array of pointers fortran

Question

Again, array of pointers in Fortran. So well, I've a derived type:

type :: t_context_pointer  
    type(t_context),pointer :: p_ctx  
end type t_context_pointer

When I do in the main program:

type(t_context_pointer),allocatable :: tab_ctx_ptr(:)  
type(t_context),allocatable,target :: ctx  
allocate(tab_ctx_ptr(1))  
allocate(ctx)  
tab_ctx_ptr(1)%p_ctx=>ctx

it works. But when I use a function call:

type(t_context_pointer),allocatable,target :: tab_ctx_ptr(:)
allocate(tab_ctx_ptr(n))
call alloc_ctx(tab_ctx_ptr,n,1)

with elsewhere:

subroutine alloc_ctx(tab_ctx_ptr,n,i)
    integer,intent(in) :: n,i
    type(t_context_pointer),intent(inout) :: tab_ctx_ptr(n)

    type(t_context),allocatable,target :: ctx

    allocate(ctx)
    tab_ctx_ptr(i)%p_ctx=>ctx
end subroutine

it seg_fault later in the code. Is there something wrong here?

Alexander Vogt · Accepted Answer · 2013-10-16 10:13:42Z

4

ctx is in the scope of the subroutine alloc_ctx and gets deallocated as soon as you leave the subroutine. Accessing it later (by means of the pointer) will cause a segfault.

In the first example, you allocate ctx in the main program, so its scope will be until the end of the program.

Why don't you allocate tab_ctx_ptr(i)%p_ctx directly:

subroutine alloc_ctx(tab_ctx_ptr,n,i)
    integer,intent(in) :: n,i
    type(t_context_pointer),intent(inout) :: tab_ctx_ptr(n)

    allocate(tab_ctx_ptr(i)%p_ctx)
end subroutine

As for deallocation: You could use something like this to deallocate all pointers (but not the array itself):

subroutine dealloc_ctx(tab_ctx_ptr,n)
    integer,intent(in) :: n
    type(t_context_pointer),intent(inout) :: tab_ctx_ptr(n)
    integer            :: i

    do i=1,n
      ! Check whether the current element has an allocated element and 
      ! deallocate if necessary. 
      if ( associated(tab_ctx_ptr(i)%p_ctx) ) deallocate(tab_ctx_ptr(i)%p_ctx)
    enddo ! i
end subroutine

edited Oct 16, 2013 at 10:13

answered Oct 16, 2013 at 9:46

Alexander Vogt

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7 Comments

user2885778 Over a year ago

Why doing that, I mean not directly? In fact it is a first step of a more complex code which plans to manage multi-threads allocations and to assure locality of data in the different memories (e.g. NUMA).

user2885778 Over a year ago

And finally, if there is an automatic deallocation: how to do as in a C code?

Alexander Vogt Over a year ago

I edited my answer to show an example... BTW: It is always a good idea to check the stat of the allocation!

Alexander Vogt Over a year ago

Now, I'm confused... What C code? Your example is in Fortran?!

user2885778 Over a year ago

Of course I check, I've just simplified the exemple. BTW: your solution works, so first, thanks, second, I'me sure I've read an example with an allocation like my example in a forum...

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