Declares Cray-style
pointers and their objects.
Syntax
POINTER (pointer1, pointee1) [, (pointer2, pointee2)]... |
- pointer
is a pointer.
- pointee
is a variable name or array declarator.
Description
HP Fortran supports both the standard Fortran 90 POINTER statement as well as the Cray-style POINTER statement. The Cray-style POINTER statement is supported for compatibility with
older, FORTRAN 77 programs. The following information applies only
to the Cray-style POINTER statement; the Fortran 90 POINTER statement is described in “POINTER (statement and attribute)”.
The following restrictions
apply to pointer:
It should be of type INTEGER(4). If it is not, the compiler interprets its type
as INTEGER(4) regardless of other implicit or explicit type
declarations.
It cannot be declared of any other data type.
Another pointer cannot point to it.
It cannot
appear in a PARAMETER or DATA statement.
It cannot be in a derived type object.
You can increase the size of pointer with the +autodbl or +autodbl4 option.
pointee may be of any type, including an array, a derived
type, a record, or a character string.
The following
restrictions apply to pointee:
It cannot be a dummy
argument, function name, function value, common block element, automatic
object, generic interface block name, or derived type.
It cannot
be used in a COMMON, DATA, EQUIVALENCE, or NAMELIST statement.
It cannot have any of the following attributes: ALLOCATABLE, EXTERNAL, INTENT, INTRINSIC, OPTIONAL, PARAMETER, POINTER, SAVE, and TARGET.
Pointees that are arrays with nonconstant bounds
can be used only in subroutines and functions, not in main programs.
Variables used in an array-bound expression that
appears in a POINTER statement must be either subprogram formal arguments
or common block variables. The value of the expression cannot change
after subprogram entry.
You associate memory with
a pointer by assigning it the address of an object. Typically, this
is done with the libU77 function, LOC. The LOC function returns the address of its argument, which
can be assigned to a pointer. The following example assigns 0 to
the pointee i:
INTEGER i, j
POINTER (p, i)
p = LOC(j)
j = 0 |
You can also
use the MALLOC intrinsic to allocate memory from the heap and
assign its return value to a pointer. Once you are done with the
allocated memory, you should use the FREE intrinsic to release the memory so that it is
available for reuse.
If you are using the pointer to manipulate a device that resides
at a fixed address, you can assign the address to the pointer, using
either an integer constant or integer expression.
Under certain circumstances, Cray-style pointers can cause
erratic program behavior—especially if the program has
been optimized. To ensure correct behavior, observe the following:
Subroutines and functions must not
save the address of any of their arguments between calls.
A function must not return the address of any of
its arguments.
Only those variables whose addresses are explicitly
taken with the LOC function must be referenced through a pointer.
Examples
In the following example, the intrinsic MALLOC returns either the address of the block of memory
it allocated or 0 if MALLOC was unable to allocate enough memory. The formal argument nelem contains the number of array elements and is multiplied
by 4 to obtain the number of bytes that MALLOC is to allocate. The FREE intrinsic returns memory to the heap for reuse.
SUBROUTINE print_iarr(nelem)
POINTER (p, iarr(nelem))
p = MALLOC( 4*nelem )
IF (p.EQ.0) THEN
PRINT *, 'MALLOC failed.'
ELSE
DO i = 1,nelem
iarr(i) = i
END DO
PRINT *, (iarr(i),i=1,nelem)
CALL FREE( p )
ENDIF
RETURN
END SUBROUTINE print_iarr |
Related statements
POINTER (standard Fortran 90)
Related concepts
For related information, see the following:
Printable version
