MPI Broadcasting dynamic 2D array to other processors - c

Question

I have searched many explanations, but I think I could not figure out how I can do this situation. I want to do something like that:
Using master processor, I am creating a dynamic 2D array. Then I want to;
1- send this array to other processors. And each processor prints this 2D array
2- send part of this array to others. And each processor prints their part to the screen.
For example; I have 2D array 11*11 and 4 processors. rank 0 is master. Others are slaves. For the first situation, I want to send all array to rank 1, rank 2, and rank 3. And for the second situation, I want to share rows to slaves. 11/3 = 3. Therefore rank 1 takes 3 rows, rank 2 takes 3 rows, and rank 3 takes 5 rows.
Here is my code:
int processorID;
int numberOfProcessors;
int main(int argc, char* argv[]){
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD ,&numberOfProcessors);
MPI_Comm_rank(MPI_COMM_WORLD ,&processorID);
double **array;
if(MASTER){
array = (double**) malloc(11*sizeof(double *));
for(i=0; i<11; i++){
array[i] = (double *) malloc(11*sizeof(double));
}
for(i=0; i<11; i++){
for(j=0; j<11; j++){
array[i][j] = i*j;
}
}
}
MPI_Bcast(array, 11*11, MPI_DOUBLE, 0, MPI_COMM_WORLD);
if(SLAVE){
for(i=0; i<11; i++){
for(j=0; j<11; j++){
printf("%f ", array[i][j]);
}
}
}
MPI_Finalize();
return 0;
}
according to these link ; MPI_Bcast a dynamic 2d array ; I need to create my array as;
if (MASTER){
array = (double**) malloc(121*sizeof(double))
for(i=0; i<11; i++){
for(j=0; j<11; j++){
array[i][j] = i*j; // this is not working.
}
}
}
but if I do this, I cannot initialize each member in the array. Inner for loops are not working. I could not find any way to solve it.
And for my second question, I followed this link sending blocks of 2D array in C using MPI . I think I need to change the inside of if(SLAVE). I should create 2D subArrays for each slave processor. And I need to use MPI_Scatterv. But I could not understand completely.
int main() {
...
...
MPI_Scatterv() // what must be here?
if(SLAVE){
if(processorID = numberOfProcessor-1){
subArray = (double**) malloc(5*sizeof(double *)); // beacuse number of row for last processor is 5
for(i=0; i<11; i++){
array[i] = (double *) malloc(11*sizeof(double));
}
}
else {
subArray = (double**) malloc(3*sizeof(double *));
for(i=0; i<11; i++){
array[i] = (double *) malloc(11*sizeof(double));
}
}
}
}

Answer 1

C does not really have multidimensional arrays. I would recommend storing your values in a regular 1D buffer, and then calculating correct indices from 1D values. Like so:
double* data = (double*)malloc(sizeof(double)*11*11);
// Now, to access data[i][j], simply do:
data[j + i*11] = ...; // this "maps" 2D indices into 1D
This will spare you all the trouble with the hierarchical malloc-ing, and it can easily be passed to the MPI APIs.

Answer 2

You can't use an array of pointers (what you are incorrectly calling a "2D array"), because each row pointer is not portable to another node's address space.
The code you quoted for creating a 2D array in a linear memory allocation is perfectly correct, all you need to do is index that memory in row major order, so that the loops become:
double* array = (double*) malloc(121*sizeof(double));
if (MASTER){
for(i=0; i<11; i++){
for(j=0; j<11; j++){
array[j+i*11] = i*j; // row major indexing here
}
}
}
/* Scatter code follows */
You can safely scatter this sort of array to multiple nodes.