Fortran constants `MPI_ARGV_NULL` and `MPI_ARGVS_NULL` are defined
in MPI-3.1 p.680 as below.
> `MPI_ARGVS_NULL`
> 2-dim. array of `CHARACTER*(*)`
> `MPI_ARGV_NULL`
> array of `CHARACTER*(*)`
`MPI_ARGV_NULL` and `MPI_ARGVS_NULL` are used as an argument of
`MPI_COMM_SPAWN` and `MPI_COMM_SPAWN_MULTIPLE` respectively and
their argument `argv` and `array_of_argv` are defined as below
for `USE mpi_f08` binding in MPI-3.1.
```
CHARACTER(LEN=*), INTENT(IN) :: argv(*)
CHARACTER(LEN=*), INTENT(IN) :: array_of_argv(count, *)
```
Defining them as `INTEGER` in `mpi_f08` module will cause
a compilation error of user programs like
"There is no specific subroutine for the generic 'mpi_comm_spawn'".
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
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