To: J3                                                     J3/19-114
From:    Bill Long
Subject: Problems with C_FUNLOC and C_F_PROCPOINTER being PURE
Date: 2019-January-18


1. Introduction

In 18.2.3.1p1 we say "The C_F_POINTER subroutine is impure; all other
procedures in the module are pure.", where the module referenced is
the ISO_C_BINDING intrinsic module, which is the topic of subclause
18.2.  This implies that 18.2.3.4 C_F_PROCPOINTER (CPTR, FPTR), and
18.2.3.5 C_FUNLOC (X) are both PURE.  However, there are undesirable
side effects of these two functions being PURE, and making them PURE
was likely an oversight.

2. Interpretation Request

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NUMBER: F18/xxxx
TITLE: Problems with C_FUNLOC and C_F_PROCPOINTER being PURE
KEYWORDS: C_FUNLOC, C_F_PROCPOINTER, ISO_C_BINDING
DEFECT TYPE: Erratum
STATUS: Submitted

QUESTIONS:

1) Regarding C_FUNLOC (X),

In 15.7 Pure procedures, constraint C1590 is:

  "C1590 The specification-part of a pure subprogram shall specify
         that all its dummy procedures are pure."

In 15.5.2.9 Actual arguments associated with dummy procedure entities,
the first paragraph says

  "If the interface of a dummy procedure is explicit, its
   characteristics as a procedure (15.3.1) shall be the same as those
   of its effective argument, except that a pure effective argument
   may be associated with a dummy argument that is not pure and an
   elemental intrinsic actual procedure may be associated with a dummy
   procedure (which cannot be elemental)."

If C_FUNLOC is PURE, then these together imply that the actual
argument, X, has to be a PURE procedure. This is not stated in the
specification of this function in 18.2.3.5.  This has the effect of
limiting the uses of C_FUNLOC compared to the specification in Fortran
2008. This is not indicated in 4.3.3 Fortran 2008 compatibility.  Was
this an intentional change, or was making C_FUNLOC PURE a mistake?

2) Regarding C_F_PROCPOINTER (CPTR, FPTR), a similar argument to that
above implies that the INTENT(OUT) procedure pointer, FPTR, is a PURE
procedure. However, there is no stated requirement that the input
argument CPTR be PURE, and indeed there is no specification of what
that even means if CPTR is a pointer to an interoperable C function.
This suggests that C_F_PROCPOINTER provides a backdoor allowing an
impure procedure to appear to be pure, invalidating the assumptions
that are associated with a pure procedure.  Was making C_F_PROCPOINTER
a PURE a mistake?


ANSWER:

Making C_FUNLOC and C_F_PROCPOINTER PURE was a mistake.  Edits are
included to correct this error.


EDITS to 18-007r1:

[469:26-27] In 18.2.3 Procedures in the module, 18.2.3.1 General,
second sentence, change "C_F_POINER subroutine is" to "C_F_POINTER and
C_F_PROCPOINTER subroutines and the C_FUNLOC function are".

Making the whole sentence read

  "The C_F_POINTER and C_F_PROCPOINTER subroutines and the C_FUNLOC
  function are impure; all other procedures in the module are pure."


SUBMITTED BY: Bill Long

HISTORY: 19-xxx   m218  Submitted


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