To: J3                                                     J3/19-130
From: Malcolm Cohen
Subject: Connection of INPUT_UNIT and *
Date: 2019-February-07


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NUMBER: F18/0005
TITLE: Does INPUT_UNIT really identify the same unit as *?
KEYWORDS: Connection, INPUT_UNIT
DEFECT TYPE: Clarification
STATUS: J3 consideration in progress

QUESTION:

According to 16.10.2.13 INPUT_UNIT [429:8-9],
    "The value of the default integer scalar constant INPUT_UNIT
     identifies the same processor-dependent external unit
     preconnected for sequential formatted input as the one
     identified by an asterisk in a READ statement;"

Consider the program:

    PROGRAM input_unit_test
        USE iso_fortran_env
        CHARACTER(80) line
        OPEN(input_unit,FILE='test.dat',ACTION='read')
        READ(input_unit,'(a)') line
        PRINT *,TRIM(line)
        READ(*,'(a)') line
        PRINT *,TRIM(line)
    END PROGRAM

Furthermore, let us suppose the file test.dat contains the
following two lines:
dat line 1
dat line 2
and let us further suppose the file that was preconnected before the
OPEN statement contains the following single line:
inp line 2

Is the output of the program (1):
 dat line 1
 dat line 2
or is it (2):
 dat line 1
 inp line 1
?

The plain meaning of the words
    "identifies the same processor-dependent external unit...
     ...as the one identified by an asterisk in a READ"
would seem to imply that (1) is expected, not (2); however, only
some Fortran processors produce output (1), while others have been
observed to produce output (2).

ANSWER:

Output (1) is correct.  According to the quoted words, using
INPUT_UNIT in a READ statement must have the same effect as UNIT=*
(which is the same effect as not having a input/output control list).

It is common for unit 5 to be effectively preconnected to stdin, but
on some processors changing the connection of unit 5 (e.g. with an
OPEN statement) does not affect unit=*.  On such a processor, the
value of INPUT_UNIT should not be equal to 5, but to whatever value,
possibly negative, that will continue to connect to unit=*.

EDIT to 18-007r1:

None.

SUBMITTED BY: Malcolm Cohen

HISTORY: 19-nnn   m218  Submitted

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