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From: 3003jalp@ucsbuxa.ucsb.edu (Applied Magnetics)
Newsgroups: comp.lang.fortran
Subject: Fun with pointers (long-ish)
Keywords: fortran pointer dynamic data structure
Message-ID: <6243@hub.ucsb.edu>
Date: 7 Sep 90 19:01:43 GMT
Sender: news@hub.ucsb.edu
Distribution: comp
Lines: 44

Preamble:  suppose I do this in a subroutine:

  integer bcbase    ! "Blank Common BASE"
  common//bcbase
  integer      link(0:3),     nwords(0:0),      content(0:0, 1)
  equivalence (link,bcbase), (nwords,link(1)), (content,link(2))

And, in the main program,

  integer scratch(1000000)  ! one million
  common//scratch

Now I can have the routine divide the blank common into unequal-sized
blocks, and link them in a list.  If a block of N words begins at offset
P in blank common, I set up

  link(P)= <offset to next block>
  nwords(P)= N
  content(P,k)= <anything I want, k= 1, N-2>

The equivalence statements force the memory layout to be what I want.
Link(P) is at offset P;  nwords(P) is at offset P+1; content(P,k) is
at offset P+2+(k-1).  More generally, I can use integer offsets into
blank common as substitute for pointers;  I can imitate C's operator
"->" and dereference mock-pointers to structure components;
I can manage nontrivial data structures dynamically, as long as the
primitive types are restricted to integer, logical, real or double
precision (more tricks needed for the latter).

Critical assessment:
The ANSI standard says that overrunning arrays like I do produces
undefined results (17.1.2).  THIS IS A CASE OF USING THE WRONG
LANGUAGE.  DO NOT TRY IT AT HOME.  I actually wrote code that way, but
only because I have to.  I'll be migrating away from it the first
chance I get.

End of preamble.

Question:
Assuming that any run-time checking of array bounds can be defeated,
are there extant implementations of Fortran that would foil my evil
designs?  Just curious.

  --Pierre Asselin, Applied Magnetics Corp.