OpenMP: Getting a fortran pointer into PRIVATE()

Question

I am trying to apply OpenMP to a large Fortran code.

Presumably to reuse memory many loops rely on work arrays that are created during initialisation. The loops access them through a long series of pointers.

Now according to the standard any PRIVATE() variables in Fortran must be either allocatable or defineable. Is their is way to dereference this pointer mess or is my best option to create a new variable with allocate(foo, source=bar)?

The complete structure I am dealing with is something like this:

type work_type
    allocatable :: bar
end type
type(work_type) :: work

pointer, type(work_type) :: w
w => work

pointer :: foo, bar
bar => w%bar
foo => bar

Or to summarise:

for => bar => w%bar
              w => work

Massimiliano · Accepted Answer · 2014-01-23 13:30:06Z

If I get your question correctly, your problem is with the initialization of private pointers to the same memory location when inside a parallel region.

If this is the case, then you should use the copyprivate clause of the single worksharing directive. This clause permits to broadcast the value of a private variable to other threads:

REAL, POINTER :: A(:)

...

!$OMP SINGLE
  ALLOCATE(A,10)
!$OMP SINGLE COPYPRIVATE(A)

! SPACE IS ALLOCATED ONLY ONCE BUT EVERY THREAD HAS A PRIVATE POINTER TO IT

The relevant quotes from the OpenMP 4.0. Standard should be:

The copyprivate clause provides a mechanism to use a private variable to broadcast a value from the data environment of one implicit task to the data environments of the other implicit tasks belonging to the parallel region.

...

If the list item has the POINTER attribute, then, in all other implicit tasks belonging to the parallel region, the list item receives, as if by pointer assignment, the same association status of the corresponding list item in the implicit task associated with the thread that executed the structured block.

The full description of the clause (in case you are interested) is in section 2.14.1.2.

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