S-Function Passing input array to c-function

Hello!

I’m trying to reproduce the example described in https://nl.mathworks.com/help/rtw/tlc/inlining-s-functions.html, but with a function that takes all the input arrays from the block. Here is my code:

wrapsfnc.c

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

extern void times_two(const real_T *inPtr, real_T *outPtr, const int_T numEls);  /* Declare times_two as extern */

/*
 * mdlInitializeSizes - initialize the sizes array
 */
static void mdlInitializeSizes(SimStruct *S)
{

    ssSetNumSFcnParams(S, 0);  // Number of expected parameters
    if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
        return; // Parameter mismatch will be reported by Simulink
    }

    if (!ssSetNumInputPorts(S, 1)) return;
    ssSetInputPortWidth(S, 0, DYNAMICALLY_SIZED);
    ssSetInputPortDataType(S, 0, SS_DOUBLE);
    ssSetInputPortDirectFeedThrough(S, 0, 1);

    if (!ssSetNumOutputPorts(S, 1)) return;
    ssSetOutputPortWidth(S, 0, DYNAMICALLY_SIZED);
    ssSetOutputPortDataType(S, 0, SS_DOUBLE);

    ssSetNumSampleTimes(S, 1);

    ssSetOptions(S, 0);
}

/*
 * mdlInitializeSampleTimes - indicate that this S-function runs
 * at the rate of the source (driving block)
 */
static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);
    ssSetOffsetTime(S, 0, 0.0);
} 


/*
 * mdlOutputs - compute the outputs by calling times_two, which
 * resides in another module, times_two.c
 */
static void mdlOutputs(SimStruct *S, int_T tid)
{
    const real_T *uPtrs = ssGetInputPortRealSignal(S, 0);
    real_T *y =   ssGetOutputPortRealSignal(S, 0);
    const int_T width = ssGetOutputPortWidth(S, 0);
    times_two(uPtrs, y, width); /* Call times_two in mdlOutputs */  
 }
/*
 * mdlTerminate - called when the simulation is terminated.
 */
static void mdlTerminate(SimStruct *S)
{
	// No termination needed
}

#ifdef MATLAB_MEX_FILE /* Is this file being compiled as a MEX-file? */
#include "simulink.c" /* MEX-file interface mechanism */
#else
#include "cg_sfun.h" /* Code generation registration function */
#endif

times_two.c

#ifdef MATLAB_MEX_FILE
#include "tmwtypes.h"
#else
#include "rtwtypes.h"
#endif
void times_two(const real_T *inPtr, real_T *outPtr, const int_T numEls)
{
    for (int_T i = 0; i < numEls; i++)
        outPtr[i] = 2.0*inPtr[i];
}

for compiling, I've used a command

mex wrapsfcn.c times_two.c

The result of the running the Simulink model was quite weird:

To by-pass this problem, I've created a temporary array and copied-by-value the input array:

InputPtrsType uPtrs = ssGetInputPortSignalPtrs(S, 0); // instead of ssGetInputPortRealSignal(S, 0);
...

real_T dblTemp[width];
for(int_T i = 0; i < width; i++)
    dblTemp[i] = *(real_T *)uPtrs[i];
times_two(dblTemp, y, width); /* Call times_two in mdlOutputs */  

and the result was OK:

Could you please explain my initial error and why my by-pass solution finally worked?

Please find attached the simulink files in the Inlined.zip

Answer 1

[Tridib]

Dear @Dimitri,

Your first implementation read the input as a single contiguous real_T* using ssGetInputPortRealSignal, but you never told Simulink that the input elements are guaranteed to be contiguous. By default, RequiredContiguous is false, so the engine may provide a non‑contiguous layout, and indexing u[i] then walks arbitrary memory, hence the tiny “garbage” doubles you observed, which is a symptom of reading the wrong addresses. The “bypass” worked because ssGetInputPortSignalPtrs returns one pointer per element specifically for the non‑contiguous case, so dereferencing each uPtrs[i] accesses the correct element regardless of layout, producing the expected results.

Hope this helps!