The value returned for MPI_HOST gets the rank of the HOST process in the group associated
with communicator MPI_COMM_WORLD, if there is such.
MPI_PROC_NULL is returned if there is no host.
MPI does not specify what it
means for a process to be a HOST, nor does it requires that a HOST
exists.
The attribute MPI_HOST has the same value on all processes
of MPI_COMM_WORLD.
MPI-Standard for MARMOT