How to use utf-8 in gfortran?

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1

Warning: command-line option ‘-finput-chartset=utf-8’ is valid for C/C++/ObjC/ObjC++ but not for Fortran.
How to fix it?

2

For Fortran if the compiler supports it you specify UTF-8 using standard code, no special compiler options are required in general.

program demo_selected_char_kind
use iso_fortran_env
implicit none

intrinsic date_and_time,selected_char_kind

! set some aliases for common character kinds
! as the numbers can vary from platform to platform

integer, parameter :: default = selected_char_kind ("default")
integer, parameter :: ascii =   selected_char_kind ("ascii")
integer, parameter :: ucs4  =   selected_char_kind ('ISO_10646')
integer, parameter :: utf8  =   selected_char_kind ('utf-8')

! assuming ASCII and UCS4 are supported (ie. not equal to -1)
! define some string variables
character(len=26, kind=ascii ) :: alphabet
character(len=30, kind=ucs4  ) :: hello_world
character(len=30, kind=ucs4  ) :: string

   write(*,*)'ASCII     ',&
    & merge('Supported    ','Not Supported',ascii /= -1)
   write(*,*)'ISO_10646 ',&
    & merge('Supported    ','Not Supported',ucs4 /= -1)
   write(*,*)'UTF-8     ',&
    & merge('Supported    ','Not Supported',utf8 /= -1)

   if(default.eq.ascii)then
       write(*,*)'ASCII is the default on this processor'
   endif

   ! for constants the kind precedes the value, somewhat like a
   ! BOZ constant
   alphabet = ascii_"abcdefghijklmnopqrstuvwxyz"
   write (*,*) alphabet

   hello_world = ucs4_'Hello World and Ni Hao -- ' &
                 // char (int (z'4F60'), ucs4)     &
                 // char (int (z'597D'), ucs4)

   ! an encoding option is required on OPEN for non-default I/O
   if(ucs4 /= -1 )then
      open (output_unit, encoding='UTF-8')
      write (*,*) trim (hello_world)
   else
      write (*,*) 'cannot use utf-8'
   endif

   call create_date_string(string)
   write (*,*) trim (string)

contains

! The following produces a Japanese date stamp.

subroutine create_date_string(string)
intrinsic date_and_time,selected_char_kind
integer,parameter :: ucs4 = selected_char_kind("ISO_10646")
character(len=1,kind=ucs4),parameter :: &
   nen =   char(int( z'5e74' ),ucs4), & ! year
   gatsu = char(int( z'6708' ),ucs4), & ! month
   nichi = char(int( z'65e5' ),ucs4)    ! day
character(len= *, kind= ucs4) string
integer values(8)
    call date_and_time(values=values)
    write(string,101) values(1),nen,values(2),gatsu,values(3),nichi
101 format(*(i0,a))
end subroutine create_date_string

end program demo_selected_char_kind

 ASCII     Supported
 ISO_10646 Supported
 UTF-8     Not Supported
 ASCII is the default on this processor
 abcdefghijklmnopqrstuvwxyz
 Hello World and Ni Hao -- 你好
 2025年7月25日

so for gfortran it looks like you want to use ISO_10646 or you will have to handle things at a lower level yourself with UTF-8, most likely using stream I/O. There have been previous discussions on this topic. See

unicode tutorial

for one

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