Reading a character string of unknown length

Question

I have been tasked with writing a Fortran 95 program that will read character input from a file, and then (to start with) simply spit it back out again. The tricky part is that

Answer 1

Here's a function for Fortran 2003, which sets an allocatable string (InLine) of exactly the length of the input string (optionally trimmed), or returns .false. if end of file

function ReadLine(aunit, InLine, trimmed) result(OK)
integer, intent(IN) :: aunit
character(LEN=:), allocatable, optional :: InLine
logical, intent(in), optional :: trimmed
integer, parameter :: line_buf_len= 1024*4
character(LEN=line_buf_len) :: InS
logical :: OK, set
integer status, size

OK = .false.
set = .true.
do
    read (aunit,'(a)',advance='NO',iostat=status, size=size) InS
    OK = .not. IS_IOSTAT_END(status)
    if (.not. OK) return
    if (present(InLine)) then
        if (set) then
            InLine = InS(1:size)
            set=.false.
        else
            InLine = InLine // InS(1:size)
        end if
    end if
    if (IS_IOSTAT_EOR(status)) exit
end do
if (present(trimmed) .and. present(InLine)) then
    if (trimmed) InLine = trim(adjustl(InLine))
end if

end function ReadLine

For example to do something with all lines in a file with unit "aunit" do

 character(LEN=:), allocatable :: InLine

 do while (ReadLine(aunit, InLine))
   [.. something with InLine]
 end do