#include "mpi.h"
#include <stdio.h>
#define SIZE 4

int main(argc,argv)
int argc;
char *argv[];  {
int numtasks, rank, source=0, dest, tag=1, i;
float a[SIZE][SIZE] = 
  {1.0, 2.0, 3.0, 4.0,  
   5.0, 6.0, 7.0, 8.0, 
   9.0, 10.0, 11.0, 12.0,
  13.0, 14.0, 15.0, 16.0};
float b[SIZE]; 

MPI_Status stat;
MPI_Datatype columntype;

MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
   
MPI_Type_vector(SIZE, 1, SIZE, MPI_FLOAT, &columntype);
MPI_Type_commit(&columntype);

if (numtasks == SIZE) {
  if (rank == 0) {
     for (i=0; i<numtasks; i++) 
       MPI_Send(&a[0][i], 1, columntype, i, tag, MPI_COMM_WORLD);
        }
 
  MPI_Recv(b, SIZE, MPI_FLOAT, source, tag, MPI_COMM_WORLD, &stat);
  printf("rank= %d  b= %3.1f %3.1f %3.1f %3.1f\n",
        rank,b[0],b[1],b[2],b[3]);
  }
else
  printf("Must specify %d processors. Terminating.\n",SIZE);
   
MPI_Type_free(&columntype);
MPI_Finalize();
}

//-----------------------------------------------------------------------------------------------

However, it doesn't work. the program just suspend there. I figure out the problem is at line MPI_Send, and MPI_Recv. the MPI_Send just send it to itself. but I don't know why it doesn't work. Is it caused by dead lock? why?

Thank you very much!

--
Best Regards,