Question

I’m try to use MPI_Scatter, sending rows of matrix(that are dynamically allocated), but it sending only one row, in others are junk. When I use static memory allocation – everything is good.

MPI_Init(&argc, &argv);
int **matrix, *matrix_stor, *row,rank, P;
MPI_Comm_size(MPI_COMM_WORLD, &P);
row = new int [P];
for(int i = 0; i < P; i++)
{
    row[i] = 0;
}
matrix = new int *[P];
for(int i = 0; i < P; i ++)
    matrix[i] = new int [P];

//int matrix[4][4], row[4], rank, size;
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

if (rank == 0){
    for(int i = 0; i < P; i++){
        for(int j = 0; j < P; j++){
            matrix[i][j] = rand()%20;
            cout << matrix[i][j] << " ";
        }
        cout << endl;
    }
}
cout << endl;

MPI_Scatter(&matrix[0][0], P, MPI_INT,&row[0], P, MPI_INT,0,MPI_COMM_WORLD);
for(int i = 0; i < P; i++)
    cout << row[i] << " ";
cout << endl;
free(matrix);
free(row);
MPI_Finalize();
return 0;

And result is:
Source matrix:
1 7 14 0
9 4 18 18
2 4 5 5
1 7 1 11
Received rows:
1 7 14 0
3626672 3626800 0 0
16 1 119 -33686019
-33686019 -572662307 524296 786765

Answer 1

This isn’t an MPI issue, it’s a C and multidimensional arrays issue.

The problem is that in C there’s no guarantee that your allocated matrix is actually P*P contiguous ints. So when you try to scatter P*P ints starting at matrix[0][0], who knows what you’re sending out.

This code:

matrix = new int *[P];
for(int i = 0; i < P; i ++)
    matrix[i] = new int [P];

Allocates first an array of P pointers-to-int, and then sets each one to a pointer where has been allocated P ints. But there’s no guarantee at all that matrix[1] starts where matrix[0] ends. If you want to do this – and you do, both for MPI and normally for multi-d arrays in technical computing applications generally – you have to manually allocate the contiguous block of memory and have matrix point into it:

matrix = new int *[P];
int *data = new int [P*P];
for(int i = 0; i < P; i ++)
    matrix[i] = &(data[P*i]);

And then the scatter should work.

Note too that you should be using delete, not free(), to deallocate memory allocated with new.