MPI_Scatter is receiving wrong values - c

Question

My goal is to take an array of 6 integers and split them among 3 processes. However, the numbers in receiveBuffer are not correct. I don't know why the three processes don't contain integers from the original array.
#include<stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>
#include <assert.h>
#include "mpi.h"
#define ARRAY_SIZE 6
// simple print array method
void printArray(int arr[], int size)
{
int i;
for (i=0; i < size; i++)
printf("%d ", arr[i]);
printf("\n");
}
int main (int argc, char *argv[])
{
srand(time(NULL));
int array[ARRAY_SIZE];
int rank, numNodes;
// fill array with random numbers and print
for(int i = 0; i < ARRAY_SIZE; i++)
array[i] = rand();
printArray(array, ARRAY_SIZE);
MPI_Init( &argc, &argv );
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size (MPI_COMM_WORLD,&numNodes);
int receiveBuffer[ARRAY_SIZE/numNodes];
if(rank == 0)
{
MPI_Scatter(array, ARRAY_SIZE/numNodes, MPI_INT, &receiveBuffer, 0, MPI_INT, rank, MPI_COMM_WORLD);
}
printf("ID: %d with %d items.\n", rank, ARRAY_SIZE/numNodes);
printArray(receiveBuffer, ARRAY_SIZE/numNodes);
MPI_Finalize();
return 0;
}
Additionally, why is the original array printing for each process? Doesn't parallelization begin after INIT?

Answer 1

There are two issues with your usage of MPI_Scatter()
MPI_Scatter() is a collective operation, and has hence to be invoked by all the ranks of the communicator (e.g. not only rank zero)
since you use MPI_INT for both send and receive datatype, the send and receive counts should be equal (e.g. use ARRAY_SIZE/numNodes instead of 0)
The MPI standard does not specify what happens before MPI_Init(), and it is very common that mpirun spawns all the tasks, so they all execute the code before MPI_Init(). That's why MPI_Init() is generally invoked at the very beginning of a MPI program.