How to use inteface blocks to pass a function to a subroutine?

Question

I understand the interface command can be used to pass a a function into a subroutine. So for example in the main program I\'d define some function and then pass it to some subr

Answer 1

A simple way to do this is to go old school and just leave the function external:

 program main
 real f,z
 external f
 call subr(f,z)
 write(*,*)z
 end

 real function f(x)
 real x
 f=x**2
 end

! below possibly in a precompiled library:

 subroutine subr(f,y)
 real f,y
 y=f(2.)
 end

out: 4

Of course with this approach you can not use advanced language features that require an explicit interface. **

On the other hand if you are interfacing with standard libraries that need function arguments this is I think the only way.

** per MSB's comment you can handle that issue with an interface block in the subroutine, for example if we want to pass a function that returns an array:

 function f(x)
 real x,f(2)
 f(1)=x
 f(2)=x**2
 end

as in the first example f is an external function, and the sub can be in a precompiled library:

 subroutine subr(g,y)
 interface
 function g(x)
 real x,g(2)
 end function
 end interface
 real y,z(2)
 z=g(2.)
 y=z(1)+z(2)
 end

out: 6

As noted, this is only strictly necessary if relying on language features that need the interface.