writing to stderr

[Beliavsky]

writing to stderr

[urbanjost]

If you think the Fortran standard says unit 5 is stdin, 6 is stdout,
and 0 is stderr -- Surprise! The standard says nothing about what units
are used for pre-assigned files. Those are just often the units used
by convention.

So how do you get the unit numbers in a portable way? Since f2003,
you can use constants from the intrinsic module ISO_FORTRAN_ENV.

program demo_stderr
use,intrinsic :: iso_fortran_env, only : &
 stdin=>input_unit, &
 stdout=>output_unit, &
 stderr=>error_unit
implicit none
character(len=256) :: line
    read(stdin,'(a)') line
    write(stdout,'(a)') line
    write(stderr,*)'Error: program will now stop'
    stop 4
end program demo_stderr