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

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);

Answer 2

You can use it:

puts("Enter the buffer size:");
gets(buf);

Answer 3

If you are using Eclipse for parallel programming, then (most probably) that is what causes the issue.
It is some kind of glitch in PTP. You can also experience this with printf, it only prints after your program has finished execution or was shut down (not while the program is running).
Then try running your code from the command line, it will work.

Else (If it still gets hanged or gives weird behaviour) then try calling
fflush(stdout) or while ((c = getchar()) != '\n' && c != EOF);
to flush everything before reading user input.