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From: Jim Xia on 13 May 2010 22:32

>
> Anyway, I have a question : does it mean that the two following
> subroutines would behave differently ?
>
>  module m
>   implicit none
> contains
>   subroutine foo1(a,b)
>     real, dimension(:), pointer :: a,b
>     a(1) = 1.0
>     b(1) = 2.0
>   end subroutine foo1
>   subroutine foo2(a,b)
>     real, dimension(:) :: a,b
>     a(1) = 1.0
>     b(1) = 2.0
>   end subroutine foo2
> end module m


The answer is possible because "call foo2(ptr1, ptr1)" is illegal by
Fortran standard. This is one of the most FAQ, and answered
repeatedly, and yet forgotten by most people or wasn't understood in
the first place.

Here is what the standard says
"
12.5.2.13 Restrictions on entities associated with dummy arguments
1 While an entity is associated with a dummy argument, the following
restrictions hold.
(1) Action that affects the allocation status of the entity or a
subobject thereof shall be taken through
the dummy argument.
(2) If the allocation status of the entity or a subobject thereof is
affected through the dummy argument,
then at any time during the invocation and execution of the procedure,
either before or after the
allocation or deallocation, it shall be referenced only through the
dummy argument.
(3) Action that affects the value of the entity or any subobject of it
shall be taken only through the
dummy argument unless
(a) the dummy argument has the POINTER attribute or
(b) the dummy argument has the TARGET attribute, the dummy argument
does not have INTENT
(IN), the dummy argument is a scalar object or an assumed-shape array
without the CONTI-
GUOUS attribute, and the actual argument is a target other than an
array section with a
vector subscript.
(4) If the value of the entity or any subobject of it is affected
through the dummy argument, then at
any time during the invocation and execution of the procedure, either
before or after the definition,
it may be referenced only through that dummy argument unless
(a) the dummy argument has the POINTER attribute or
(b) the dummy argument has the TARGET attribute, the dummy argument
does not have INTENT
(IN), the dummy argument is a scalar object or an assumed-shape array
without the CONTI-
GUOUS attribute, and the actual argument is a target other than an
array section with a
vector subscript."

The text is pretty hard to read, but basically it says if two dummy
arguments are referring to the same entity, you'd better NOT to modify
their values, allocation or association status. The ONLY exceptions
are 1.) the POINTER dummy whose value can be changed, 2.) a modifiable
TARGET dummy argument associated with a TARGET actual argument (i.e.
no copy-in situation).

I would steer clear off overlapping dummy arguments in code.

Cheers,

Jim
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