Confusing double precision real in Fortran

Have this burning question on my mind right now: What is the "accepted" way to declare double precision real in modern Fortran? In order from oldest to newest, the story seems to go like this: DOUBLE PRECISION, then REAL(kind=8), then INTEGER, PARAMETER :: dp=kind(1.d0) with REAL(kind=dp)--Metcalf now says dp=kind(0.d0)--and now float32=selected_real_kind(6,37) or float64=selected_real_kind(15,307). So...

1. How should I be declaring double precision real now?
2. Is kind redundant in REAL(kind=dp)?
3. Are there any special flags needed at compile time to invoke double precision real with gfortran or ifort?

Solution 1:

Personally I now write

use, intrinsic :: iso_fortran_env

which includes parameters such as int32,real64 which have the obvious meanings, and can be used like this:

real(real64) :: a_64_bit_real_scalar

Note that kind=8 is not guaranteed, by the standard, to deliver an 8-byte kind. The values that kind parameters take are not standardised and do vary from compiler to compiler.

You could, if you want, write statements such as

use, intrinsic :: iso_fortran_env, dp=>real64
...
real(dp) :: a_64_bit_real_scalar

Solution 2:

1)How should I be declaring double precision real now?

I personally prefer using the

integer, parameter :: wp = selected_real_kind(15,307)
real(wp) :: var

method for this. But as Mark points out, the iso_fortran_env is another straight-forward method. If you plan on using interfaces to C, you may want to try the ISO_C_BINDING module and use

use iso_c_binding
real(c_double) :: var

And get the double precision you want.

2) Is kind redundant in REAL(kind=dp)?

Yes it is.

3) Are there any special flags needed at compile time to invoke double precision real with gfortran or ifort?

For gfortran you can use the compilation flag -fdefault-real-8. For ifort, you can use the -real-size=64 flag.