OpenMPI program stuck on reading stdin when stdin is directed to any process except 0

Ask Fedora
1

Here is a minimal example:

#include <mpi.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
    MPI_Init(&argc, &argv);
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    if (rank == 1)
    {
        printf("rank %d\n", rank);
        getchar();
    }
    MPI_Finalize();
    return 0;
}

It does not terminate unless with “rank == 0” and “–stdin 0” below. It is stuck on getchar(). The issue can be reproduced on Live USB.

$ module load mpi
$ mpirun -V
mpirun (Open MPI) 5.0.2
$ mpicc -Wall -pedantic -o test test.c
$ mpirun -n 6 --stdin 1 ./test
rank 1
a
b
...

Is this a bug or am I doing something wrong?

EDIT: Nope does not happen on 4.1.6 (Ubuntu). I think it might either be the newer version or the package.

2

You have not changed the stdin mode.

It is in line mode, so getchar() will not return anything until you type return.

3

Hmm with --stream-buffering 0 it’s still the same.
p.s. I also tried typing just return.

4

Also happens on 5.0.5 on Arch. Guess this is upstream then…

5

That is not documented as a python option as far as I can see 1. Command line and environment — Python 3.12.5 documentation

You need to write code to put stdin into a mode that will work.

It is your code that is not using an API that does do what you want.
That is not a python issue.

6

Wait what which is part is Python…? The code itself is in C.
I got the option from here:
https://docs.open-mpi.org/en/v5.0.x/man-openmpi/man1/mpirun.1.html#the-stream-buffering-option

7

Doh! My bad I have been switching between python and fedora forums.

The same issue is that C runtime STDIN FILE object is in line buffered mode and getchar() will not read on one key press.
You can use termios(3) - Linux manual page to set the germinsl into 1 char mode.

The MPI software changes the stdin mode? That is surprising.

1 Like
8

Once you have the terminal in one char mode then you can use read() on stdin FD to get 1 char. getchar() is not the right API to use.