Question

I doing an application using C and MPI that makes a vector by Matrix multiplication, but I keep having errors like Error posting readv , and An existing connection was forcibly closed by the remote host (10054)
Here is the code:

#include "stdio.h"
#include "mpi.h"

#define W 5
#define H 5

void make_matrix(int[]);
void make_vector(int []);

void main(int argc, char* argv[])
{

  int myrank,size,k;
  int matrix[H*W];
  int vec[W];
  int res[W];
  static int col_count = 0;

  MPI_Init(&argc,&argv);
  MPI_Comm_rank(MPI_COMM_WORLD,&myrank);
  MPI_Comm_rank(MPI_COMM_WORLD,&size);

  MPI_Bcast(vec,W,MPI_INT,0,MPI_COMM_WORLD);

  if(myrank != 0)
  {
    for(k=0; k<W; k++)
    {
      vec[k]+= matrix[k*W+col_count];
    }
    col_count++;
    printf("%d ",vec[col_count]);
  }

  MPI_Finalize();
}

void make_matrix(int a[])
{
  int i;
  for(i=0; i<H*W; i+=1)
  {
    a[i] = i;
  }
};

void make_vector(int v[])
{
  int i;
  for(i=0; i<H; i++)
    v[i] = i*2;
};

Answer 1

MPI_Bcast() is a collective function, which means that every process in the communicator must call it. In other words, don’t call MPI_Recv(). So get rid of your if(myrank == 0) conditional and have all processes call:

MPI_Bcast(vec,W,MPI_INT,0,MPI_COMM_WORLD);

Note that I have vec above, not &vec; since vec is already an array, it is the pointer that MPI needs. Also, your result will appear in vec on the non-root processes; there is no need for the separate arr array.

I recommend that you read some examples of MPI and try to make your code look more like them.