Redirecting unit * to a file

Help
1

I would like to have a program read from standard in by default or from a file if a specified. I have thought about using unit 5 for input, which I can redirect to a file in an open statement, but I understand 5 isn’t designated as standard input by the Fortran standard. Is that really a problem? Am I likely to run in to users where unit 5 is not standard in? I mean, someone with a Linux, WIndows or Mac machine. I don’t care about anything older than f2c. The program is used by random unsophisticated users, I’d like to put the redirection in the program rather than rely on a shell script I would have to explain.

As it is, the best I can think of to do that fits the standard is to replace each read statement with 2 read statements, one with a * and one with a unit number. Is that the best I can do?

I notice that some compilers such as
Oracle Fortran allow a * in the open statement:

[UNIT=] u

  • u is an integer expression or an asterisk (*) that specifies the unit number. u is required

but does that modify read or write statements? I suppose it doesn’t matter, since so few compilers support this. Still, it seems odd if there is no way to redirect unit *.

1 Like
2

Why not use input_unit from module ISO_FORTRAN_ENV of Fortran 2003?

1 Like
3

If you have not used INPUT_UNIT before, here is an example that reads one line from stdin
and echoes it unless a filename is given on the command line, in which case that file is read from instead. This assumes your input file is a text file; reading binary files from stdin is problematic.

program readfrom
use,intrinsic :: iso_fortran_env, only : &
 & stdin=>input_unit, &
 & stdout=>output_unit,&
 & stderr=>error_unit
implicit none
integer                      :: count, argument_length, lun, ios
character(len=:),allocatable :: filename
character(len=256)           :: line, msg
   ! get number of arguments
   count = command_argument_count()
   if(count.ne.0)then
       call get_command_argument(number=1,length=argument_length)
       ! allocate string array big enough to hold command line argument 
       allocate(character(len=argument_length) :: filename)
       call get_command_argument(1, filename)
       open(file=filename,newunit=lun)
   else
       lun=stdin
   endif
   read(lun,'(a)',iostat=ios,iomsg=msg)line
   if(ios.eq.0)then
      write(*,*)'READ  :',trim(line)
   else
      write(*,*)'<ERROR>',trim(msg)
   endif

end program readfrom`

so if you entered “readfrom myfile” it would read the first line of file “myfile”; otherwise it would
read a line from stdin. You do not have to rename the variables, but I prefer the names stdin, stdout, and stderr myself.

4

Thanks. Another demonstration that there is no problem in computer science that can not be solved with an additional level of indirection.

5

If you are using Intel Fortran and the ISO_FORTRAN_ENV constants, be sure to compile with -standard-semantics. If you don’t, the compiler will use various negative unit numbers for unit * (or PRINT).

1 Like
6

Would that make any difference in a program like above @urbanjost’s sample?

7

No - my comment related to @feenberg 's mention of unit *, which is part of the standard language. Specifically, if you expected to open unit INPUT_UNIT and then do READ(*) and have that be the unit you opened, it would not work in Intel Fortran without the standard-semantics option (or -assume noold_unit_star)