double-precision (MOD001)

What it does

Checks for use of double precision and double complex types.

Why is this bad?

The double precision type does not guarantee a 64-bit floating point number as one might expect, and instead is only required to have a higher decimal precision than the default real, which may vary depending on your system and can be modified by compiler arguments.

In modern Fortran, it is preferred to use real and complex and instead set the required precision using 'kinds'. For portability, it is recommended to use real(dp), with dp set in one of the following ways:

• use, intrinsic :: iso_fortran_env, only: dp => real64
• integer, parameter :: dp = selected_real_kind(15, 307)

For code that should be compatible with C, you should instead use real(c_double), which may be found in the intrinsic module iso_c_binding.

References

• Metcalf, M., Reid, J. and Cohen, M., 2018, Modern Fortran Explained: Incorporating Fortran 2018, Oxford University Press, Appendix A 'Deprecated Features'
• Fortran-Lang Best Practices on Floating Point Numbers