MPI_scatter of 1D array - c

Question

I new to MPI and I am trying to write program that uses MPI_scatter. I have 4 nodes(0, 1, 2, 3). Node0 is master, others are slaves. Master asks user for number of elements of array to send to slaves. Then it creates array of size number of elements * 4. Then every node prints it`s results.
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#define MASTER 0
int main(int argc, char **argv) {
int id, nproc, len, numberE, i, sizeArray;
int *arrayN=NULL;
int arrayNlocal[sizeArray];
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (id == MASTER){
printf("Enter number of elements: ");
scanf("%d", &numberE);
sizeArray = numberE * 4;
arrayN = malloc(numberE * sizeof(int));
for (i = 0; i < sizeArray; i++){
arrayN[i] = i + 1;
}
}
MPI_Scatter(arrayN, numberE, MPI_INT, &arrayNlocal, numberE,MPI_INT, MPI_COMM_WORLD);
printf("Node %d has: ", id);
for (i = 0; i < numberE; i++){
printf("%d ",arrayNlocal[i]);
}
MPI_Finalize();
return 0;
}
And as error i get:
BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES
PID 9278 RUNNING AT 192.168.100.100
EXIT CODE: 139
CLEANING UP REMAINING PROCESSES
YOU CAN IGNORE THE BELOW CLEANUP MESSAGES

Answer 1

In arrayNlocal[sizeArray];, sizeArray is not initialized. The best way to go is to broadcast numberE to every processes and allocate memory for arrayNlocal. Something like:
MPI_Bcast( &numberE, 1, MPI_Int, 0, MPI_COMM_WORLD)
arrayN is an array of size sizeArray = numberE * 4, so:
arrayN = malloc(sizeArray * sizeof(int));
MPI_Scatter() needs pointers to the data to be sent on root node, and a pointer to receive buffer on each process of the communicator. Since arrayNlocal is an array:
MPI_Scatter(arrayN, numberE, MPI_INT, arrayNlocal, numberE,MPI_INT,MASTER, MPI_COMM_WORLD);
or alternatively:
MPI_Scatter(arrayN, numberE, MPI_INT, &arrayNlocal[0], numberE,MPI_INT,MASTER, MPI_COMM_WORLD);
id is not initialized in id == MASTER: it must be rank==MASTER.
As is, the prints at the end might occur in a mixed way between processes.
Try to compile your code using mpicc main.c -o main -Wall to enable all warnings: it can save you a few hours in the near future!