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Integrate C Code Using C Caller Blocks

You can integrate new or existing C code into Simulink® using the C Caller block. To create custom blocks in your Simulink models, the C Caller block allows you to call external C functions specified in external source code and libraries. The advantages of the C Caller block are:

  • Automated integration of simple C functions

  • Integration with Simulink Coverage™, Simulink Test™, and Simulink Design Verifier™

  • Integration with Simulink Coder™

The C Caller block allows you to bring C algorithms into Simulink. To model dynamic systems, use the S-Function Builder instead. Next steps describe the workflow to integrate C code into Simulink using the C Caller block.

Specify Source Code and Dependencies

Specify your external source code file that contains your C functions.

  1. From Simulink toolstrip, open the Configuration Parameters.

  2. In the left pane, select Simulation Target.

  3. To enable code parsing by the C Caller block, ensure that the Import custom code box is selected.

    The directories and file paths can be absolute and relative file paths to model directories or to the current working directory. See Specify Relative Paths to Your Custom Code (Stateflow).

  4. Select Header file and enter the name of your header file with the #include tag.

  5. Under Additional build information, select Include directories, and enter the folders where additional build information, such as header files, are stored.

  6. Select Source files and enter the path and the name of the source file. If the model and the source files are in different directories, enter the directory that contains the source file before the file name.


If a function is declared in the header file but not implemented in the source code, an empty stub function is automatically generated to simulate and compile the model.

Define Default Function Array Layout

You can specify the order of how your matrix data is stored in Simulink. Matrix data passed to and from your C functions is converted to the default function array layout you specify. If the function array layout is not specified, the matrix data is passed through the C Caller in the same order of your Simulink data, and computational errors may occur due to row-column major disarrangement. Ensure that you follow the same default function array layout for all Simulink data.

  • Column-Major — The C Caller block handles Simulink data in column-major order. Suppose that you have a 3-by-3 matrix. In the C Caller block, this matrix is stored in this sequence: first column, second column, and third column.

  • Row-Major — The C Caller block handles Simulink data in row-major order. Suppose that you have a 3-by-3 matrix. In the C Caller block, this matrix is stored in this sequence: first row, second row, and third row.

  • Any — Array data can be stored both in column-major and row-major order in the C Caller block. As a result, you can generate code both in column-major and row-major settings.

  • Not specified — Array data can be stored in both column-major and row-major order. Compared to Any setting, you can only generate code in column-major setting.

To learn more about the row-major and column-major array layouts in Simulink, see Default function array layout.

  1. Select an array layout option under Default Array Function Layout.

  2. If you need to apply a specific array layout to some of the functions in your code, click Specify by Function to select these functions.

  3. Click Apply to accept your changes.

  4. Click OK to close the Configuration Parameters.

Call C Caller Block and Specify Ports

You can start your custom C code integration into Simulink by typing C Caller in the Simulink canvas. Alternatively, drag a C Caller block from the Library Browser > User-Defined Functions. Double-click the block to open the Block Parameters dialog box to see the names of your functions and port specifications.

  1. Click on the Refresh custom code to import your source code and its dependencies.

  2. Your C functions are displayed under Function Name. If you can't see your full list of functions, click on the to reimport your source code.

  3. To view function declarations or input/output variables to your functions in the header file, click the Go to function declaration to navigate the source files.

  4. To change source files and their dependencies, or to define and select function array layouts, click the Custom code settings to open the Simulation Target tab in Configuration Parameters.

Map C Function Arguments to Simulink Ports

You can map C function arguments from your source code to Simulink ports using the Port specification table in the C Caller block and by creating a FunctionPortSpecification object through the command line . In your source code, the header file includes the C function arguments to be connected to Simulink ports.

extern void mean_filter(const unsigned char* src,
                           unsigned char* dst,
                           unsigned int width, unsigned int height,
                           unsigned int filterSize);

Port specification shows the details of your arguments and how they connect to your C Caller block in Simulink.

Name — Specifies the name of input and output arguments. Name is the function argument or parameter name as defined in your C functions from source code. This column is for reference purposes only.

Scope — Specifies how C function arguments map to the Simulink scope. Your arguments have default scopes depending on the function definition and you can change the scopes depending your function definition in the source code.

Simulink ScopeScope to Block Mapping
InputBlock input port
OutputBlock output port
InputOutputBlock input and output port
GlobalGlobal variable used by the block
ParameterBlock tunable parameter
ConstantConstant value

When you have a constant qualifier definition such as const double *u, the argument can only be an input or a parameter. When there is no constant qualifier, the argument is an output by default, and you can change it to an Input, InputOutput or to a Parameter scope. In this case, ensure that the C function does not modify the memory pointed by the pointer. If the argument is of an Output type, every element pointed by this pointer should be reassigned in every call for this function.

C Argument

Simulink Scope

Function return


double u

Input, Parameter, Constant

double u[]

double u[][2]

double u[2][3]

Output (default), Input, Parameter

double *u

Output (default), InputOutput, Input, Parameter

const double *u

const double u[]

const double u[][2]

const double u[2][3]

Input (default), Parameter

Use the InputOutput port to map an input passed by a pointer in your C functions. Ports created using an InputOutput port have the same name for input and output ports. InputOutput ports enables reuse of buffer for input and output ports. This may optimize the memory use depending on the signal size and the block layout.

