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I've always been using the -fdefault-real-8 option of gfortran to automatically promote every single REAL declared anywhere in the program to double precision, along with any constant, e.g. 1.23. If I ever wanted to switch back to single precision, I only had to remove that option and recompile, without changing a single character in the source code.

At a point I started using ISO_FORTRAN_ENV module, since it allows me to use constants like INPUT|OUTPUT|ERROR_UNIT, as well as IOSTAT_END and IOSTAT_EOR and others (which seemed to be a good and easy move in the direction of portability, am I wrong?). From then on, I've been seeing and ignoring the following warning

Warning: Use of the NUMERIC_STORAGE_SIZE named constant from intrinsic module ISO_FORTRAN_ENV at (1) is incompatible with option -fdefault-real-8

since such incompatibility seems to have no effect so far.

Now I'd like to get rid of this warning if it is possible and worth it.

If I correctly understood, to avoid this warning I should give up on -fdefault-real-8 option and change every REAL to REAL(real64) and/or to REAL(dp) (provided that, in the latter case, the statement USE, INTRINSIC :: ISO_FORTRAN_ENV, dp => real64 is put in that unit), which is not a difficult task for sed or vim.

Nevertheless, it seems to me that this change wouldn't be the same as using -fdefault-real-8 option, since all constants would stay single precision as long as I don't add d0 to them. Assumed the -fdefault-real-8 option is removed and ISO_FORTRAN_ENV is used anywhere, is there any way to make any constant across the program behave as each had d0 suffix? Whether or not this is possible, have I correctly extrapolated that I can put the following lines in a single module which is used by all others program units, each of which can then use dp as kind type parameter?

USE, INTRINSIC :: ISO_FORTRAN_ENV
INTEGER, PARAMETER :: dp = real64

I would prefer this way since I could switch to real32 or real128 or whatever by changing only that line.

Enlico
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2 Answers2

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If you just want to silence the warning and you do not care about the implications -fdefault-real-8 has on storage association and some Fortran standard requirements, just do not import NUMERIC_STORAGE_SIZE from the module. For example,

USE, INTRINSIC :: ISO_FORTRAN_ENV, only: INPUT_UNIT,OUTPUT_UNIT,ERROR_UNIT
Vladimir F Героям слава
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  • Your solution seems quite immediate, but your words make me worry about _the implications `-fdefault-real-8` has on storage association and some Fortran standard requirements_. Could you tell me more about it? – Enlico Apr 01 '18 at 10:48
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    Standard Fortran requires that default logical, integer and real are of the same size (the numeric storage unit). If you increase the real but not the others you are breaking this assumption and if some code relies on that it will be broken. – Vladimir F Героям слава Apr 01 '18 at 12:58
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Assumed the -fdefault-real-8 option is removed and ISO_FORTRAN_ENV is used anywhere, is there any way to make any constant across the program behave as each had d0 suffix?

No.

By the way, d0 is exactly the same as double precision, so that doesn't fixate much either, since the meaning of double precision is allowed to vary as much as real.

Whether or not this is possible, have I correctly extrapolated that I can put the following lines in a single module which is used by all others program units, each of which can then use dp as kind type parameter?

Yes. That is a common practice.

André Chalella
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