Matlab S-Function example: Read double input and write to file

#define S_FUNCTION_NAME  single_input_write_to_file
#define S_FUNCTION_LEVEL 2

#include "simstruc.h"
#include <cstdio>  // For file handling

// Define constants
#define SAMPLING_RATE 1000 // 1kHz

static void mdlInitializeSizes(SimStruct *S)
{
    ssSetNumSFcnParams(S, 0);  // No parameters
    ssSetNumContStates(S, 0);  // No continuous states
    ssSetNumDiscStates(S, 0);  // No discrete states
    // 1 input port with 1 element
    if (!ssSetNumInputPorts(S, 1)) return;
    ssSetInputPortWidth(S, 0, 1);  // Input port width = 1
    ssSetInputPortDataType(S, 0, SS_DOUBLE);
    ssSetInputPortComplexSignal(S, 0, COMPLEX_NO);

    ssSetInputPortRequiredContiguous(S, 0, 1);  // Require contiguous input port memory
    // ssSetInputPortDirectFeedThrough(S, 0, 1);  // Direct feedthrough

    // No output ports
    if (!ssSetNumOutputPorts(S, 0)) return;
    // Sample times
    ssSetNumSampleTimes(S, 1);  // Single sample time

    // Work vectors
    ssSetNumRWork(S, 0);  // Real work vector
    ssSetNumIWork(S, 0);  // Integer work vector
    ssSetNumPWork(S, 1);  // Pointer work vector for file pointer
    ssSetNumModes(S, 0);  // Mode vector
    ssSetNumNonsampledZCs(S, 0);  // Zero crossings

    // Make the S-function block capable of being used in a Real-Time Workshop-generated model
    ssSetOptions(S, 0);
}

#define MDL_START
static void mdlStart(SimStruct *S) {
    // Open the file "out.txt" for writing
    FILE *file = fopen("out.txt", "w");
    if (file == NULL) {
        ssSetErrorStatus(S, "Failed to open file 'out.txt'");
        return;
    }

    const real_T *u = (const real_T*) ssGetInputPortSignal(S, 0);  // Input pointer
    real_T input_value = u == nullptr ? -1 : u[0];  // Read the input value

    ssSetPWorkValue(S, 0, file);
}

// Function: mdlInitializeSampleTimes =========================================
// Abstract:
//    Initialize the sample times to be 1ms (1kHz)
static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, 1.0 / SAMPLING_RATE);  // 1kHz sampling rate
    ssSetOffsetTime(S, 0, 0.0);
}

// Not needed for this example
static void mdlOutputs(SimStruct *S, int_T tid) {
    // Do nothing
}

#define MDL_UPDATE
static void mdlUpdate(SimStruct *S, int_T tid)
{
    real_T time = ssGetT(S);  // Get the current simulation time

    // Get the input value
    const real_T *u = (const real_T*) ssGetInputPortRealSignal(S, 0);  // Input pointer
    real_T input_value = u == nullptr ? -1 : u[0];  // Read the input value

    // Get the file pointer from the work vector
    FILE *file = (FILE*) ssGetPWorkValue(S, 0);
    if (file != NULL) {
        fprintf(file, "%f,%f\n", time, input_value);  // Write the input value to the file
    }
}

static void mdlTerminate(SimStruct *S)
{
    // Close the file
    FILE *file = (FILE*) ssGetPWorkValue(S, 0);
    if (file != NULL) {
        fclose(file);
    }
}

// Required S-function trailer
#ifdef MATLAB_MEX_FILE
#include "simulink.c"   // MEX-file interface mechanism
#else
#include "cg_sfun.h"    // Code generation interface
#endif