mpirun not passing command line arguments

Question

mpirun (and mpiexec) do not seem to be passing command line arguments to my c code.

Running my exeutable "gecko" directly from the command line gives expected:

$ ./gecko  -np 2

main:There are 3 arguments:
arg=./gecko
arg=-np
arg=2

But running the same through mpirun is different:

$ mpirun -np 2 ./gecko

main:There are 1 arguments:
arg=./gecko

which means MPI_init(argc,argv) doesn't have any arguments to work with. I'm on Ubuntu 12.04 with MPICH 2.

Can anyone see a reason why this is not happening?

Thanks.

--------------------------- EDIT ---------------------------------

There are many examples on the net that say the way to initialize MPI is via the command line arguments, eg.:

#include <stdio.h>
#include “mpi.h”                                
int main(int argc, char* argv[])
{
 int size, rank;
 MPI_Init(&argc, &argv);                       
 MPI_Comm_size(MPI_COMM_WORLD, &size);           
 MPI_Comm_rank(MPI_COMM_WORLD, &rank);
 printf(“Greetings from process %i\n”, rank);
 MPI_Finalize();                                
 return 0;
}

and that the way to execute the mpi code is to use:

mpirun -np 2 ./code_name

So, if mpirun does not pass the arguments np and 2 to the c code, how does the c code ever get to know how many processors it should be run on?

Well, at least it is consistent. With OpenMPI a have some processes receiving the arguments and other part not receiving them (in Fortran, though). — Vladimir F
You should use instead mpirun -np 2 ./gecko -np 2 if you'd like to pass -np 2 to gecko. — Hristo Iliev
MPI-1 required that argc and argv should be passed to MPI_Init. MPI-2 removed that requirement. Most examples use MPI_Init(&argc, &argv) purely of historical reasons. — Hristo Iliev

Hristo Iliev Hristo Iliev · Accepted Answer · 2014-10-14T14:45:06

Ever since circa 1997 MPI_Init() also works if both its arguments are NULL. This means that practically all MPI implementations use a different mechanism to pass configuration information, namely environment variables and files. For example, Open MPI passes the following variables:

$ cat print_ompi_vars
#!/bin/bash
printenv | grep ^OMPI | sort
$ mpiexec -np 1 ./print_ompi_vars
OMPI_COMM_WORLD_LOCAL_RANK=0
OMPI_COMM_WORLD_LOCAL_SIZE=1
OMPI_COMM_WORLD_NODE_RANK=0
OMPI_COMM_WORLD_RANK=0
OMPI_COMM_WORLD_SIZE=1
OMPI_MCA_btl=^tcp
OMPI_MCA_btl_openib_ib_timeout=24
OMPI_MCA_btl_openib_warn_default_gid_prefix=0
OMPI_MCA_btl_tcp_if_include=ib0
OMPI_MCA_carto=file
OMPI_MCA_carto_file_path=/opt/MPI/openmpi/carto/carto_2K-64C.txt
OMPI_MCA_ess=env
OMPI_MCA_mpi_yield_when_idle=0
OMPI_MCA_oob_tcp_if_include=ib0
OMPI_MCA_orte_app_num=0
OMPI_MCA_orte_cpu_model=Intel(R) Xeon(R) CPU           X5675  @ 3.07GHz
OMPI_MCA_orte_cpu_type=GenuineIntel
OMPI_MCA_orte_daemonize=1
OMPI_MCA_orte_ess_jobid=4114219009
OMPI_MCA_orte_ess_node_rank=0
OMPI_MCA_orte_ess_num_procs=1
OMPI_MCA_orte_ess_vpid=0
OMPI_MCA_orte_hnp_uri=4114219008.0;tcp://1.2.3.4:48206
OMPI_MCA_orte_local_daemon_uri=4114219008.1;tcp://1.2.3.4:59277
OMPI_MCA_orte_num_nodes=1
OMPI_MCA_orte_num_restarts=0
OMPI_MCA_orte_precondition_transports=1f4c3cf87403b137-a8e3173542efb9c3
OMPI_MCA_plm=rsh
OMPI_MCA_shmem_RUNTIME_QUERY_hint=mmap
OMPI_UNIVERSE_SIZE=1

It works similarly with other MPI implementations.

Anyway, if you wish to pass arguments to your MPI executable, put them as usual after the name of the executable:

$ mpiexec -n 16 <other mpiexec args> ./gecko -np 2 <other program args>

mpirun not passing command line arguments

2 Answers