Path: utzoo!attcan!uunet!wuarchive!julius.cs.uiuc.edu!apple!amdahl!key!sjc
From: sjc@key.COM (Steve Correll)
Newsgroups: comp.lang.fortran
Subject: Re: record length for direct access files (corrected)
Message-ID: <2152@key.COM>
Date: 1 Oct 90 23:50:19 GMT
References: <MCCALPIN.90Sep24090852@pereland.cms.udel.edu> <2146@key.COM> <2151@key.COM>
Organization: Key Computer Labs, Fremont, CA
Lines: 58

I may or may not have posted a half-finished message on this subject (postnews
somehow grabbed the wrong file, placed it in our /usr/spool/news directory, and
then gave me a "no message posted" message). Here's a more concise version,
with errors corrected. Please ignore the first one if you got it, and accept my
apology for whatever it was I did wrong:
=======================================

Thanks for bringing INQUIRE/IOLENGTH to my attention. I have a couple of
questions for Fortran 90 experts:

1. Is there anything in the Fortran 90 standard which forbids the following?
  (The intent is to write an element-count "icnt" followed by that number of
  elements of an array).

	dimension x(5000)
	integer record_length
	...
	read *, icnt
	inquire(iolength=record_length) icnt, (x(j), j=1, icnt)
	open(unit=io, file='myfile', form='unformatted', &
	  access='direct', recl=record_length, &
	  status='new', action='readwrite')
	write(unit=io,rec=1) icnt, (x(j), j=1, icnt)
	close(unit=io)

2. If that example is legal, then I believe it's not possible to compute at
  compilation time how long the records will actually be. Is it intended that
  the INQUIRE/IOLENGTH construct be computed at execution time rather than at
  compilation time?

3. Now suppose another program wants to read the file back in. Is there
  anything in the Fortran 90 standard that forbids the following example?
  
	inquire(iolength=record_length) icnt, (x(j), j=1, icnt)
	open(unit=ii, file='myfile', form='unformatted', &
	  access='direct', recl=record_length, &
	  status='old', action='readwrite', iostat=iostat)
	read(unit=ii) icnt, (x(j), j=1, icnt)

4. If that example is legal, then is the compiler allowed to give the "wrong"
  answer by using the value of "icnt" at the time of the INQUIRE/IOLENGTH
  statement even though that value will change during the READ statement?
  S8.115 section 9.6.3 says INQUIRE/IOLENGTH computes a suitable record length
  for "input/output statements with the same input/output list", as if it were
  always possible to do so correctly.

(Incidentally, a less perverse example which _is_ computable at compilation
time, but not trivially so, is the following, which writes a triangular portion
of a matrix. Any guesses on how many Fortran 90 compilers will get this stuff
wrong the first time out?)

	dimension x(5000, 5000)
	inquire(iolength=il) ((x(i, j), i=1, j), j=1, 5000)
	...
	write(unit=io) ((x(i, j), i=1, j), j=1, 5000)

-- 
sjc@key.com or ...{sun,pyramid}!pacbell!key!sjc 		Steve Correll