edw@ius2.cs.cmu.edu.UUCP (03/13/87)
Question to thoses of you that are familiar with Sun's Common Lisp and
its foreign function call feature.
I am designing an interface in lisp to a library written in C. There is
no problem calling the C functions, the problem is that this library
extensively uses malloc and free. As I have been told by other people,
when lisp goes to garbage collect, it also collects the memory that
was allocated by malloc (all memory up to sbrk).
One, is this true?
Second if so are there any plans to fix it so malloc will work right
or am I stuck re-implementing malloc.
Eddie Wyattmccaugh@uiucdcsm.cs.uiuc.edu (03/18/87)
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)