openmpi: printf not waiting for scanf - c

Question

int main( int argc, char *argv[])
{
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
int x;
printf("Rank: %d\n", rank);
fflush(stdout);
scanf("%d", &x);
MPI_Finalize();
return 0;
}
when i run it on 2+ tasks it prints everything, but scanf used once in the end
(base) curer:hw4 user$ mpic++ -o a ./-.cpp
(base) curer:hw4 user$ mpirun -oversubscribe -np 2 ./a
Rank: 0
Rank: 1
7
scanf in the end and i entered 7
I want to be like that
(base) curer:hw4 user$ mpic++ -o a ./-.cpp
(base) curer:hw4 user$ mpirun -oversubscribe -np 2 ./a
Rank: 0
8
Rank: 1
7
printf Rank i then scanf any number

Answer 1

If you really really want each MPI tasks to do a job one after another, in order, you could synchronize the jobs by a series of MPI_Send and MPI_Recv.
This a working example:
#include <stdio.h>
#include <mpi.h>
int read(int rank)
{
int x;
printf("Rank: %d\n", rank);
fflush(stdout);
scanf("%d", &x);
return x;
}
int main( int argc, char *argv[])
{
int rank, size, send, receive, x;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
send = 1;
if (rank == 0)
{
x = read(rank);
if (size > 1)
MPI_Send(&send, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
}
else
{
MPI_Recv(&receive, 1, MPI_INT, rank - 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
x = read(rank);
MPI_Send(&send, 1, MPI_INT, rank + 1, 0, MPI_COMM_WORLD);
}
MPI_Finalize();
return 0;
}
In the code above, each task send a message to the next one, and (except the root) wait a message from the previous task before doing the job.
However, as noted in the comments, it is much more common to let a task, typically the root, to do the input job and then distribute the inputs among the task, using MPI_Bcast or MPI_Scatter.