MPI_Gather send and recieving array error - c

Question

#include<mpi.h>
#include<stdio.h>
int main(int argc,char * argv[])
{
int rank,size,m;
int arr[1000];
int b[100];
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&size);
if(rank == 0)
{
scanf("%d",&m);
for(int i=0;i<size*m;i++)
{
scanf("%d",&arr[i]);
}
}
MPI_Barrier(MPI_COMM_WORLD);
MPI_Scatter(arr,m,MPI_INT,b,m,MPI_INT,0,MPI_COMM_WORLD);
printf("in process %d \n",rank);
for(int i=0;i<m;i++)
{
printf("%d ",b[i]);
}
printf("\n");
MPI_Finalize();
return 0;
}
Input
mpiexec -n 4 ./three
Actual Output
in process 2
in process 1
in process 3
2
1 2 3 4 5 6 7 8
in process 0
1 2
Expected Output
2
1 2 3 4 5 6 7 8
in process 0
1 2
in process 1
3 4
in process 2
5 6
in process 3
7 8

Answer 1

You have not broadcast the value of m so only rank 0 has a correct value - it looks like it is defaulting to m=0 on other ranks, hence their lack of output.
The barrier is not required - MPI collectives do all the necessary synchronisation automatically.
If you replace your barrier call with:
MPI_Bcast(&m, 1, MPI_INT, 0, MPI_COMM_WORLD);
then your code works as expected (subject to the output not being as clean as you might expect as pointed out by Gilles above).