MATLAB: Can we use matlab engine from a inside a function in C++

cenginefunction

I compiled a C++ program to sort double values(matlab does the sorting). it works fine. however when i move the code related to matlab computation to a function, it gives erroneous results.
following is the code with both the formats in a single file. when i execute matlab functions from main it works, when i execute the matlab part from a function(call_matlab_processing) it gives random results.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#include <iostream>
#include <vector>
#define  BUFSIZE 256
int call_matlab_processing(double double_array[], int size_double_array,double * sorted, double * indices_cpp)
{
  for (int i=0;i<size_double_array;i++)
  {
    std::cout << "double array at "<< i << " "<< double_array[i]<<std::endl;
  }
  Engine *ep;
  mxArray *T = NULL, *result = NULL, *indices=NULL;
  /*
   * Call engOpen with a NULL string. This starts a MATLAB process
     * on the current host using the command "matlab".
   */
  if (!(ep = engOpen(""))) {
    fprintf(stderr, "\nCan't start MATLAB engine\n");
    return 1;
  }
  T = mxCreateDoubleMatrix(1,size_double_array, mxREAL);
  memcpy((void *)mxGetPr(T), (void *)double_array, sizeof(double_array));
  /** Place the variable T into the MATLAB workspace */
  engPutVariable(ep, "T", T);
  engEvalString(ep, "[D I] = sort(T,'descend')");
  engEvalString(ep, "dlmwrite('myFile.txt',D)");
  result = engGetVariable(ep,"D");
  indices = engGetVariable(ep,"I");
  sorted=(double *)mxGetData(result);
  indices_cpp=(double *)mxGetData(indices);
/*  for (int i=0;i<10;i++)
  {
    std::cout << "sorted= " <<*sorted << " indices= " << *indices_cpp <<std::endl;
    sorted++;
    indices_cpp++;
  }*/
  mxDestroyArray(T);
  mxDestroyArray(result);
  mxDestroyArray(indices);
  engEvalString(ep, "close;");
  engClose(ep);
  return 0;
}
int main()
{
  int size_double_array=10;
  double double_array[10]={ 0, 5, 2, 7, 3, 9, 1, 6, 8, 4 };
  double * sorted;
  double * indices_cpp;
  Engine *ep;
  mxArray *T = NULL, *result = NULL, *indices=NULL;
  if (!(ep = engOpen(""))) {
    fprintf(stderr, "\nCan't start MATLAB engine\n");
    return 1;
  }
  T = mxCreateDoubleMatrix(1,size_double_array, mxREAL);
  memcpy((void *)mxGetPr(T), (void *)double_array, sizeof(double_array));
  engPutVariable(ep, "T", T);
  engEvalString(ep, "[D I] = sort(T,'descend')");
  engEvalString(ep, "dlmwrite('myFile.txt',D)");
  result = engGetVariable(ep,"D");
  indices = engGetVariable(ep,"I");
  sorted=(double *)mxGetData(result);
  indices_cpp=(double *)mxGetData(indices);
  mxDestroyArray(T);
  mxDestroyArray(result);
  mxDestroyArray(indices);
  engEvalString(ep, "close;");
  engClose(ep);
//  int result=call_matlab_processing(double_array,10,sorted,indices_cpp);
    for (int i=0;i<10;i++)
    {
      std::cout << "sorted= " <<*sorted << " indices= " << *indices_cpp <<std::endl;
      sorted++;
      indices_cpp++;
    }
  std::cout << "Done"<< std::endl;
  return EXIT_SUCCESS;
}

Best Answer

You have a fundamental flaw in your code. You free the memory behind the pointers that you use downstream in your code. E.g.,
  result = engGetVariable(ep,"D");  // <-- Creates the mxArray result, including the data area
  indices = engGetVariable(ep,"I");  // <-- Creates the mxArray indices, including the data area
  sorted=(double *)mxGetData(result);  // <-- Gets pointer to data area of result
  indices_cpp=(double *)mxGetData(indices);  // <-- Gets pointer to data area of indices
  mxDestroyArray(T);
  mxDestroyArray(result);  // <-- Wipes out result, *and* the memory behind sorted
  mxDestroyArray(indices);  // <-- Wipes out indices, *and* the memory behind indices_cpp
// So at this point in the code, both of the pointers sorted and indices_cpp are *invalid*
  engEvalString(ep, "close;");
  engClose(ep);
// Code downstream of here uses *invalid* pointers. Anything can happen.
// Could get lucky good results, or random bad results, or even a program bomb.
//  int result=call_matlab_processing(double_array,10,sorted,indices_cpp);
    for (int i=0;i<10;i++)
    {
      std::cout << "sorted= " <<*sorted << " indices= " << *indices_cpp <<std::endl;
      sorted++;
      indices_cpp++;
    }
You need to delay executing these two lines until after you are done using their data areas:
  mxDestroyArray(result);
  mxDestroyArray(indices);
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