Difference between c function and s function

Is there any difference between c function and s function block?In the mathlab documentation I saw as

"The C Function block supports initializing persistent data and calling external functions from the block dialog. The C Function block supports only initializing and terminating persistent data; the block does not support updating the data during simulation. To model a dynamic system with continuous states, use an S-Function block. "

Can anyone explain me this difference with some example? Or whether this difference is no longer there in new version(2024a) of Matlab?

Answer 1

[Jaimin]

Hello @AKHILA

The distinction between the “C Function block” and the “S-Function block” in Simulink can be clarified by examining their capabilities and intended use cases:

C Function Block

• Purpose: The C Function block is intended for embedding custom C code into a Simulink model. It is generally used for straightforward computations and invoking external C functions.
• Initialization: It allows for the initialization of persistent data, enabling data to maintain its value across function calls.
• Limitations: The block does not allow for updating persistent data during simulation, meaning you cannot have persistent states that evolve over time within the block.
• Use Case: Best suited for static computations or functions that do not need state updates.

S-Function Block

• Purpose: The S-Function block is highly versatile and robust, enabling the creation of custom blocks that can manage complex behaviors, including dynamic systems with states.
• Initialization and Termination: It allows for both the initialization and termination of persistent data.
• State Updates: In contrast to the C Function block, the S-Function block can handle both continuous and discrete states, making it ideal for modelling dynamic systems.
• Use Case: Perfect for modelling intricate dynamic systems that necessitate state updates throughout the simulation.

Example Scenario

• C Function Block Example: Imagine you have a C-written mathematical function that computes the square root of a number. The C Function block can be used to call this function within a Simulink model. The computation is static, with no need for state updates.
• S-Function Block Example: Consider simulating a simple harmonic oscillator, a system with continuous states (position and velocity) that evolve over time. An S-Function block would be used to implement the differential equations that describe the oscillator's motion, as it supports state updates during the simulation.

To check “C-Function Block” and “S-Function Block” working in MATLAB R2024a please refer this MathWorks Documentation

C-Function Block: https://www.mathworks.com/help/simulink/integrate-external-c-algorithms-using-c-function-block.html

S-Function Block: https://www.mathworks.com/help/simulink/sfg/what-is-an-s-function.html

I hope this will be helpful.

[AKHILA]

Thanks @Jaimin, thank you for your answer. I was experimneting the block with an example of integrater

double integrate1(double input)

{

static double state = 0; // Persistent state variable

state = state+ input; // Update state

return state; // Output state

}

Here in this code, we are updating the value of state, and it's working in c function. That's why I got confused. Can you help me in clarifying my doubt in this case