C Function Block Configuration

When to Use

• Calling or wrapping C/C++ code or libraries in Simulink
• Creating, configuring, or using a C Function block
• Integrating custom code, legacy code, or external C/C++ into a model
• Integrating a C++ class into a Simulink model or test bench
• Configuring inputs, outputs, or parameters for a C++ algorithm in a block

When NOT to Use

• Building general Simulink models without C/C++ integration
• MATLAB Function blocks (Embedded MATLAB) — those use MATLAB syntax, not C/C++
• Code generation workflows (Embedded Coder, Simulink Coder) — this skill is for simulation-time configuration only

Rules

• Derive each symbol's Simulink type from the corresponding C/C++ function parameter/return type (e.g. float → single, bool → boolean, char*/char[] → string (R2024b+)). However, when replacing an existing block (e.g. an S-Function), use the types already on that block first — for example, what looks like int32 from C may actually be fixdt(1,32,3) or another fixed-point type in the existing model.
• MANDATORY for C++ classes: When the user's code defines a C++ class, you MUST first attempt to declare a Persistent symbol of Class: ClassName and call class methods directly in the code sections (e.g. obj.Method(args)). If the class has an init method (not a constructor), call it in StartCode. Only fall back to wrapper/handler functions or opaque pointer patterns if the class approach fails to compile.
• Prefer relative paths for custom code files (sources, headers, libraries, search directories). Use paths relative to the model file location (e.g. 'myFunctions.h', './src/myLib.cpp') rather than absolute paths. This ensures portability across machines and users.

Workflow Overview

1. Add the C Function block — use model_edit with type "C Function" and set CustomCodeSettingLocation (default: "BlockSettings")
2. Configure custom code settings — point to headers/sources either on the block or model config (see references/custom-code-config.md)
3. Set code sections — configure OutputCode, StartCode, etc. on the block
4. Configure symbols via SymbolSpec — use model_query_params to get the SymbolSpec, then evaluate_matlab_code for object operations

SymbolSpec API (Port & Symbol Specification)

The SymbolSpec object defines the block's inputs, outputs, persistent state, constants, and parameters.

Getting the object

Query with model_query_params, then operate via evaluate_matlab_code:

obj = get_param('myModel/MyCFunction', 'SymbolSpec');

Adding, getting, deleting symbols

symObj = obj.addSymbol('varName');       % adds with default: Input, double, size '1'
symObj = obj.getSymbol('varName');       % get one by name
allSyms = obj.Symbols;                  % get all symbols
obj.deleteSymbol('varName');

Note: addSymbol always creates with defaults (double, Input, size '1'). When re-adding a deleted symbol, explicitly set all non-default properties (Type, Scope, Size) — prior properties are not retained.

Symbol Properties

Property	Values	Default	Notes
Name	identifier string	(from addSymbol)	Variable name in code; for class types includes constructor args
Scope	'Input', 'Output', 'InputOutput', 'Persistent', 'Constant', 'Parameter'	'Input'	See Scope Details below
Type	'double', 'single', 'int8'...'uint64', 'Boolean', 'string', 'Bus: BusName', 'Enum: EnumName', 'Class: ClassName', 'AliasTypeName', or fixedpoint	'double'
Size	dimension string	'1'	e.g. '1', '3', '[2,3]', 'size(u1)'
Label	port label / value	same as Name	For Constant: the literal value
PortNumber	uint32	auto-assigned	Port ordering

Scope Details

Scope	Description	Port?
Input	Block input port	Yes (inport)
Output	Block output port	Yes (outport)
InputOutput	Read-write pass-through (always first in port numbering)	Yes (both)
Persistent	State that persists across time steps (also used for class instances)	No
Constant	Compile-time constant (scalar only)	No
Parameter	Tunable parameter from workspace/mask	No

Setting Parameter values

After setting Scope to 'Parameter', set the value by configuring the block with the parameter name as the key.

C++ Class Types

Class instances are declared as Persistent scope symbols with Type = 'Class: ClassName'.

Constructor arguments are specified in the Name property:

obj = get_param('myModel/CFunction', 'SymbolSpec');

% Default constructor (no args)
sym = obj.addSymbol('myObj');
sym.Scope = 'Persistent';
sym.Type = 'Class: MyClass';

% Constructor with arguments (args must be Parameter or Constant scope symbols)
sym = obj.addSymbol('myObj(p)');
sym.Scope = 'Persistent';
sym.Type = 'Class: MyClass';

Rules for class types:

• Class must be defined in an external header (included via custom code settings) — NOT inside code sections
• Cannot create class instances as local variables in code sections — applies when CustomCodeSettingLocation = 'ModelConfigurationParameters' (always parsed), or 'BlockSettings' with ParseBlockCode = 'on'
• Constructor arguments must be Parameter or Constant scoped symbols only
• Function calls in constructor arguments are not allowed
• new operator is not allowed when block code is parsed
• Private/protected members cannot be accessed from code sections
• Class type does not support save/restore (SimSnapshot)

Access class methods in code sections: y = myObj.compute(u1);

Size Expressions

• Scalar: '1'
• Vector: '3' or '[3,1]'
• Matrix: '[2,3]'
• Dynamic from input: 'size(u1)', 'size(u1,1)', 'size(u1) + 1'

Rules:

• size() IS allowed in output dimensions (compute output size from inputs)
• size() is NOT allowed in input dimensions
• Output size CANNOT be '-1' (inherited) — must be explicit or use size() expression
• Input size CAN be '-1' (inherited)
• Size must evaluate to integer values
• Nested size(size(...)) is NOT supported

Restrictions

• #include directives NOT allowed inside code sections (Start/Output/Terminate) — use custom code settings
• Start/Terminate code CANNOT access Input or Output scoped symbols
• Constant scope must be scalar
• Library functions (e.g. isalnum) not supported inside code sections when ParseBlockCode = 'on' or CustomCodeSettingLocation = 'ModelConfigurationParameters' — use external calls via header. No restriction under default R2026a+ settings (BlockSettings + ParseBlockCode = 'off')
• When using a Simulink.NumericType with DataScope = 'Exported' in symbols, or any Simulink.NumericType in code sections, its IsAlias property must be true
• When CustomCodeSettingLocation = 'UpdateBuildInfo', GenerateCodeAsIs is forced 'on'
• Class definitions NOT allowed inside code sections — define in external header
• Variable-size signals are not supported

Full Example: Block-Level Custom Code with C++ Function

% 1. Add C Function block with block-level custom code
%    Use model_edit: add_block type "C Function", set CustomCodeSettingLocation,
%    SimCustomHeaderFile, SimCustomSourceFile, and OutputCode on the block.

% 2. Configure symbols
obj = get_param('mMyTest/C Function', 'SymbolSpec');
obj.addSymbol('u1');
obj.addSymbol('u2');
u2obj = obj.getSymbol('u2');
u2obj.Type = 'int32';
u2obj.Size = '3';
obj.addSymbol('y');
yobj = obj.getSymbol('y');
yobj.Scope = 'Output';
yobj.Size = 'size(u2)';
obj.addSymbol('state');
stateobj = obj.getSymbol('state');
stateobj.Scope = 'Persistent';
save_system('mMyTest');
close_system('mMyTest', 0);

See references/examples.md for additional patterns (model-level custom code, C++ class with constructor args). See references/custom-code-config.md for detailed parameter tables and custom code location settings.

---

Copyright 2026 The MathWorks, Inc.

---