J3/04-313

Date: 5 May 2004
To:   J3
From: JOR/interp/Matthijs van Waveren
Subj: F95 interpretation #000017


NUMBER: 000017
TITLE: Characteristics of an array function result
KEYWORDS: Characteristics, function
DEFECT TYPE: Interpretatation
STATUS: J3 consideration in progress

Sections 12.2.1.1 and 12.2.2 of the Fortran 95 standard define rules
for dummy arguments and function returns in the presence of explicit
interfaces that allow array bounds expressions to be evaluated on
either the calling side or the called side of a call.  The
interpretation provided for RFI 000049 on the X3J3 website clarifies
and reinforces the conditions that permit evaluation on either side
of the call.  The erratum published in the same file for RFI 000070
changes the semantics of dummy arguments.  The erratum does not
explicitly mention function results, but it might be considered to
imply a similar change to the semantics of function results.

The erratum provided for RFI 000070 refers to altered text for the
interpretation provided for RFI 000049.  The version of the
interpretation that appears in the file on the X3J3 website does not
contain the altered text.

The definition of characteristics of function results as stated in
Section 12.2.2 of the Fortran 95 standard permits more efficient
implementation of array-valued functions, at least for some
architectures, than the definition implied by the erratum provided
for RFI 000070.  For Sun, it is more efficient for the calling
routines to allocate space for array results than to have the called
routines allocate the space.  In order for the calling routine to
allocate space for an array result, it must know the size of the
array.  To determine the size of the array, it must evaluate the
array bounds expressions given in the explicit interface for the
function.

Section 12.2.2 of the Fortran 95 standard requires the values of the
nonconstant bounds expressions given in an explicit interface to be
the same as the values of the bounds expressions given in the
corresponding function definition.  Thus, the values of the
nonconstant bounds expressions used to determine the size of the
array result can be passed to the called routine, avoiding any need
for the called routine to re-evaluate those expressions. Because
Fortran 95 allows user-defined routines to appear in bounds
expressions, evaluating the bounds expressions more than once per
call could prove inefficient and confusing.

The change implied by the erratum provided for RFI 000070 would
remove the nonconstant bounds expressions from the characteristics of
function results. The shape would still be a characteristic, but the
same shape can be produced by many different values of the bounds
expressions.  Thus, the values of the nonconstant bounds expressions
used in the called routine may differ from the values of the
corresponding expressions in the explicit interface.

For example, consider the explicit interface

      INTERFACE
        FUNCTION FOO(F, G, H)
          INTERFACE
            PURE INTEGER FUNCTION F()
            END FUNCTION
            PURE INTEGER FUNCTION G()
            END FUNCTION
            PURE INTEGER FUNCTION H()
            END FUNCTION
          END INTERFACE
          CHARACTER*(F()) FOO(G():H())
        END FUNCTION
      END INTERFACE

The definition given in Section 12.2.2 of the Fortran 95 standard
requires the values of the length and bounds expressions in the
interface to be the same as the values of the corresponding length
and bounds expressions in the function definition.

Under the definition implied by the erratum provided for RFI 000070,
the shapes must match, but the values of the nonconstant bounds
expressions need not. Thus, the function definition might be

      FUNCTION FOO(F, G, H)
        INTERFACE
          PURE INTEGER FUNCTION F()
          END FUNCTION
          PURE INTEGER FUNCTION G()
          END FUNCTION
          PURE INTEGER FUNCTION H()
          END FUNCTION
        END INTERFACE
        CHARACTER*(F()) FOO(G()-1:H()-1)
        . . .
      END FUNCTION

In this case, the values of the bounds expressions used in the called
routine must be the values of the expressions specified in the
function definition, not the values of the expressions specified in
the interface block.

QUESTION:

If a bound of a function result array is not a constant expression,
is the exact dependence on the entities in the expression a
characteristic of the function result?

ANSWER:

No. According to Corrigendum 1, the correct statement is now:

"If a bound of a function result array is not an initialization
expression, the exact dependence on the entities in the expression is
a characteristic of the function result."

EDITS: None

REFERENCES: 95-006a, F90/000027, F90/000049 & F90/000070, 04-295,
N1416 (Corrigendum 1).

SUBMITTED BY:  Robert Paul Corbett

HISTORY:  J3/98-114 submitted
          J3/04-313 draft answer