Function doesn't return correct value

Question

I am trying to write a Fortran 77 program where a subroutine makes a function call, with the function being provided as an argument (to the subroutine).

The problem I am facing is that function 'bar' doesn't return the correct result. Here is a minimal (not) working example:

% cat mwe.f
      real*8 function bar()
      print *,"bar:",bar
      bar = 101.0d00
      print *,"bar:",bar
      end

      subroutine foo(func)
      real*8 rv
      rv = func()
      print *,"rv:",rv
      end

      program tsig
      external bar
      call foo(bar)
      end
% gfortran mwe.f && ./a.out
 bar:   0.0000000000000000
 bar:   101.00000000000000
 rv:   0.0000000000000000
%

Answer 1

It was stated in the comments, but perhaps it should be said explicitly, because, you still appear to struggle. Keeping (pseudo) Fortran 77 you must do

  subroutine foo(func)
    real*8 rv
    real*8 func
    rv = func()
    print *,"rv:",rv
  end

The reason is that the type of func is assumed to be implicitly real inside foo. You must declare it explicitly if it returns some other type.

I strongly recommend to place implicit none at the start of each program and subroutine. It isn't part of Fortran 77 standard, but neither is real*8. Both are just common extensions. implicit none is standard in Fortran 90, real*8 is not standard Fortran at all.