title: Processes read data from the same file
body: <p>I have a mesh file and I did a partitioning of it using METIS(in 4 parts/processes).METIS provided me with the partitioning file of the mesh(gave me a file with the number of process where each element of the mesh belongs to).My job now is to input these information to my parallel code.I tried to do it by letting each process to have access to the same mesh file and read the data that it wants based on partitioning file.</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;fstream&gt;
  #include &lt;sstream&gt;
  #include "mpi.h"
  using namespace std;
  //each process stores the partitioning
  int* PartitionFile(){
   ifstream file ("epart.txt");
   int NE=14;
   int part,i=0;
   int *partition=new int[14];
   while(file&gt;&gt;part){
     partition[i]=part;
     i++;
   }
   file.close();
   return partition;
  }
  int FindSizeOfLocalElements(int *epart,int rank){
    int size=0;
    for (int i=0;i&lt;14;i++){
     if(epart[i]==rank){
      size+=1;
     }
    }
    return size;
  }
  //stores the elements of each process
  int * LocalPartition(int* epart,int size,int rank){
  
    int *localPart=new int[size];
    int j=0;
   for(int i=0;i&lt;14;i++){
      if (epart[i]==rank){
          localPart[j]=i+1;//+1 because elements start from 1(global numbering)
          j+=1;
      }
    }
  return localPart;
  }
  int **ElementConnectivityMeshFile(int* localPart,int size){
    ifstream file ("mesh.txt");
    int node1,node2,node3;
    int elem=1;
    int i=0;
    int **elemConn=new int*[size];
    for(int j=0;j&lt;size;j++){
      elemConn[j]=new int[3];//each element has 3 nodes.Here elements has local numbering.Global numbering is stored in localPart
   }
    while(file&gt;&gt;node1&gt;&gt;node2&gt;&gt;node3){
      if (elem==localPart[i]){
          elemConn[i][0]=node1;
          elemConn[i][1]=node2;
          elemConn[i][2]=node3;
          i+=1;
      }
      elem+=1;
      if(elem&gt;14){break;}
    }
    file.close();
    return elemConn;
  }
  
  
  
  
  int main(){
    MPI_Init(NULL, NULL);
    int numProc;
    MPI_Comm_size(MPI_COMM_WORLD, &amp;numProc);
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
    int *epart=PartitionFile();
    int size=FindSizeOfLocalElements(epart,rank);
    int *elem=LocalPartition(epart,size,rank);
    int **elemConn=ElementConnectivityMeshFile(elem,size);
    MPI_Finalize();
    return 0;
  }
  </code></pre>
  
  <p>This part of code gives me the desired results,however I want to know how efficient is letting MPI processes read the same file,by using c++ standard functions, and if that can affect scalability and performance.For this demostration i used a mesh of 14 elements and 4 processes.</p>
  
  <pre><code>mesh file
  1 3 2
  2 3 4
  3 5 4
  4 5 6
  5 7 6
  8 7 5
  3 8 5
  9 7 8
  9 8 3
  1 9 3
  10 9 1
  11 10 1
  11 1 12
  12 1 2
  
  epart file
  2
  2
  0
  0
  0
  1
  0
  1
  1
  3
  3
  3
  2
  2
  </code></pre>