MPI Segmentationi fault MPI_Scatter using C - c

Question

I am new in this area and use OpenMPI and C. I try to find out why my code leads to an Segmentatioin fault. I red already a lot about MPI but I did not find any help. It took me already hours. So I decided to ask here for help.
I get the expected result of my code. But I also get every time an error message.
Is it right how I use MPI_Scatter in my case?
Here is my simple code:
#include <stdio.h>
#include "mpi.h"
#include <stdlib.h>
const int MASTER_RANK = 0;
#define DIM 3
int main(int argc, char* argv[])
{
int numProc, rank;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &numProc);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int n = 9;
double *m;
double *m_client;
m_client = (double *)malloc(3);
if(rank == MASTER_RANK)
{
m = (double* )malloc(n);
for(int i=0; i<n; i++)
{
m[i] = (double)i+1.0;
}
}
MPI_Scatter(m, 3, MPI_DOUBLE, m_client, 3, MPI_DOUBLE, MASTER_RANK, MPI_COMM_WORLD);
printf("Process %d:\n", rank);
for(int i=0; i < 3; i++)
{
printf(" (%lf", m_client[i]);
m_client[i] += 1000*rank;
printf(" -> %lf)", m_client[i]);
printf("\n");
}
printf( "\n" );
MPI_Gather(m_client, 3, MPI_DOUBLE, m, 3, MPI_DOUBLE, MASTER_RANK, MPI_COMM_WORLD);
if(rank == MASTER_RANK)
{
printf("Master: Received= \n");
for(int i=0; i<numProc; i++)
{
for(int j=0; j < 3; j++)
{
int idx = i*3 + j;
printf("%lf ", m[idx]);
}
printf("from Process %d\n", i);
}
}
free(m);
free(m_client);
MPI_Finalize();
exit(0);
}
I build my MPI file by using mpicc mpifile.c -o mpifile and run it with mpirun -np 3 ./mpifile. I use 3 processes.
The error I get is:
[Samuel-Z97-HD3:14361] *** Process received signal ***
[Samuel-Z97-HD3:14361] Signal: Segmentation fault (11)
[Samuel-Z97-HD3:14361] Signal code: (128)
[Samuel-Z97-HD3:14361] Failing at address: (nil)
I am using Ubuntu and vim / Geany.

Answer 1

Your code has two problems.
Both your malloc() calls are having wrong sizes. You should allocate number of bytes instead of number of doubles. E.g. instead of calling malloc(3), call malloc(3*sizeof(double)).
Another problem is that your variable m should be initialized to NULL. Alternatively, you could surround free(m) with if(rank == MASTER_RANK). As is, a non-master process calls free(m) where m is uninitialized and could contain arbitrary value.