Sharing a dynamically allocated 2D array with MPI [duplicate] - c

Question

This question already has answers here:
Sending and receiving 2D array over MPI
(3 answers)
Closed 2 years ago.
I am trying to share a dynamically allocated 2D array from a master thread to several other threads using MPI in c, from within a function.
A simplified representation of the relevant code is as follows:
//Initialize program, start up the desired number of threads.
//Master thread takes input from user, dynamically allocates and constructs 2d array.
//All threads call method analyze_inputs(**array), which takes the array as input (all threads other than master simply pass NULL as argument)
//The master thread shares the array, along with work division to all other threads:
{//Master thread
MPI_Send(&x, 1, MPI_INT, recievingThread, 0, MPI_COMM_WORLD);
MPI_Send(&y, 1, MPI_INT, recievingThread, 0, MPI_COMM_WORLD);
MPI_Send(&(array[0][0]), x*y, MPI_INT, recievingThread, 0, MPI_COMM_WORLD);
}
{//Subthreads
MPI_Recv(&x, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
MPI_Recv(&y, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
MPI_Recv(&(array[0][0]), x*y, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
}
This is a soulution i found on this site for sending dynamically allocated 2d arrays, but i get segmentation error for the array recieve.
How can i do this?
edit: Minimal reproducible example
#include <mpi.h>
#include <stdlib.h>
#include <stdio.h>
int analyze_inputs (int x, int y, int** array);
int main (int argc, char **argv)
{
int x = 10;
int y = 8;
int rank;
int **array = NULL;
MPI_Init (&argc, &argv);
MPI_Comm_rank (MPI_COMM_WORLD, &rank);
if (rank == 0)
{
array = malloc(x * sizeof(int*));
for (int i = 0; i < x; i++)
{
array[i] = malloc(y * sizeof(int));
}
for (int i = 0; i < x; i++)
{
for (int j = 0; j < y; j++)
{
array[i][j] = rand();
}
}
}
analyze_inputs(x,y,array);
MPI_Finalize ();
}
int analyze_inputs(int x,int y, int** array)
{
int rank, x_temp, y_temp, **array_temp;
MPI_Comm_rank (MPI_COMM_WORLD, &rank);
if (rank == 0)
{
MPI_Send(&x, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
MPI_Send(&y, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
MPI_Send(&(array[0][0]), x*y, MPI_INT, 1, 0, MPI_COMM_WORLD);
}
else
{
MPI_Recv(&x_temp, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
MPI_Recv(&y_temp, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
printf("Works to here.\n");
MPI_Recv(&(array_temp[0][0]), x_temp*y_temp, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
printf("Crashes before here.\n");
}
}

Answer 1

Each row of array is allocated separately in your code, so simple
MPI_Send(&(array[0][0]), x*y, MPI_INT, 1, 0, MPI_COMM_WORLD);
won't work in this case.
An simple solution is to allocate a single block of memory like this:
array = malloc(x * sizeof(int*));
array[0] = malloc(y * x * sizeof(int));
for (int i = 1; i < x; i++)
{
array[i] = array[0] + y * i;
}
And freeing this array will be
free(array[0]);
free(array);
Do not free array[1], array[2], ... in this case because they are already freed by free(array[0]);.