How can I send rows of a matrix to all the processes using MPI_Scatterv?

Question

I am working with the MPI interface. I want to split a matrix (by rows) and distribute the parts among every process.

For example, I have this 7x7 square matrix M.

Answer 1

MPI_Scatterv needs a pointer to the data and the data should be contiguous in memory. Your program is fine on the second part, but MPI_Scatterv receives a pointer to pointers to data. So it would be a good thing to change for :

MPI_Scatterv(&m[0][0], sendcounts, displs, MPI_DOUBLE, &mParts[0][0], sendcounts[myrank], MPI_DOUBLE, root, MPI_COMM_WORLD);

There are also a couple of things to change for sendcounts and displs : to go 2D, these counts should be multiplied by n. And the count of receive in MPI_Scatterv is not rows anymore, but sendcouts[myrank].

Here is the final code :

#include 
#include 
#include 

#define BLOCK_LOW(id,p,n) ((id)*(n)/(p))
#define BLOCK_HIGH(id,p,n) ((id+1)*(n)/(p) - 1)
#define BLOCK_SIZE(id,p,n) ((id+1)*(n)/(p) - (id)*(n)/(p))
#define BLOCK_OWNER(index,p,n) (((p)*((index)+1)-1)/(n))

void **matrix_create(size_t m, size_t n, size_t size) {
    size_t i; 
    void **p= (void **) malloc(m*n*size+ m*sizeof(void *));
    char *c=  (char*) (p+m);
    for(i=0; i

If you want to know more about 2D arrays and MPI, look here

Look also at the DMDA structure of the PETSc library here and there