module test1

implicit none

contains

subroutine x(ff,y)

    interface
        real function ff(y)
            real, intent(in) :: y
        end function ff
    end interface
    real, intent(in) :: y
    integer z

    z=ff(y)

end subroutine x

end module test1

program tester
use test1
implicit none

call x(f,1.0)

contains

real function f(y)
    real, intent(in) :: y

    write(*,*) y
    f=y*y
end function f

end program tester

gfortran-7 -ggdb test_fun_passing.f90 -o test

(gdb) bt
#0  0x00007ffffffde320 in ?? ()
#1  0x0000000000400734 in test1::x (ff=0x7ffffffde320, y=1) at test_fun_passing.f90:17
#2  0x0000000000400829 in tester () at test_fun_passing.f90:31
#3  0x0000000000400860 in main (argc=1, argv=0x7ffffffde64f) at test_fun_passing.f90:27
#4  0x00007ffffec70830 in __libc_start_main (main=0x40082c <main>, argc=1, argv=0x7ffffffde448,             init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffffffde438)        at ../csu/libc-start.c:291

#5  0x0000000000400669 in _start ()

module test1

implicit none

contains

subroutine x(ff,y)

    interface
        real function ff(y)
            real, intent(in) :: y
        end function ff
    end interface
    real, intent(in) :: y
    integer z

    z=ff(y)

end subroutine x

end module test1

module test2

implicit none

contains

real function f(y)
    real, intent(in) :: y

    write(*,*) y
    f=y*y
end function f

end module test2

program tester
use test1
use test2
implicit none

call x(f,1.0)

end program tester


gfortran-7 -ggdb test_fun_passing.f90 -o test && ./test
1.00000000