How to set the initial value for C caller block?

Hi, I'm using C caller blocks to simulate my C code in simulink.

There is a variable which updates its state with its previous value.(It works the same as integral term in PID controller) The code is below.

void GetVar(double a, double* res)
{
    res = res+a;
}

To do this, I need to set the initial value of res, however, I don't know how to set the initial value.

Thanks.

[Walter Roberson]

That code does not update a state. That code takes the value of a and adds sizeof(double) times a bytes to the pointer address stored in res storing the result (locally) in res with nothing outside the routine getting updated.

Were you thinking of code more like

void GetVar(double a, double* res)
{
    *res = *res+a;
}

Answer 1

[Riya]

Hello lim daehee,

As per my understanding, the initial value of the variable ‘res’ in your C code is to be set.

In this case, you can modify the code as follows:

void GetVar(double a, double* res) 
{ 
    static double initial_value = 0.0; // Set the initial value here 
    static int initialized = 0; // Flag to check if the initial value is set 
    
    if (!initialized) 
        { 
            *res = initial_value; 
            initialized = 1; 
            } 
        
        *res = *res + a; 
        } 

In the modified code, we introduce a ‘static’ variable ‘initial_value’ to hold the initial value of ‘res’. By default, ‘initial_value’ is set to 0.0, but you can change it to any desired initial value.

Additionally, we introduce a ‘static’ flag variable initialized to ensure that the initial value is set only once. The first time ‘GetVar’ is called, it checks if ‘initialized’ is 0. If it is, the initial value is assigned to ‘res’, and initialized is set to 1 to indicate that the initial value has been set.

By using this approach, the initial value of ‘res’ will be set only once, and subsequent calls to ‘GetVar’ will update ‘res’ based on its previous value.

In case you further require a more specific solution tailored to your case, you can provide more information.

You can also refer the following documentation about how to initialize static variable in C caller blocks during simulation for more details:

https://www.mathworks.com/matlabcentral/answers/1678144-how-to-initialize-static-variable-in-c-caller-blocks-during-simulation

I hope it helps!