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From: fox@DASHER.NSCL.MSU.EDU
Newsgroups: comp.lang.fortran
Subject: Re: Dimensioning arrays at run-time, best way?
Message-ID: <1990Sep18.115704.20642@msuinfo.cl.msu.edu>
Date: 18 Sep 90 14:53:24 GMT
Sender: news@msuinfo.cl.msu.edu
Organization: National Superconducting Cyclotron Lab
Lines: 50

In article <1990Sep17.150155.10220@alchemy.chem.utoronto.ca>, mroussel@alchemy.chem.utoronto.ca (Marc Roussel) writes...

>In article <614@keele.keele.ac.uk> phd11@.uk.ac.keele (Zipzoid) writes:
>>I think this question may have been asked many times before,
>>but what is the best way of dimensioning an array whose maximum
>>memory requirement (size) would be read in at run-time?
> 
>Let your PROGRAM block be a dummy.  (I.E. don't put any code into it
>that does anything but set N (the array size) and calls a subroutine
>which does the real work.)  Then call your subroutine.  Most FORTRAN
>compilers don't check for the right number of parameters in a subroutine
>call, so the following may not be portable, but it should do the work on
>most computers.
> 
>Now for the example:
> 
>      PROGRAM EXAMPLE
>      read(*,*)n
>      call sub1(n)
>      END
> 
>      SUBROUTINE SUB1(N,X)
>      real X(N)
>C     Do the work in this subroutine.
>C     Of course you can't use common blocks so you'll have to pass X
>C      around as an argument.
>      END

NO NO NO A thousand times NO!!!!!

  In most machines this will cause a very quick failure.  No storage has
actually been allocated for X.  This is because it is passed in as a dummy
parameter.  FORTRAN expects the storage to have been allocated by the caller
which is not the case.  What the caller has allocated instead is an argument 
list with a single entry.  The second entry is just garbage and referencing
it will at *best* get you garbage, and, on most memory protected machines
just cause your program to bus error or access violate depending on the O/S.
  In subroutine/functions, the dimension of a dummy parameter does *NOT*
allocate storage.  It simply is a device to tell the compilation unit how to
treat the parameter.  Storage for the parameter is expected to have been
allocated (statically or dynamically depending on the compiler) earlier in the
call tree.

		Ron

Ron Fox                     | FOX@MSUNSCL.BITNET      | Where the name 
NSCL                        | FOX@CYCVAX.NSCL.MSU.EDU | goes on before
Michigan State University   | MSUHEP::CYCVAX::FOX     | the quality
East Lansing, MI 48824-1321 |                         | goes in.
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