external code at the top and bottom of header and source files that the code generator produces for a root model. These blocks have two text fields into which you can type or paste code. One field specifies code that you want to place at the top of the generated header or source file. The second field specifies code that you want to place at the bottom of the file.

The remaining blocks add external code to functions that the code generator produces for the root model or atomic subsystem that contains the block. The blocks display text fields into which you can type or paste code that customizes functions that the code generator produces. The text fields correspond to the declaration, execution, and exit sections of code for a given function.

To Customize Code That	Use This Block
Computes continuous states	System Derivatives
Disables state	System Disable
Enables state	System Enable
Resets state	System Initialize
Produces output	System Outputs
Executes once	System Start
Saves data, free memory, reset target hardware	System Terminate
Requires updates at each major time step	System Update

The block and its location within a model determines where the code generator places the external code. For example, if the System Outputs block is at the root model level, the code generator places the code in the model Outputs function. If the block resides in a triggered or enabled subsystem, the code generator places the code in the subsystem Outputs function.

If the code generator does not need to generate a function that corresponds to a Custom Code block that you include in a model, the code generator does one of the following:

Omits the external code that you specify in the Custom Code block.
Returns an error, indicating that the model does not include a relevant block. In this case, remove the Custom Code block from the model.

For more information, see Block Target File Methods.

Note

To avoid a potential mismatch between simulation and code generation results, do not use custom code blocks to read from and write to global Simulink data (signals, states, and block parameters). Instead, use the proper modeling pattern (for example, Data Store Read, Data Store Write, State Reader, and State Writer blocks.)

Add Custom Code Blocks to the Modeling Canvas

To add the Custom Code library blocks to a model:

In the Simulink Library Browser, expand the Simulink Coder node then select the Custom Code block library.
Drag the blocks that you want into your model or subsystem. Drag Model Header and Model Source blocks into root models only. Drag function-based Custom Code blocks into root models or atomic subsystems.

You can use models that contain Custom Code blocks as referenced models. The code generator ignores the blocks when producing code for a simulation target. When producing code for a code generation target, the code generator includes and compiles the custom code.

Place External Code in Generated `start function`

This example shows how to use the System Start block to place external C code into the declaration and execution sections of the start function that the code generator produces for a model containing a Discrete filter.

Create the following model.
Configure the model for code generation.
Double-click the System Start block.
In the block parameters dialog box, in the System Start Function Declaration Code field, enter this code:
```
unsigned int *ptr = 0xFFEE;
```
In the System Start Function Execution Code field, enter this code:
```
/* Initialize hardware */
*ptr = 0;
```
Click OK.
Generate code and a code generation report.

View the generated model.c file. Search for the string start function. You should find the following code, which includes the external code that you entered in steps 4 and 5 into the right places.

  {
    {
      /* user code (Start function Header) */
      /* System '<Root>' */
      unsigned int *ptr = 0xFFEE;
  
      /* user code (Start function Body) */
      /* System '<Root>' */
      /* Initialize hardware */
      *ptr = 0;
    }
  }

Integrate External Code by Using Model Configuration Parameters

Model configuration parameters provide a way to place external C or C++ code into specific locations and functions within code that the code generator produces.

To	Select
Insert external code near the top of the generated `model.c` or `model.cpp` file	Additional code, and enter the external code to insert. If you generate subsystem code into separate files, that code does not have access to external code that you specify with the Additional code parameter. For example, if you specify an include file as a Additional code setting, the code generator inserts the `#include` near the top of the `model.c` or `model.cpp` file. The subsystem code that the code generator places in a separate file does not have access to declarations inside your included file. In this case, consider specifying your external code with the Include headers parameter.
Insert external code near the top of the generated `model.h` file	Include headers, and enter the external code to insert.
Insert external code inside the model initialize function in the `model.c` or `model.cpp` file	Initialize code, and enter the external code to insert.
Insert external code inside the model terminate function in the `model.c` or `model.cpp` file	Terminate code, and enter the external code to insert. Also select the Terminate function required parameter on the Interface pane.
Add preprocessor macro definitions	Defines, and enter a space-separated list of preprocessor macro definitions to add to the generated code. The list can include simple definitions (for example, `-DEF1`) and definitions with a value (for example, `-DDEF2=1`). Definitions can omit the `-D` (for example, `-DFOO=1` and `FOO=1` are equivalent). If a definition includes `-D`, the toolchain can override the flag if the toolchain uses a different flag for defines.
Use the same custom code parameter settings as the settings specified for simulation of MATLAB Function blocks, Stateflow^® charts, and Truth Table blocks	Use the same custom code settings as Simulation Target This parameter refers to the Simulation Target pane in the Configuration Parameters dialog box.
Enable a library model to use custom code settings unique from the parent model to which the library is linked	Use local custom code settings (do not inherit from main model) This parameter is available only for library models that contain MATLAB Function blocks, Stateflow charts, or Truth Table blocks.

To include a header file in an external header file, add #ifndef code. Using this code avoids multiple inclusions. For example, in rtwtypes.h, the following #include guards are added:

#ifndef RTW_HEADER_rtwtypes_h_ 
#define RTW_HEADER_rtwtypes_h_ 
... 
#endif /* RTW_HEADER_rtwtypes_h_ */

For more information on how to add files names and locations of header, source, and shared library files to the build process, see Build Integrated Code Within the Simulink Environment.

Note

The code generator includes external code that you include in a configuration set when generating code for software-in-the-loop (SIL) and processor-in-the-loop (PIL) simulations. However, the code generator ignores external code that you include in a configuration set when producing code with the S-function, rapid simulation, or simulation system target file.

Integrate External C Code into Generated Code by Using Model Configuration Parameters

Open Script

This example shows how to place external code in generated code by using model configuration parameters.

1. Open the model CustomCode.

open_system('CustomCode')

2. Open the Simulink Coder or Embedded Coder app.

3. Open the Model Configuration Parameters dialog box and navigate to the Custom Code pane.

4. On the Additional source code tab, examine the settings for model configuration parameters Additional code and Initialize code.

Additional code specifies a comment and sets the variable GLOBAL_INT2 to -1.
Initialize code initializes the variable GLOBAL_INT2 to 1.

5. Close the dialog box.

6. Generate code and a code generation report.

7. Examine the code in the generated source file CustomCode.c. At the top of the file, after the #include statements, you find the following declaration code.

int_T GLOBAL_INT2 = -1;

The following assignment appears in the model initialize entry-point function. The example specifies this assignment with the Initialize code configuration parameter.

GLOBAL_INT2 = 1;