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From: ddh@hare.cdc.com (Dan Horsfall)
Newsgroups: comp.lang.fortran
Subject: Re: passing DIMs via COMMON??
Message-ID: <26872@shamash.cdc.com>
Date: 11 Oct 90 14:26:26 GMT
References: <MARTIN.90Oct9111140@slsiris.harvard.edu> <BURLEY.90Oct11030305@world.std.com>
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Reply-To: ddh@dash.udev.cdc.com (Dan Horsfall)
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In article <BURLEY.90Oct11030305@world.std.com> burley@world.std.com (James C Burley) writes:
> In article <MARTIN.90Oct9111140@slsiris.harvard.edu> martin@slsiris.harvard.edu (Mr. Science) writes:
> 
>    Ive noticed that when I try to specify LUNs or adjustable array
>    parameters by passing the value in a common block, f77 doesnt know
>  [ ... ]
>    ....................................................................
>    Patrick Martin
> 
> 
> Make sure you are passing the arrays themselves as dummies; you can
> not put adjustable arrays in common areas.

Yes, this is the essence of the problem

> 
> However, if you are doing this right,  ... then I think the compiler is
> broken.
> 
> I.e. you should be able to do both
>     SUBROUTINE X(A,N)
>     INTEGER A(N)
>     ...
> 
> and
> 
>     SUBROUTINE X(A)
>     INTEGER A(N)
>     COMMON /FOO/N
>     ...
> 
> This is, I believe, standard for ANSI 77.

Technically, the second form IS permitted, but it is a rarely used form.
In my experience, I've never seen it (and had to look it up in the
standard to write this).  Given its low use, I'd hesitate to depend
on it in case a particular implementation hasn't done a foolproof job.

> 
> Whether a particular implementation (such as f77) supports it is another
> issue!

Absolutely!

Other tricks that have been used in the past are to declare the array
in a common block to have length 1 (or some other small value, read on)
in all routines except the Main.  At some point, however, the actual
amount of space actually needed must be allocated, and when using
COMMON, you have to use COMMON to do so (No, that's not the department
of redundancy department -- it means you can't fake out COMMON with
other forms of dynamic allocation.)

If your arrays are one-dimensional, you caan pass them through arglists
and declare them with
    DIMENSION A(*)
in all routines _except_ the main, since, again, somebody somewhere has
to allocate the actual amount of space needed.  Here, unlike COMMON, you
may allocate this space in some other fashion, and pass its address thru
the arglist of each subprogram that needs the array.  If you need higher
order arrays, only the last dimension can float in this manner, but this
is still an adequate solution for some problems.

If you have further questions on this issue, contact me directly and
we'll talk.  I don't describe myself as "the walking flint", but in
my current position I write FORTRAN (dialect) translators in FORTRAN
and consider myself pretty well versed in the predominate extension
and implementation dependencies.

In case Pnews botches my sig, I'm ddh@dash.udev.cdc.com.

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