J3/04-299

Date:  4-May-2004
To:    J3
From:  JOR/interp/Stan Whitlock
Subj:  F95 interpretation #000006


NUMBER: 000006
TITLE: Character length specification of a function result
KEYWORDS: character, function result
DEFECT TYPE: Interpretation
STATUS: J3 consideration in progress

QUESTION:

Consider the following program:

      PROGRAM main
      CHARACTER(8) arg, func
      EXTERNAL func
      CALL sub(arg, func)
      END PROGRAM

      SUBROUTINE sub(arg, func)
      CHARACTER(*) arg, func
      CHARACTER(8) str
      str = func(arg)
      ...
      END SUBROUTINE

      CHARACTER(8) FUNCTION func(arg)
      CHARACTER(*) arg
      ...
      END FUNCTION

Some implementations reject the reference to FUNC in subroutine SUB
and some do not.  The implementations that do not reject the reference
to FUNC use essentially the following as the argument for allowing
the reference:

  The rules for when an asterisk can be used to specify character
  length are specified on in section 5.1.1.5 (middle of page 51 of
  the Fortran 95 standard).

    * The first rule states that an asterisk may be used to declare a
      dummy argument of a procedure.  It is clear that FUNC is a
      dummy argument of SUB.

    * The third rule says that in an external function, the length of
      the result may be specified within the function itself with an
      asterisk but if the result length is so specified, any invoker
      of the function must specify the character length of the
      function result as something other than an asterisk.  But the
      function that is being invoked in the above example does not
      declare its result length using the asterisk, so this rule does
      not apply.

Is the above sample program a standard conforming program with
respect to the concept of declaring the function result length in the
invoking program as an asterisk?

ANSWER:

Yes. When item (1) on page 51 is applied to the argument func of sub,
it tells us that its length is 8 for the call from the main program.

NOTE:

The NO votes in 01-380 raised the concern that this answer introduced a
binary incompatibility with FORTRAN 77.  FORTRAN 77 features continue to
work in Fortran 95;  Fortran 95 features may not always work in FORTRAN 77.

EDITS: None

SUBMITTED BY: Larry Rolison

HISTORY: J3/97-242  m143  submitted
         WG5-N1456        Draft answer
         01-305r1   m158  Passed by J3 meeting
         01-380     m159  Failed J3 letter ballot
         04-299     m168  Submitted for J3 meeting vote