MPI: Printf Statement is not executed at the right time

Question

I have a small program.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
{
int rank, size;
int buf;
int err;
MPI_Status status;

err = MPI_Init(&argc, &argv);
if(err == MPI_SUCCESS) {
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

if(rank == 0) {
printf("Buffer size is less than 10\n");
printf("Enter the buffer size: ");
scanf("%d", &buf);
MPI_Send(&buf, 1, MPI_INT, 1, 10,
MPI_COMM_WORLD);
}
else {
MPI_Recv(&buf, 1, MPI_INT, 0, 10,
MPI_COMM_WORLD, &status);
}
printf("process[%d]: buffer size : %d\n", rank,buf);

}
err = MPI_Finalize();
return 0;
}

The output is:

[veda@home-pc hpc]$ mpicc test.c
[veda@home-pc hpc]$ mpiexec -np 2 a.out
Buffer size is less than 10
3
Enter the buffer size: process[0]: buffer size : 3
process[1]: buffer size : 3

In this output, you can notice that

Enter the buffer size:

is printing/prompting after I entered the size of the buffer.

Can anyone tell me how to solve this problem.

Answer 1

Well thats what you have.

printf("Buffer size is less than 10\n");
printf("Enter the buffer size: ");

I mean what do you suspect for it to do? Look right after: if(rank==0) to see what I mean. In fact, you don't even use the %d you use 10.

You probably want:

if(rank == 0) {
   printf("Enter the buffer size: ");
   scanf("%d", &buf);
   printf("Buffer size is less than %d\n", buf);
   //...

Answer 2

I suspect your standard output is line-buffered. The "Enter the buffer size: " is not printed until "process[%d]: buffer size : %d\n" (containing a newline) is printed. The solution is to explicitly flush the standard output between the printf and the scanf:

printf("Enter the buffer size: ");
fflush(stdout);
scanf("%d", &buf);