4.3 Cray Pointers

4.3Cray Pointers

A Cray pointer is a variable whose value is the address of another entity, called the pointee.

f95 supports Cray pointers; Standard Fortran 95 does not.

4.3.1Syntax

The Cray POINTER statement has the following format:

POINTER ( pointer_name, pointee_name [array_spec] ), …

Where pointer_name, pointee_name, and array_spec are as follows:

For example, we can declare Cray pointers to two pointees:

POINTER ( p, b ), ( q, c )

The above example declares Cray pointer p and its pointee b, and Cray pointer q and its pointee c.

We can also declare a Cray pointer to an array:

POINTER ( ix, x(n, 0:m) )

The above example declares Cray pointer ix and its pointee x; and declares x to be an array of dimensions n by m+1.

4.3.2Purpose of Cray Pointers

You can use pointers to access user-managed storage by dynamically associating variables to particular locations in a block of storage.

4.3 Cray Pointers

Cray pointers allow accessing absolute memory locations.

4.3.3Declaring Cray Pointers and Fortran 95 Pointers

Cray pointers are declared as follows:

POINTER ( pointer_name, pointee_name [array_spec] )

Fortran 95 pointers are declared as follows:

POINTER object_name

The two kinds of pointers cannot be mixed.

4.3.4Features of Cray Pointers

■Whenever the pointee is referenced, f95 uses the current value of the pointer as the address of the pointee.

■The Cray pointer type statement declares both the pointer and the pointee.

■The Cray pointer is of type Cray pointer.

■The value of a Cray pointer occupies one storage unit on 32-bit processors, and two storage units on 64-bit processors.

■The Cray pointer can appear in a COMMON list or as a dummy argument.

■The Cray pointee has no address until the value of the Cray pointer is defined.

■If an array is named as a pointee, it is called a pointee array. Its array declarator can appear in:

■A separate type statement

■A separate DIMENSION statement

■The pointer statement itself

If the array declarator is in a subprogram, the dimensioning can refer to:

■Variables in a common block, or

■Variables that are dummy arguments

The size of each dimension is evaluated on entrance to the subprogram, not when the pointee is referenced.

176 Oracle Developer Studio 12.6: Fortran User's Guide • July 2017

4.3 Cray Pointers

4.3.5Restrictions on Cray Pointers

■pointee_name must not be a variable typed CHARACTER*(*).

■If pointee_name is an array declarator, it must be explicit shape, (constant or non-constant bounds), or assumed-size.

■An array of Cray pointers is not allowed.

■A Cray pointer cannot be:

■Pointed to by another Cray pointer or by a Fortran pointer.

■A component of a structure.

■Declared to be any other data type.

A Cray pointer cannot appear in:

■A PARAMETER statement or in a type declaration statement that includes the PARAMETER attribute.

■A DATA statement.

4.3.6Restrictions on Cray Pointees

■A Cray pointee cannot appear in a SAVE, DATA, EQUIVALENCE, COMMON, or PARAMETER statement.

■A Cray pointee cannot be a dummy argument.

■A Cray pointee cannot be a function result variable.

■A Cray pointee cannot be a structure name or a structure component.

■The type of a Cray pointee cannot be finalizable.

4.3.7Usage of Cray Pointers

Cray pointers can be assigned values as follows:

■Set to an absolute address Example: q = 0

■Assigned to or from integer variables, plus or minus expressions

Example: p = q + 100

■Cray pointers are not integers. You cannot assign them to a real variable.

■The LOC function (nonstandard) can be used to define a Cray pointer.