Storing values starting from a particular location in MPI_Recv - c

Question

I am testing an example, where I am trying to send an array of 4 elements from process 0 to process 1 and I am doing so using MPI_Type_contiguous
This is the code for the same
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main( int argc, char *argv[] )
{
MPI_Init(&argc, &argv);
int myrank, size; //size will take care of number of processes
MPI_Comm_rank(MPI_COMM_WORLD, &myrank) ;
MPI_Comm_size(MPI_COMM_WORLD, &size);
//declaring the matrix
double mat[4]={1, 2, 3, 4};
int r=4;
double snd_buf[r];
double recv_buf[r];
double buf[r];
int position=0;
MPI_Status status[r];
MPI_Datatype type;
MPI_Type_contiguous( r, MPI_DOUBLE, &type );
MPI_Type_commit(&type);
//sending the data
if(myrank==0)
{
MPI_Send (&mat[0], r , type, 1 /*dest*/ , 100 /*tag*/ , MPI_COMM_WORLD);
}
//receiving the data
if(myrank==1)
{
MPI_Recv(recv_buf, r, type, 0 /*src*/ , 100 /*tag*/, MPI_COMM_WORLD,&status[0]);
}
//printing
if(myrank==1)
{
for(int i=0;i<r;i++)
{
printf("%lf ",recv_buf[i]);
}
printf("\n");
}
MPI_Finalize();
return 0;
}
As one can see the recv_buf size is same as the size of the array. And the output that is being printed is 1 2 3 4
Now what I trying to do is that, say the recv_buf size is 10 and I want to store the elements starting from location 6 to 9. For which I have written this code, but to my surprise it is giving no output
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main( int argc, char *argv[] )
{
MPI_Init(&argc, &argv);
int myrank, size; //size will take care of number of processes
MPI_Comm_rank(MPI_COMM_WORLD, &myrank) ;
MPI_Comm_size(MPI_COMM_WORLD, &size);
//declaring the matrix
double mat[4]={1, 2, 3, 4};
int r=4;
double snd_buf[r];
double recv_buf[10]; //declared it of size 10
double buf[r];
int position=0;
MPI_Status status[r];
MPI_Datatype type;
MPI_Type_contiguous( r, MPI_DOUBLE, &type );
MPI_Type_commit(&type);
//packing and sending the data
if(myrank==0)
{
MPI_Send (&mat[0], r , type, 1 /*dest*/ , 100 /*tag*/ , MPI_COMM_WORLD);
}
//receiving the data
if(myrank==1)
{
MPI_Recv(&recv_buf[6], r, type, 0 /*src*/ , 100 /*tag*/, MPI_COMM_WORLD,&status[0]);
}
//printing
if(myrank==1)
{
for(int i=6;i<10;i++)
{
printf("%lf ",recv_buf[i]);
}
printf("\n");
}
MPI_Finalize();
return 0;
}
Where am I going wrong?

Answer 1

From this SO Thread one can read:
MPI_Type_contiguous is for making a new datatype which is count copies
of the existing one. This is useful to simplify the processes of
sending a number of datatypes together as you don't need to keep track
of their combined size (count in MPI_send can be replaced by 1).
That being said, in your MPI_Send call:
MPI_Send (&mat[0], r , type, 1 /*dest*/ , 100 /*tag*/ , MPI_COMM_WORLD);
you should not send an array with 'r' elements of type 'type', but rather send 1 element of type 'type' (which is equal to 4 doubles). One of the goals of the MPI_Type_contiguous is to abstract away the count to 1 instead of keeping track of the number of elements.
The same applies to your recv call:
MPI_Recv(&recv_buf[6], 1, type, 0 /*src*/ , 100 /*tag*/, MPI_COMM_WORLD,&status[0]);
Finally, you should also free the custom type accordingly:
MPI_Type_free(&type);
The entire code:
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main( int argc, char *argv[] )
{
MPI_Init(&argc, &argv);
int myrank, size;
MPI_Comm_rank(MPI_COMM_WORLD, &myrank) ;
MPI_Comm_size(MPI_COMM_WORLD, &size);
double mat[4]={1, 2, 3, 4};
int r=4;
double snd_buf[r];
double recv_buf[10];
MPI_Status status;
MPI_Datatype type;
MPI_Type_contiguous( r, MPI_DOUBLE, &type );
MPI_Type_commit(&type);
if(myrank==0)
MPI_Send (&mat[0], 1 , type, 1, 100, MPI_COMM_WORLD);
else if(myrank==1)
{
MPI_Recv(&recv_buf[6], 1, type, 0, 100, MPI_COMM_WORLD, &status);
for(int i=6;i<10;i++)
printf("%lf ",recv_buf[i]);
printf("\n");
}
MPI_Type_free(&type);
MPI_Finalize();
return 0;
}
The Output:
1.000000 2.000000 3.000000 4.000000