Why do Functions and Subroutines compile differently?

[scriabin3]

Please help me understand this:

MAIN program "contains" routines F1 and F2. MAIN calls F1 which in turn calls F2.

If F1 and F2 are Subroutines, then the program compiles and runs just fine. But if F1 and F2 are Functions, then the program gives a compilation error: Undefined symbols: "_F2_", referenced from: _MAIN__

In the latter case, what do I need to do to make the compiler "see" F2?

Here are example programs that display the above behavior:

!-------------------------

program test1

real :: x,a1

x=5.

call f1(x,a1)

print*, a1

contains

subroutine f1(x,a1)

real, intent(in) :: x

real, intent(out) :: a1

call f2(x,a1)

end subroutine f1

subroutine f2(x,a1)

real, intent(in) :: x

real, intent(out) :: a1

a1=2.*x

end subroutine f2

end program test1

!-------------------------

program test2

real :: x,a1

x=5.

a1=f1(x)

print*, a1

contains

function f1(x)

real :: f1,f2

real, intent(in) :: x

f1=f2(x)

end function f1

function f2(x)

real :: f2

real, intent(in) :: x

f2=2.*x

end function f2

end program test2

!-------------------------

[forcpp]

Interestingly your both versions of the same code work fine with gfortran. Also if we write the second code like below, we get the right result by ifort too.

[fortran]program test2
  real :: x,a1
  real, external :: f1
  x=5.
  a1=f1(x)
  print*, a1
end program test2

  function f1(x)
    real :: f1,f2
    real, intent(in) :: x
    f1=f2(x)
  end function f1

  function f2(x)
    real :: f2
    real, intent(in) :: x
    f2=2.*x
  end function f2[/fortran]

[mecej4 · Accepted solution]

In the second program, the one which uses functions f1 and f2, replace

real:: f1,f2

by

real :: f1

and run the program. Then, read the manuals on the topic of "host-association".

What happens is that in the body of internal function f1(x), the declaration of f2 as real overwrites the host-association of f2. Since f2 is already available and its attributes are known to the compiler by host-association, a redeclaration of f2 as real tells the compiler to override the previous instance of f2.

[scriabin3]

Thank you! So obvious, after your explanation.