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From: mccaugh@uiucdcsm.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: Lisp-C interface
Message-ID: <4700007@uiucdcsm>
Date: Wed, 18-Mar-87 02:56:00 EST
Article-I.D.: uiucdcsm.4700007
Posted: Wed Mar 18 02:56:00 1987
Date-Received: Sat, 21-Mar-87 07:13:41 EST
References: <1067@ius2.cs.cmu.edu>
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Nf-From: uiucdcsm.cs.uiuc.edu!mccaugh    Mar 18 01:56:00 1987


 I hesitate to submit this response, since I am unfamiliar with common-lisp
 and really know only Franz and Xlisp, but what it seems you may need is an
 un-lisp-like interface between lisp and C: I'm sure you don't wish to re-
 write the C library to "go after" the mallocs and frees if they are that
 extensively employed, and you may not have access to the garbage-collector
 source. 

 Without further info, it is difficult to comment and I wouldn't have butted
 in here but for the fact your note is almost a week old with no comments to
 it, so at least the following is better than nothing:

 If Common Lisp is anything like Franz or Xlisp -- and you wish to hide some
 data-structures (the C-malloc'd ones, presumably, since you indicate they
 get free'd by C) from the garbage-collector, don't let those data-structures
 get to the symbol-table where garbage-collection initiates its sweep. This is
 just one peon's solution: others involve twiddling with the mark-bits so that
 even if garbage-collection finds a node it doesn't try to recover it with the
 right bit set, but the simplest way is to prevent it from accessing your data-
 structures in the first place. 
 Of course, this crudest of solutions carries with it the onus that if the data-
 structure doesn't reside in the symbol-table, Lisp never finds it to begin with
 so other steps need to be taken to ensure Lisp finds it, and that is indeed a
 problem. If your data-structures aren't parameters, you can't install them on
 the A-list.
 However, constants are symbols to Lisp and so enjoy places on the symbol-table.
 The trick, then, is to find a constant and attach your data-structures to its
 property-list. The fact that it's constant should prevent garbage-collection
 from ever trying to recover it, and thus the structures hanging off its prop-
 erty-list should be safe -- of course, all this presumes ease of their access.

 -- Scott McCaughrin (mccaugh @ uiucmsl)