S-Function String Parameter C++

Hey,

I've trouble reading the value of a string passed to MATLAB as an S-Function parameter in C++:

std::string str(mxArrayToString(ssGetSFcnParam(S,0)));

Throws the following exception:

Program: C:\windows\SYSTEM32\MSVCP140D.dll
File: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\xstring
Line: 1181

Expression: invalid null pointer

For information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application)
MATLAB.exe has triggered a breakpoint.

Exception thrown at 0x00007FFB6C9A0A63 (test.mexw64) in MATLAB.exe: 0xC0000005: Access violation reading location 0x0000000000000000.
> throw_segv_longjmp_seh_filter()
throw_segv_longjmp_seh_filter(): invoking THROW_SEGV_LONGJMP SEH filter
> throwSegvLongjmpFilter()
MATLAB.exe has triggered a breakpoint.

The program '[19672] MATLAB.exe' has exited with code 0 (0x0).

For debugging purposes I splitted it up:

const mxArray* pArrayValue = ssGetSFcnParam(S,0);
const char* pCharArray = mxArrayToString(pArrayValue);
std::string str(pCharArray);

pCharArray points to:

0x000000000000000

I am using VS 14.0.25431.01 Update 3 on Win 10 Pro 17134.165 with Matlab 2018a. I found code in our companies repository where both of the above code snippets are working, in MATLAB 2013b, what am I doing wrong? Is this syntax invalid in 2018a?

Edit 31.08.18:

I added an example:

#define S_FUNCTION_LEVEL 2
#define S_FUNCTION_NAME  sfun_pass_string

#include "simstruc.h"
#include <string>

static void mdlInitializeSizes(SimStruct *S)
{
    ssSetNumSFcnParams(S, 1);

      ssSetSFcnParamTunable(S, 0, SS_PRM_NOT_TUNABLE);

      if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
          return;
      }

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

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

      ssSetNumSampleTimes(S, 1);

      ssSetOptions(S, SS_OPTION_CALL_TERMINATE_ON_EXIT);

}

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

#define MDL_START
static void mdlStart(SimStruct *S)
{
    mexPrintf("class of parameter: %s", mxGetClassName(ssGetSFcnParam(S,0)));
    const mxArray* pArrayValue = ssGetSFcnParam(S,0);
    const char* pCharArray = mxArrayToString(pArrayValue);
    std::string string_1(pCharArray);
    mexPrintf(string_1.c_str());
}

static void mdlOutputs(SimStruct *S, int_T tid)
{

}                                                

static void mdlTerminate(SimStruct *S)
{
}                                      

#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

[James Tursa]

The obvious question is, what class of mxArray is pArrayValue? Do you even know if it is of class char?

[Nils Gerke]

You might be right.

mxArray test = *pArrayValue;

Gives me a compiler error:

Error  C2079  'test' uses undefined struct 'mxArray_tag'

I've only one Parameter that I pass to the S-Function, in the model I write the string in double quotes.

I'm a C++ beginner, but still I really don't get whats wrong with that code. If I pass a string to the simulink model I would suspect (based on the documentation) that

ssGetSFcnParam(S,0)

gives me back my string as mxArray and that

mxArrayToString()

would convert it to a std::string. Or is this assumption wrong?

[Titus Edelhofer]

Nils,

mxArray test = ...

will not work, since you always have to use pointers ...

Titus

Answer 1

[Titus Edelhofer]

Hi Nils,

I didn't take a look into the zip file but could it be that you did the same mistake as I, namely, really passing a string ("hello") to the S-function instead of the char array ('hello')? If I use the string class, then I see the same problem. When passing an old fashioned char array, then everything works as expected.

Titus

[Titus Edelhofer]

PS: yes, you did. I just tried again and now the class is "char" instead of "string" ... :)

[Nils Gerke]

That did the trick, I marked your answer as accepted. But never the less I would suspect

mxArrayToString()

to be able to process string input as well as char array input. If this is not possible it would be nice to throw a proper exception. Now all that happens is that MATLAB crashes with a Fatal Exception.

[Titus Edelhofer]

Yes and no: yes, the fatal exception is indeed "undesirable" :). And yes again: would be good if the function also accepts strings and not only char arrays. Now for the no: this is the C world, and it's (unfortunately?) the users responsibility to check return values.

It's the further processing of the NULL pointer that causes the exception. In the doc you see:

Returns C-style string. Returns NULL on failure. Possible reasons for failure include out of memory and specifying an array that is not an mxCHAR array.

Titus

Answer 2

[Titus Edelhofer]

Hi Nils,

just to be sure: did you check the number of parameters you passed? I guess you are sure that there is at least one parameter, but may helps to check, because otherwise it looks fine. Try

int nParam = ssGetSFcnParamsCount(S);
mexPrintf("Number of parameters: %d\n", nParam);

Titus

[Nils Gerke]

Yes I'm sure that I'm passing one parameter. Today I'll work on a code snippet which hopefully helps you to reproduce my problem.

[Titus Edelhofer]

And coming back to James comment: you are probably also sure, that you indeed pass a string? You could add

mexPrintf("class of parameter: %s\", mxGetClassName(ssGetSFcnParam(S,0)));

to verify.

Titus

[Nils Gerke]

Returns:

class of parameter: string

[Titus Edelhofer]

O.K., then I guess we need to wait to see a reproducing example ...

[Nils Gerke]

I added a reproducing example, see code and .zip of post 1