CODEWITHSUNDEEP
cmpi
Peter Kažmirský
Peter Kažmirský

Reputation: 31

MPI_Scatter and MPI_Gather dont work

Hallo Iam trying to make a simlpe parralel program in C language uing MPI. Program should find maximum in array. Root process should send chunks of array to all processes using MPI_Scatter and then gather results by MPI_Gather. When I run the program i get general error like this:

Perhaps this Unix error message will help: Unix errno: 14 Bad address

I know that there is some problem with MPI_Scatter and MPI_Gather or with the values I am sending to this functions.

I was trying to find the solution, but I found nothing what could be useful.

Here is my code:

  #include <mpi.h>
  #include <stdio.h>
  #include <stdlib.h> 
  #define BUFSIZE 9

  int max(int *buf, int N){
      int i;
      int value = 0;
      for(i=0; i<N; i++){
         if (buf[i]>value){
             value = buf[i];
         }
      }
 return value;
 }
 int main(int argc, char** argv)
 { int size, rank;
   int slave;
   int *buf;
   int *buf1; 
   int *buf2;       
   int i, n, value;
   MPI_Status status;
   /* Initialize MPI  */
   MPI_Init(NULL, NULL);
   /* 
    * Determine size in the world group.
    */
   MPI_Comm_size(MPI_COMM_WORLD, &size);

   if ((BUFSIZE % size) != 0) {
     printf("Wrong Bufsize ");
     return(0);
   }


  MPI_Comm_rank(MPI_COMM_WORLD, &rank); 

  if (rank==0) { 
     buf = (int *)malloc(BUFSIZE*sizeof(int));
     buf2 = (int *)malloc(size*sizeof(int));

     printf("\n Generated array: \n");
     for(i=0; i<BUFSIZE; i++){
       buf[i] = rand() % 20;
       printf("%d, ", buf[i]);
     }
     printf("\n");
     printf("\n Sending values to processes:"); 
     printf("\n -----------------------------"); 
  }

  buf1 = (int *)malloc((BUFSIZE/size)*sizeof(int));

  MPI_Scatter(buf, BUFSIZE/size, MPI_INT, buf1, BUFSIZE/size, MPI_INT, 0, MPI_COMM_WORLD);

  value = max(&buf1[0], BUFSIZE/size);

  printf("\n Max from rocess %d : %d \n", rank, max(&buf1[0], BUFSIZE/size));

  MPI_Gather(&value, 1, MPI_INT, buf2, 1, MPI_INT, 0, MPI_COMM_WORLD);

  if (rank == 0){
     printf("\n Max value: %d", max(&buf2[0], size));
  }

  MPI_Finalize();
  return(0);
  }

Upvotes: 0

Views: 1732

Answers (1)

moralejaSinCuentoNiProverbio
moralejaSinCuentoNiProverbio

Reputation: 987

Initialize your pointers to NULL, and track them. use buf1 instead of &buf1[0], is more clear. free your buffers before MPI_Finalize() with:

if(bufferPionter != NULL) free(bufferPionter);

If something is wrong with a pointer will crash in the free call. In the max function, If all your numbers are less than zero the maximun is zero. i fix that.

 int max(int *buf, int N){
      int i;
      int value = N? buf[0] : 0;
      for(i=0; i<N; i++){
         if (buf[i]>value){
             value = buf[i];
         }
      }
 return value;
 }

Best regards!

Upvotes: 1

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