How can I detect if my application is started by MS MPI mpiexec?

Question

My application is sequential and I want to make it parallel. Hence I want to check if I am in MPI environment or not to adapt to parallel processing.

Answer 1

Sequential application is a special case of a parallel application (when number of processes = 1), so you should not need any special adaptation. Just call MPI_Init and, as suggested in the comments, determine the communicator size.

Some MPI implementations (e.g., Open MPI and MS MPI) allow you execute your program directly, without mpiexec, and still the call to MPI_Init will not fail, so your parallel application can then be used equally well as a serial one.

To actually detect whether your application is really executed using mpiexec, you can typically inspect the environment variables. I used the following program to print the environment variables of my Windows application (linked to MS MPI v10):

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

int main (void)
{
    for (const char* envs = GetEnvironmentStrings();
         *envs != 0;
         envs += strlen(envs) + 1)
    {
        printf("%s\n", envs);
    }

    return 0;
}

When I execute the application using mpiexec -n 1, I see that several new environment variables are set on top of those already present when not using the launcher:

PMI_APPNUM=0
PMI_DOMAIN=a96606eb-0e17-46fc-ba6e-d8ca3f462bc6
PMI_HOST=localhost
PMI_KVS=4d648866-1723-4f0d-b2dc-450eae52e410
PMI_NODE_IDS=smp_region_5540
PMI_PORT=03fd2819-719a-43a0-ad9a-d8b9fb0443fd
PMI_RANK=0
PMI_RANK_AFFINITIES=affinity_region_5540
PMI_SIZE=1
PMI_SMPD_ID=1
PMI_SMPD_KEY=0
PMI_SPAWN=0

Hence, presence of these environment variables may indicate use of MS MPI. Similarly, presence of OMPI_COMM_WORLD_SIZE would indicate use of Open MPI. Etc.