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stdinfile-descriptoropenmpitermios

openmpi, termios and stdin

I've a question regarding openmpi and the standard input file descriptor (fd) and the use of termios. Is actually the standard fd used in ompi programs ?

There's the mpirun -stdin option for standard input redirection to a certain rank. I assume something is done behind the scene.

For example, the following code snippet fails at tcgetattr for STDIN_FILENO only with mpirun (but works in sequential).

#include <iostream>
#include <mpi.h>
#include <termios.h>
#include <unistd.h>
#include <stdio.h>
#include <assert.h>
using namespace std;

int main(int argc, char* argv[]) {
  MPI_Status status;
  MPI_Init(&argc, &argv);

  int in = dup( STDIN_FILENO );
  struct termios org_opts, new_opts;
  res = tcgetattr( in, &org_opts);
  assert(res==0);

  MPI_Finalize( );
}
Solution

  • In Open MPI, stdin is read by the mpirun process.

    Then the data read is forwarded to one MPI task (task 0 unless explicitly requested otherwise), and that could involve the orted daemon(s).

    Bottom line, and from the point of view of the MPI task that is forwarded stdin, file descriptor zero is a pipe to the local daemon (mpirun or orted), and that is why tcgetattr() and friends fail.

    Note all other MPI tasks have file descriptor zero pointing to /dev/null.

    IIRC, SLURM provides a srun option (e.g. direct-run) in which stdin is a pseudo-terminal.