Oh great Open MPI Gurus,
I'm slowly trying to learn and transition to 'use mpi_f08'. So, I'm writing
various things and I noticed that this triggers an error:
program hello_world
use mpi_f08
implicit none
type(MPI_Comm) :: comm = MPI_COMM_NULL
end program hello_world
when compiled (Open MPI 2.0.0 with GCC 6.1):
(380) $ mpifort test1.F90
test1.F90:7:27:
type(MPI_Comm) :: comm = MPI_COMM_NULL
1
Error: Parameter ‘mpi_comm_null’ at (1) has not been declared or is a
variable, which does not reduce to a constant expression
Why is that? Obviously, I can just do:
type(MPI_Comm) :: comm
comm = MPI_COMM_NULL
and that works just fine (note MPI_COMM_NULL doesn't seem to be special as
MPI_COMM_WORLD triggers the same error).
I'm just wondering why the first doesn't work, for my own edification. I
tried reading through the Standard, but my eyes started watering after a
bit (though that might have been the neon green cover). Is it related to
the fact that when one declares:
type(MPI_Comm) :: comm
that the comm == MPI_COMM_WORLD evaluates to .TRUE.? I discovered that once
when I was printing some stuff.
Thanks for helping me learn,
Matt
--
Matt Thompson
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