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

[Bader, Reinhold]

Hello Steve, 

"same image" as used in the standard document seems to me always to 
apply to a single program execution. The same applies for "another image"
or "different image"

So you cannot deduce that for two execution instances the two instances
of image 1 are "different images" or "same images" in the sense the standard
uses the term ...

For practical reasons, I would think "same image" refers to equality of the
image index
(of the initial team). 

Regards
Reinhold

> -----Urspr?ngliche Nachricht-----
> Von: J3 [mailto:j3-bounces at mailman.j3-fortran.org] Im Auftrag von Steven
> G. Kargl
> Gesendet: Sonntag, 21. Januar 2018 01:57
> An: J3 Fortran <j3 at j3-fortran.org>
> Betreff: (j3.2006) What does "same image" mean?
> 
> In the description of the dummy argument REPEATABLE for the
> RANDOM_INIT intrinsic subroutine, one finds the sentence
> 
> 
>    REPEATABLE ... If it 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.
> 
> What does "same image" mean here?  There are two case to consider.
> For this discussion consider a Fortran program that does not use
co-arrays, so
> the execution of a compiled program creates a single image.
> 
>    program same_image1
>       real x(2), y(2)
>       call random_init(.true., .false.)
>       call random_number(x)
>       print *, x
>       call random_init(.true., .false.)
>       call random_number(y)
>       print *, y
>    end program same_image1
> 
> Here, with the implied assumption that a seed yields a unique sequence of
> random numbers, one can expect that x == y.
> 
> Now, consider
> 
>    program same_image2
>       real x(2)
>       call random_init(.true., .false.)
>       call random_number(x)
>       print *, x
>    end program same_image2
> 
> Compile this program to create an executable and run it to create an
image.
> This will produce two numbers and then terminate.  Wait an hour and re-run
> the executable.  This will create a new image, two numbers will be
produced,
> and the image terminates.  Should one expect that the two sets of numbers
> are the same?  Clearly, the new image cannot be the previous image as the
> previous image terminated an hour ago, so the call to RANDOM_INIT() in the
> new image cannot occur in the "same image" as the previous image.  So,
> does this mean that the second program's use of RANDOM_INIT does not
> guarantee repeatable sequences of random numbers for multiple
> executions of the same executable?
> 
> --
> steve
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