Re: reading back a partially-written record on direct access

glen herrmannsfeldt <gah@xxxxxxxxxxxxxxxx> wrote:

Thomas König wrote:

Is the following program standard-conforming?

PROGRAM test
IMPLICIT NONE
CHARACTER*8 as_written, as_read
INTEGER irec
INTEGER i
as_written = "12345678"
inquire (iolength=irec) as_written, i
OPEN (76, FILE="test.txt", ACCESS="DIRECT", STATUS="NEW",
& RECL=irec)
WRITE(76, REC=1) as_written
READ(76, REC=1) as_read, i
PRINT *, "as_written = ", as_written, " as_read = ", as_read
CLOSE(76)
END PROGRAM test

I don't think it is, because it reads something that wasn't written
(the variable i). On the other hand, i isn't used, so it may be OK.


9.5.3.4.1 "On output to a file connected for unformatted direct access,
the output list shall not specify more values than can fit
into the record. If the file is connected for direct access
and the values specified by the output list do not fill the
record, the remainder of the record is undefined."

The value of I is at least undefined.

I think Glenn has the core of it here. You also need the bit somewhere
else that says the records in a direct access file are all the same
length; otherwise one might imagine a short record being written, in
which case the read would be invalid. I think that the read is valid,
but the variable "i" becomes undefined. But I'll admit there might be
room to question that. I'd have to research a bit further to be
completely sure. There might be a requirement somewhere that the record
contain an integer to correspond with "i". If so, the rest of the record
being undefined would violate that. I'm too lazy to look further right
now, but that's at least a hint of the kind of thing I'd look for.

Do note one picky and slightly quirky side point. Although I think the
read is ok, your inquire is not. That's because inquire is defined to
use an output list. Variables in an output list must be defined. Your
"i" variable isn't. I think I recall actually getting bit by a more
complicated case of this once. I tend to think of the inquire statement
as like an inquiry intrinsic in that it only needs to know properties
(in particular, sizes) and not values. Thus you might think that the
values don't need to be defined. But the way it is specified in the
standard, they do need to be defined for the inquire statement.

--
Richard Maine | Good judgement comes from experience;
email: last name at domain . net | experience comes from bad judgement.
domain: summertriangle | -- Mark Twain
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