c++ - error using libmatio / reading mat-files

Question

I'm reading data from mat-files with code below. The code starts to search all subdirectories for mat-files like Dog_1/Dog_1_interictal_segment_1/0001.mat and transposing the matrix data into a csv file.

The problem occurs at line 139 when memory is beeing freed twice using Mat_VarFree(). The program aborts with

*** Error in `./a.out': double free or corruption (fasttop): 0x0000000000b66470 ***

as mentioned, when Mat_VarFree is called twice. If I remove one of the Mat_VarFree's a memory leak is born.

What can I do??

Thanks

#include <regex>
#include <iostream>
#include <boost/filesystem.hpp>
#include <matio.h>
#include <string>
#include <fstream>

using namespace std;
using namespace boost::filesystem;

// global def
smatch m;

regex r (
  "^"
  "(Dog|Person)_"
  "([0-9])_"
  "(interictal|preictal|test)_"
  "(segment)_"
  "(0{0,3}([1-9][0-9]{0,3}))"
  "(\\.mat)"
  "$"
);

path p("csv");

int main(int argc, char *argv[])
{
  // mr proper
  if (is_directory(p))
  {
    for (directory_iterator i(p), j; i!=j; ++i)
    {
        remove_all(i->path());
    }
  }
  else
  {
    create_directory(p);
  }

  // work
  for (recursive_directory_iterator i("."),j; i!=j; ++i)
  {
    if (!is_directory(i->path()))
    {
      if (regex_match(i->path().filename().string(), m, r))
      {
        // for later use
        string m0 = m[0];
        string m1 = m[1];
        string m2 = m[2];
        string m3 = m[3];
        string m4 = m[4];
        string m5 = m[5];
        string m6 = m[6];

        // mat file
        auto mfilename = i->path().string();
        auto mfile = Mat_Open(mfilename.c_str(), MAT_ACC_RDONLY);
        if (NULL == mfile)
        {
          cerr
            << "Error opening MAT file '" 
            << i->path().filename().string()
            << "'"
            << endl;

            return EXIT_FAILURE;
        }

        // mat var
        auto mvarname = m3 + "_" + m4 + "_" + m6;
        auto mvar = Mat_VarReadInfo(mfile, mvarname.c_str());
        if (NULL == mvar)
        {
          cerr
            << "Error reading MAT var '"
            << mvarname
            << "' in file '"
            << mfilename
            << "'"
            << endl;

           Mat_Close(mfile);
           mfile = NULL;

           return EXIT_FAILURE;
        }

        // mat struct
        auto mstructname = "data";
        auto mstruct = Mat_VarGetStructFieldByName(mvar,mstructname,0);
        if ( NULL == mstruct )
        {
          cerr
            << "Error reading MAT struct in var '"
            << mvarname
            << "' in file '"
            << mfilename
            << "'"
            << endl;

          Mat_VarFree(mvar);
          mvar = NULL;

          Mat_Close(mfile);
          mfile = NULL;

          return EXIT_FAILURE;
        }

        // mstruct to array
        int rows = mstruct->dims[0];
        int cols = mstruct->dims[1];
        int start[2] = {0,0};
        int stride[2] = {1,1};
        int edge[2] = {rows, cols};
        double * data = (double *)malloc(sizeof(double)*rows*cols);

        Mat_VarReadData(mfile, mstruct, data, start, stride, edge);

        // csv file
        auto cfilename = p.string() + "/" + m3 + "_" + m4 + "_" + m5;
        std::ofstream cfile(cfilename, std::ofstream::out);
        cout << cfilename << endl;
        for (int i=0; i!=cols; ++i) {
                for (int j=0; j<rows; ++j) {
                        cfile << data[j+(rows*i)] << "\t";
                }
                cfile << "\n";
        }
        cfile.close();

        // mr proper
        Mat_VarFree(mstruct);
        mstruct = NULL;

        Mat_VarFree(mvar);
        mvar = NULL;

        Mat_Close(mfile);
        mfile = NULL;
      }
    }
  }
  return EXIT_SUCCESS;
}