To map C function arguments to an InputOutput port, define the variable as a pointer in your function definitions.

extern void mean_filter(unsigned char* src,
                           unsigned int width, unsigned int height,
                           unsigned int filterSize);

Then, select the port specification to the InputOutput scope in the Port Specification table, and assign the resulting function output to the input variable in the custom function.

You can use global variables in your custom code map them to the appropriate Simulink scope. To enable the use of global variables in your model, select Enable custom code globals as function interface from Model Settings > Configuration Parameters > Simulation Target. You can map the global variables to an Input, Output, InputOutput or Global scope on the C Caller block. The availability of the these scopes depend on the use of the global variable in your custom code.

A Global scope enables you to transfer data between custom code and the C Caller block and lets you use the global variable during calculations on the block. Values transferred using Global scope are not visible on the block interface. This table shows example code snippets and their default and available ports.

Example CodeSimulink Scope

double data;

void foo(void)
        int temp = data;  

Global variable data only reads the variable data. Available scopes are:

Input (default)


double data;

void bar(void)
        data = 0;

Data is written to a global variable. Available scopes are:

Output (default)



double data;

void foo2(void)
        data = data + 1;

Data is both read and written on a global variable. Available scopes are:

Global (default)



Label — Indicates the label for the corresponding argument in a Simulink block. By default, your argument label is the same as the argument name, unless you change it.

Simulink ScopeSimulink Port Label

input, output

Port name
inputoutputPort name in both input and output port
GlobalPort name and global variable name


Parameter name


Expression for the constant value.

size expressions using input argument names, for example size(in1,1)

Type — Demonstrates the match between the Simulink data type and the C function argument data type.

C Argument Data TypeSimulink Data Type
signed charint8
unsigned charuint8
charint8 or uint8, depending on the compiler
unsigned int*uint32
short *int16
long *int32 or fixdt(1,64,0), depending on the operating system
typedef struct {…} AStruct**Bus: AStruct
typedef enum {..} AnEnum**Enum: AnEnum

* If the C Caller takes an integer type, for example, int16_t, you can modify it to a fixed-point type with matching base type, for example to fixdt(1, 16, 3).

** The C Caller sync button prompts you to import struct or enum types used by a C function as Simulink bus and enumeration types.

Size — Specifies the data dimensions in the argument.

C Argument DimensionsSimulink Port Dimensions

double u

scalar (1)

double u[]

double u[][2]

inherited (-1) (default)

If the argument is for an output port, the size should be specified. The size of an output port cannot be inherited.

double *u

inherited (-1) (default)

If the argument is for an inputoutput port, the size cannot be inherited even though the size in the output port can be inherited.

For global variables, size is scalar (1).

double u[2][3]

Size is [2, 3].

Create a FunctionPortSpecification Object and Edit C Caller Block Properties

To change Port Specification table properties programmatically, you can create a FunctionPortSpecification object and modify its properties. To create a FunctionPortSpecification object for a selected C Caller block in a model, type in the command line:

myCCallerConfigObj = get_param(gcb, 'FunctionPortSpecification')
myCCallerConfigObj = 

  FunctionPortSpecification with properties:

        CPrototype:  'real_T add(real_T u1, real_T u2);'
    InputArguments:  [1×2 Simulink.CustomCode.FunctionArgument]
    ReturnArgument:  [1×1 Simulink.CustomCode.FunctionArgument] 
    GlobalArguments: [1×0 Simulink.CustomCode.FunctionArgument]
The CPrototype property is read-only, and shows the declaration of C function input variables. The InputArgument and ReturnArgument properties create a FunctionArgument object that you can further edit its properties according to the rules defined for Port Specification table above. You can See FunctionPortSpecification to learn more.

To modify the global arguments in a C Caller block, create a handle to the GlobalArguments object using getGlobalArg and modify its properties.

Create a Custom C Caller Library

You can create a library model to group your C Caller blocks and keep your models organized.

  1. Open a new library model. On the Simulation tab, select New > Library.

  2. On the Modeling tab, under Design, click Simulation Custom Code.

  3. Select C or C++ in the Language option, depending on your code, and ensure the Import custom code box is selected.

  4. Follow the instructions in Specify Source Code and Dependencies to add your source files and their dependencies.

  5. Create C Caller blocks to call C functions.

  6. To insert a block from your library model to a Simulink model, simply drag the block into your model.

Generate Debug Symbols for Custom Code

To attach an external debugger to the MATLAB® process, and debug external C code, generate the debug symbols using:

After you turn this setting on and update your model, your debug symbols are generated and you can attach an external debugger to your MATLAB process.

Turn this setting off using:



  • Global Variables — Global variables as function input outputs do not support multi dimensional arrays.

  • Initialization/Termination of Custom Code Settings — If you need to allocate and deallocate memory for your custom code, insert allocate and deallocate in the Initialize function and Terminate function fields of custom code settings.

  • Complex Data Support — The C Caller block does not support complex data types in Simulink.

  • Continuous Sample Time — The C Caller block does not support continuous sample time.

  • Variable Arguments — Variable arguments in C are not supported, for example, int sprintf(char *str, const char *format, ...).

  • C++ Syntax — The C Caller block does not support native C++ syntax directly. You need to write a C function wrapper to interface with C++ code.

To test your models that includes C Caller blocks, see Test Integrated C Code (Simulink Test).

See Also

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