#include "mpi.h" int MPI_Comm_group ( MPI_Comm comm, MPI_Group *group )
It is an error to use MPI_COMM_NULL as one of the arguments to MPI_Comm_group. The relevant sections of the MPI standard are
.(2.4.1 Opaque Objects) A null handle argument is an erroneous IN argument in MPI calls, unless an exception is explicitly stated in the text that defines the function.
.(5.3.2. Group Constructors) <no text in MPI_COMM_GROUP allowing a null handle>
Previous versions of MPICH allow MPI_COMM_NULL in this function. In the interests of promoting portability of applications, we have changed the behavior of MPI_Comm_group to detect this violation of the MPI standard.
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.
All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Errhandler_set; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error.