A quick question regarding fortran pointers

[Wentao_Z_]

Hi,

I am quite new to fortran and have some simple questions regarding fortran pointer. The code piece is shown below:

1. subroutine pointer_test(p_in)
2.   Implicit None
3.   
4.   !…Incoming varibale
5.   real(kind=8), pointer :: p_in
6.  
7.   !…Local variable
8.   real(kind=8), pointer :: p_local(:,:,:)
9.  
10.   p_local => p_in
11.  
12. end subroutine

My questions are: 
If p_in points to a 3D-array of size 10*20*30, does p_in contain only one (the initial) address of the 3D-array or 6000 addresses for all the floating point numbers?

What's the data movement in p_local => p_in (line 10) ? I am just moving one address from p_in into p_local or I am moving 6000 addresses?

Thanks!
 

[Steven_L_Intel1 · Accepted solution]

The code you show is not legal and won't compile. You can't point a rank-3 pointer to a rank-1 array. But in general, pointer assignment at most moves one address and some other "descriptor" information - it doesn't move data. Even in a multidimensional array case, it moves the address and bounds, not data.

There are other ways to get this sort of rank remapping - are you sure you need to pass pointers around? In modern Fortran, pointers have very limited applicability.

[Wentao_Z_]

Steve Lionel (Intel) wrote:

The code you show is not legal and won't compile. You can't point a rank-3 pointer to a rank-1 array. But in general, pointer assignment at most moves one address and some other "descriptor" information - it doesn't move data. Even in a multidimensional array case, it moves the address and bounds, not data.

There are other ways to get this sort of rank remapping - are you sure you need to pass pointers around? In modern Fortran, pointers have very limited applicability.

Hi Steve,

Thanks for your reply. Your explanations truly help me a lot. I am sorry that in line 5 it should be p_in(:,:,:) rather than p_in.

This is quite a big code that I just start learning. We will consider changing the data structure for improving performance. 

Many thanks for your kind suggestions!

[Steven_L_Intel1]

Ok. In that case, it is as I said - just one address and some bounds information is copied. There is only one data pointer anyway, not one for each element.