How to pass a string parameter to a Simulink S-function using the S-Function Builder

Hi all,

I've created a Simulink S function that calls a DLL file containing the model functions in C using the S-Function Builder in Simulink 2021a

However, during the initialization of this model, I need to pass a string that points to a JSON file containing all the model parameters. Currently, I've hard-coded the file path for testing, and it works. However, I would like to be able to pass this as a parameter.

    // TODO: make params_file_path a parameter
    const char* params_file_path = "default_parameters.json";
    params = load_parameters(params_file_path);

When I click on the parameter data types in the S-Function Builder, I see options for double, single, int8, int16, int32, uint8, uint16, uint32, and boolean, but I do not see an option for strings or characters.

I assume that I will need to manually modify the autogenerated wrapper.c file, but I would appreciate some guidance on how to do this.

Regards,

Answer 1

[Sarthak]

Hi Marco,

As per my understanding, the data type of the ports and parameters for the S-Function, can only be Simulink built-in numeric data types. You can try to create a character array and pass the parameter value as you have done since there is no direct option for a string datatype.

const char* params_file_path = "default_parameters.json"; 
params = load_parameters(params_file_path); 

There can also be fixdt data types and bus types for ports. The "int64" and "uint64" data types are not supported for parameters as mentioned in the following documentation.

https://www.mathworks.com/help/simulink/sfg/s-function-builder-dialog-box.html

[Marco Furlan]

Hi Sarthak,

Thanks for your answer. I also found very useful the answer to this other question:

https://www.mathworks.com/matlabcentral/answers/362410-how-do-i-use-the-character-or-string-data-type-in-a-simulink-s-function

I was able to pass in the full path to the file using a uint8 array of characters. I just had to convert the string to uint8 usign the uint8 function.

Then, inside the S-Funcition Builder Editor for C code, I convert it back to a character array:

const char* params_file_path_char = params_file_path;