(j3.2006) What does "same image" mean?

[Steven G. Kargl]

John,

I agree that there is an ambiguity in the language of the
Standard.  I disagree with the assessment that the below
behavior is non-conforming (at least with regards to the
draft of F2018 that I had). 

The crux of the problem is 3.84
image
instance of a Fortran program

Execution of a program causes an image to occur (ie., an
instance of the Fortran program).  RANDOM_INIT(.true., .false.)
will use a processor-dependent value of the seed.  If
RANDOM_INIT(.true., .false.) were called again in image, then
the Standard requires that the same seed value to be used.

Now, when the image completes and exits, the instance of
the Fortran program no longer exists.  Execution of the program
a second time will cause a new different image to occur.  As thisr
is a new, different image, it cannot possibly be the *same image*
as an image that no longer exists.  In this new different image,
RANDOM_INIT(.true., .false.) will use a processor-dependent value
of the seed.  The Standard (at least the draft I had) does not
require that this new seed be the same as the previous.

For the record, I do believe the Standard ought to say what I
preceive to be your preference.

-- 
steve


On Tue, Feb 20, 2018 at 07:50:12PM +0000, John Reid wrote:
> Steven,
> 
> I would say that this behaviour does not conform, but there does appear 
> to be an ambiguity. Should we add "on the same processor" at the 
> sentence end so that it reads "If it (the argument repeatable) has the 
> value true, the seed accessed by the pseudorandom number generator is 
> set to a processor-dependent value that is the same each time 
> RANDOM_INIT is called from the same image on the same processor."?
> 
> John Reid.
> 
> 
> 
> Steven G. Kargl wrote:
> > Bill,
> >
> > I agree with your assessment with the observation that I'm
> > concern with what a mere mortal user thinks the standard
> > says.  As far as I know, the following conforms
> >
> > program foo
> >    real x(3)
> >    call random_init(.true., .false.)
> >    call random_number(x)
> >    write(*,'(3F15.7)') x
> > end program foo
> >
> > % gfcx -o z c.f90
> > % ./z
> >       0.9166497      0.0343677      0.9250018
> > % ./z
> >       0.3294057      0.3634282      0.6506106
> > % ./z
> >       0.8441532      0.5690981      0.7679952
> >
> > but, is this what a user expects and wants?  As the person that
> > made 'call RANDOM_SEED()' return the same sequence of RN with
> > gfortran, I am well aware that a number of users were unhappy
> > because other vendors choose a different behavior.
> >
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-- 
Steve