7.1.1.2 Host rank

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