To: J3                                                     J3/17-246r1
From: Steve Lionel
Subject: Assignment to specifier variables
Date: 2017 October 18

Reference: 17-007r2, 10-007r1, 14-194r1


Introduction
------------

Paper 14-194r1 introduced a change to the language used to describe
assignment to specifier variables, such as ERRMSG=, IOMSG= and
INQUIRE specifiers. For example, Fortran 2008's description of
ERRMSG= in ALLOCATE said:

[132:23-25, 6.7.5p2]
If an error condition occurs during execution of an ALLOCATE or
DEALLOCATE statement, the processor shall assign an explanatory message
to errmsg-variable.

In the Fortran 2015 draft, this paragraph reads:

[147:26-28, 9.7.5p2]
If an error condition occurs during execution of an ALLOCATE or
DEALLOCATE statement with an ERRMSG= specifier, the errmsg-variable
is assigned an explanatory message, as if by intrinsic assignment.

The change was to use the phrase "as if by intrinsic assignment"; the
primary motivation being to get the right semantics for
padding/truncation of character values. Unfortunately, as do so many
seemingly harmless changes, this had unintended side-effects when
the variable provided for the specifier is a deferred-length
character allocatable.

Discussion
----------

Consider the following program:

character(:), allocatable :: errmsg_var
real, allocatable :: arr(:)
integer :: status

! Assume that an attempt to allocate an array of HUGE(0) elements
! fails and that the error message the processor provides is
! "Out of memory"

errmsg_var = "initial" ! Allocates errmsg_var to length 7
ALLOCATE (arr(HUGE(0)), STAT=status, ERRMSG=errmsg_var)
PRINT *, status, errmsg_var !A

DEALLOCATE(errmsg_var)
ALLOCATE (arr(HUGE(0)), STAT=status, ERRMSG=errmsg_var)
PRINT *, status, errmsg_var !B

END

Intrinsic assignment to a deferred-length allocatable character
variable is described in 10.2.1.3. In case A, the length of the
message (13) is different from the errmsg-variable's length (7),
so the variable would be deallocated and then reallocated to length
13 before assignment. In case B, the variable is not allocated
so it would get allocated to length 13.

This is a significant technical change and it is clear from the earlier
papers that it was not considered. In the long term,
automatic reallocation here would be useful to programmers but it is
too late in the development cycle to include this.

Edits are provided to eliminate the potential rellocation. The
committee may want to consider reintroducing this as part of a
broader expansion of allocatable variables in a future revsion.

Edits
-----

[147:27, 9.7.5p2, ERRMSG= specifier]
Replace:
    "as if by {intrinsic assignment}"
with:
    "truncated or padded according to the rules of {intrinsic
    assignment}"

[218:31, 11.6.11p12, STAT= and ERRMSG= specifiers in image control
statements]
Replace:
    "as if by {intrinsic assignment}"
with:
    "truncated or padded according to the rules of {intrinsic
    assignment}"

[255:27, 12.10.1p1, Forms of the INQUIRE statement
Replace:
    "performed as if by {intrinsic assignment}"
with:
    "converted, truncated, or padded according to the rules of
    {intrinsic assignment}"

[264:27, 12.11.6p1, IOMSG= specifier]
Replace:
    "as if by {intrinsic assignment}"
with:
    ", truncated or padded according to the rules of {intrinsic
    assignment}"