Fortran terminal ISO standard units

iso_fortran_env is respected by modern Fortran compilers, as implied by its name. iso_fortran_env is part of the Fortran 2003 specification and popular Fortran compilers added it by calendar year 2010 generally.

Why use iso_fortran_env terminal I/O: legacy programs written before Fortran 2003 often write to terminal with:

write(*,*) 'The value of X is ',x

or

write(6,*) 'The value of X is ',x

This is a problem when trying to debug with text output to terminal, especially where someone has used “6” for file I/O unit by mistake. Or, if trying to write to a file with an uninitialized unit number, stderr gets redirected to file fort.0.

For operating systems including macOS, Windows, and Linux the standard terminal units correspond like:

Legacy code making use of open(unit=) number in this range can cause unexpected behavior. Please use open(newunit=...) to avoid conflicts with the standard terminal units. Of note, GFortran can use environment variables to change the default unit numbers for standard input, output, and error as in the table above.

Example

Print repeatedly to the same line, and combine prompt text on the same line with input.

iso_fortran_env terminal I/O: example prints to stdout, then stderr and finally asks for user input with a prompt on the same line.

program myterm
use iso_fortran_env
implicit none (type, external)
character(1000) :: usertxt  ! 1000 is an arbitrarily large number
integer :: ios

! could also just use print *,'printed to stdout'
write(output_unit,*) 'Printed to stdout'

write(error_unit,*) 'printed to stderr'

! prompt with caret on same line as input, here using a greater than sign >
write(output_unit,'(A)',advance='no') ' >'
flush(output_unit)

read(input_unit,"(A)", iostat=ios) usertxt
! trap Ctrl-D EOF on Unix-like systems to avoid crashing program
if (ios/=0) backspace(input_unit)  ! ctrl D gobble

write(output_unit,*) usertxt

end program

If stderr goes to file fort.0

If stderr from error_unit gets written to a file fort.0 instead of being printed to screen, this is an indication of open(u)ing a file without first setting a value for u, which might default to 0.

Unless needing to persist a file opening between calls of a function/subroutine, normally open a file with newunit.

program myfile
use iso_fortran_env
implicit none (type, external)

integer :: u, ios
character(1000) :: fn ! 1000 is an arbitrarily large number
character(1000) :: dat

print *, "please input file to open"

read(input_unit, '(a)', iostat=ios) fn
if(is_iostat_end(ios)) stop

! open file, using better to ask forgiveness than permission principle
! status='old' means generate error if file doesn't exist
open(newunit=u,file=fn, status='old',action='read',iostat=ios)
if (ios /= 0) then
    write(error_unit,*) 'could not open file ',trim(fn)
    error stop 'file IO error'
endif

read(u,'(A)') dat
print *,'first two lines of ',trim(fn),' are:'
print *,trim(dat)
read(u,'(A)') dat
print *,trim(dat)

close(u)  ! implicitly closed at end of program, but as good practice...
end program

Related:

• Fortran iostat error code
• Fortran stream buffering