MPI_Gather 2D array

Question

N is 4, so is N_glob. It happens to be of the same size. p is 4.

Here is a small portion of the code:

float **global_grid;
float **gridPtr; 
lengthSubN = N/pSqrt;
subN = lengthSubN + 2;
grid = allocate2D(grid, subN, subN);
..
MPI_Type_contiguous(lengthSubN, MPI_FLOAT, &rowType);
MPI_Type_commit(&rowType);
..
gridPtr = grid;
..
MPI_Barrier(MPI_COMM_WORLD);
if(id == 0) {
    global_grid = allocate2D(global_grid, N_glob, N_glob);
}
MPI_Barrier(MPI_COMM_WORLD);
MPI_Gather(&(gridPtr[0][0]), 1, rowType,
           &(global_grid[0][0]), 1, rowType, 0, MPI_COMM_WORLD);
MPI_Barrier(MPI_COMM_WORLD);
if(id == 0)
    print(global_grid, N_glob, N_glob);

where I have p submatrices and I am trying to gather them all in the root process, where the global matrix waits for them. However, it will just throw an error, any ideas?

I am receiving a seg fault:

BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES PID 29058 RUNNING AT linux16 EXIT CODE: 139 YOUR APPLICATION TERMINATED WITH THE EXIT STRING: Segmentation fault (signal 11)

---

EDIT:

I found this question MPI_Gather segmentation fault and I initialized global_grid to NULL, but no luck. However, if I do:

//if(id == 0) {
    global_grid = allocate2D(global_grid, N_glob, N_glob);
//}

then everything works. But shouldn't the global matrix live only in the root process?

---

EDIT_2:

IF I do:

if(id == 0) {
    global_grid = allocate2D(global_grid, N_glob, N_glob);
} else {
    global_grid = NULL;
}

then it will crash here:

MPI_Gather(&gridPtr[0][0], 1, rowType,
                global_grid[0], 1, rowType, 0, MPI_COMM_WORLD);

Answer 1

The variable global_grid is not initialized in ranks other than rank 0. Thus, this equation

&(global_grid[0][0])

or this one:

global_grid[0]

leads to a segmentation fault, because it tries to access the first element of global_grid.

Just make two calls to MPI_Gather, one for rank 0 and one for the other ones:

if(id == 0) {
    MPI_Gather(gridPtr[0], 1, rowType, global_grid[0], 1, rowType, 0, MPI_COMM_WORLD);
} else {
    MPI_Gather(gridPtr[0], 1, rowType, NULL, 0, rowType, 0, MPI_COMM_WORLD);
}