generating a string as output of an S-function

Hello,

I'm trying to implement a C-S-function which outputs a string rather than a numerical value, but I couldn't find a way to do it. The length of the string is fixed.

How can i do it? Thanks in advance!

Answer 1

[Purvaja]

Hi,

To display strings as an S-Function output in Simulink, I came across another MATLAB Answer that describes the answer for it.

You can refer it from here: https://www.mathworks.com/matlabcentral/answers/515831-is-it-possible-to-have-strings-as-i-o-for-s-function-blocks

Above answer specifies that input/output of string is a limitation of S-Function for C. You can find the documentation for same in below links:

1. https://www.mathworks.com/help/simulink/ug/simulink-strings.html#mw_588e9a27-6f3e-48f4-ad7a-4c66f6b0526c
2. https://in.mathworks.com/help/simulink/sfg/integrating-existing-c-functions-into-simulink-models-with-the-legacy-code-tool.html#bq4g1es-10:~:text=work1%5B3%5D%5B5%5D)-,Supported%20Data%20Types,-Data%20Type

Instead of directly trying to output in string format, we can use other data type that are compatible. One workaround which worked for me was to output in array of “uint8_T” data type and then using “ASCII to String” block to get in string format.

You can refer following C-Code:

#define S_FUNCTION_NAME displayStringSFunction
#define S_FUNCTION_LEVEL 2
#include "simstruc.h"

#define STR_LEN 16

static void mdlInitializeSizes(SimStruct *S)
{
    ssSetNumSFcnParams(S, 0);           
    ssSetNumInputPorts(S, 0);          
    ssSetNumOutputPorts(S, 1);
    ssSetOutputPortWidth(S, 0, STR_LEN);  /* Output is a 1x16 vector */
    ssSetOutputPortDataType(S, 0, SS_UINT8); /* Data type: uint8 */
    ssSetNumSampleTimes(S, 1);
    ssSetNumRWork(S, 0);
    ssSetNumIWork(S, 0);
    ssSetNumPWork(S, 0);
    ssSetNumModes(S, 0);
    ssSetNumNonsampledZCs(S, 0);
}

static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);
    ssSetOffsetTime(S, 0, 0.0);
}

static void mdlOutputs(SimStruct *S, int_T tid)
{
    uint8_T *y = (uint8_T *)ssGetOutputPortSignal(S,0);
    /* Define the fixed string; note: length must be <= STR_LEN */
    char s[STR_LEN+1] = "Hello, Simulink!";
    int i = 0;
    
    /* Copy characters until the null terminator or STR_LEN is reached */
    while (s[i] != '\0' && i < STR_LEN) {
         y[i] = (uint8_T)s[i];
         i++;
    }
    /* Pad any remaining positions with 0 */
    while (i < STR_LEN) {
         y[i++] = 0;
    }
}

static void mdlTerminate(SimStruct *S) { }

#ifdef MATLAB_MEX_FILE
#include "simulink.c"
#else
#include "cg_sfun.h"
#endif

In Simulink after calling S-function, I got output like below:

For more clarification on the functions used, you can refer to the following resources,

1. S-Function: https://www.mathworks.com/help/simulink/sfg/what-is-an-s-function.html
2. ASCII to string: https://www.mathworks.com/help/simulink/slref/asciitostring.html

Or you can access release specific documentation using these commands in your MATLAB command window respectively:

web(fullfile(docroot, 'simulink/sfg/what-is-an-s-function.html')) 
web(fullfile(docroot, 'simulink/slref/asciitostring.html')) 

Hope this solves your doubt!