Re: ALLOCATABLE arrays

From: Gus Gassmann (hgassmann_at_mgmt.dal.ca)
Date: 01/06/04 

Date: Tue, 06 Jan 2004 12:39:28 -0400

Tony Jay wrote:

> > Do I read this right?
> >
> > Suppose I declare
> >
> > subroutine test
> > integer :: i
> > dimension i(k)
> >
> > where k is set in a module somewhere. Is it then the case that
> > a) i gets allocated automatically on entry to test and deallocated
> > on exit
> > b) this operation takes (essentially) zero time?
> >
> > Is that really all there is to it? I'm afraid to look a gift horse
> > in the mouth, but this sounds too good to be true!
> >
> >
> Gus,
>
> This technique can cause problems if you have insufficient stack size
> (especially if ou add in a second array at a later date)
>
> My understanding is ...
>
> If try to create a very large array on the stack and you do not have enough
> stack (set up in the linking step of the compiler i think) then you program
> will fail at run time.
>
> Allocating on the heap gives you access to a hell of a lot more memory (well
> as much as your OS can find !)
>
> The typical allocate on the stack problem is to test with small problems and
> then end users use large problems bomb out. You then look a fool (speaking
> from personal experience)

Hmmmh. 'Nother question, then: Do I get any control in where the array is
allocated? Here is my current understanding from reading the various
responses, many of which are over my head, I'm afraid:

Allocatable arrays are allocated on the heap. They can be pretty damn large,
and I can inquire whether things went OK. The downside is that it is slow.
Automatic arrays are always allocated on the stack. They are very fast, but
if the stack overflows, I have no recourse, so I am SOOL. So why use them,
except for very small arrays? Is there a compiler directive that moves
automatic arrays to the heap?