1 /* Copyright (C) 1998,1999,2000,2001 Free Software Foundation, Inc.
3 * This program is free software; you can redistribute it and/or modify
4 * it under the terms of the GNU General Public License as published by
5 * the Free Software Foundation; either version 2, or (at your option)
8 * This program is distributed in the hope that it will be useful,
9 * but WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11 * GNU General Public License for more details.
13 * You should have received a copy of the GNU General Public License
14 * along with this software; see the file COPYING. If not, write to
15 * the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
16 * Boston, MA 02111-1307 USA
18 * As a special exception, the Free Software Foundation gives permission
19 * for additional uses of the text contained in its release of GUILE.
21 * The exception is that, if you link the GUILE library with other files
22 * to produce an executable, this does not by itself cause the
23 * resulting executable to be covered by the GNU General Public License.
24 * Your use of that executable is in no way restricted on account of
25 * linking the GUILE library code into it.
27 * This exception does not however invalidate any other reasons why
28 * the executable file might be covered by the GNU General Public License.
30 * This exception applies only to the code released by the
31 * Free Software Foundation under the name GUILE. If you copy
32 * code from other Free Software Foundation releases into a copy of
33 * GUILE, as the General Public License permits, the exception does
34 * not apply to the code that you add in this way. To avoid misleading
35 * anyone as to the status of such modified files, you must delete
36 * this exception notice from them.
38 * If you write modifications of your own for GUILE, it is your choice
39 * whether to permit this exception to apply to your modifications.
40 * If you do not wish that, delete this exception notice. */
44 /* This is an implementation of guardians as described in
45 * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
46 * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
47 * Programming Language Design and Implementation, June 1993
48 * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
50 * By this point, the semantics are actually quite different from
51 * those described in the abovementioned paper. The semantic changes
52 * are there to improve safety and intuitiveness. The interface is
53 * still (mostly) the one described by the paper, however.
55 * Original design: Mikael Djurfeldt
56 * Original implementation: Michael Livshin
57 * Hacked on since by: everybody
61 #include "libguile/_scm.h"
62 #include "libguile/ports.h"
63 #include "libguile/print.h"
64 #include "libguile/smob.h"
65 #include "libguile/validate.h"
66 #include "libguile/root.h"
67 #include "libguile/hashtab.h"
68 #include "libguile/weaks.h"
70 #include "libguile/guardians.h"
73 /* The live and zombies FIFOs are implemented as tconcs as described
74 in Dybvig's paper. This decouples addition and removal of elements
75 so that no synchronization between these needs to take place.
78 typedef struct t_tconc
84 #define TCONC_EMPTYP(tc) (SCM_EQ_P ((tc).head, (tc).tail))
86 #define TCONC_IN(tc, obj, pair) \
88 SCM_SETCAR ((tc).tail, obj); \
89 SCM_SET_CELL_WORD_1 (pair, SCM_EOL); \
90 SCM_SET_CELL_WORD_0 (pair, SCM_BOOL_F); \
91 SCM_SETCDR ((tc).tail, pair); \
95 #define TCONC_OUT(tc, res) \
97 (res) = SCM_CAR ((tc).head); \
98 (tc).head = SCM_CDR ((tc).head); \
102 static scm_t_bits tc16_guardian
;
104 typedef struct t_guardian
108 struct t_guardian
*next
;
112 #define GUARDIAN_P(x) SCM_SMOB_PREDICATE(tc16_guardian, x)
113 #define GUARDIAN_DATA(x) ((t_guardian *) SCM_CELL_WORD_1 (x))
116 #define F_LISTED (1L << 1)
117 #define F_DESTROYED (1L << 2)
119 #define GREEDY_P(x) (((x)->flags & F_GREEDY) != 0)
120 #define SET_GREEDY(x) ((x)->flags |= F_GREEDY)
122 #define LISTED_P(x) (((x)->flags & F_LISTED) != 0)
123 #define SET_LISTED(x) ((x)->flags |= F_LISTED)
124 #define CLR_LISTED(x) ((x)->flags &= ~F_LISTED)
126 #define DESTROYED_P(x) (((x)->flags & F_DESTROYED) != 0)
127 #define SET_DESTROYED(x) ((x)->flags |= F_DESTROYED)
129 /* during the gc mark phase, live guardians are linked into the lists
131 static t_guardian
*greedy_guardians
= NULL
;
132 static t_guardian
*sharing_guardians
= NULL
;
134 static SCM greedily_guarded_whash
= SCM_EOL
;
136 /* this is the list of guarded objects that are parts of cycles. we
137 don't know in which order to return them from guardians, so we just
138 unguard them and whine about it in after-gc-hook */
139 static SCM self_centered_zombies
= SCM_EOL
;
143 add_to_live_list (t_guardian
*g
)
150 g
->next
= greedy_guardians
;
151 greedy_guardians
= g
;
155 g
->next
= sharing_guardians
;
156 sharing_guardians
= g
;
162 /* mark a guardian by adding it to the live guardian list. */
164 guardian_mark (SCM ptr
)
166 add_to_live_list (GUARDIAN_DATA (ptr
));
168 /* the objects protected by the guardian are not marked here: that
169 would prevent them from ever getting collected. instead marking
170 is done at the end of the mark phase by guardian_zombify. */
176 guardian_free (SCM ptr
)
178 scm_gc_free (GUARDIAN_DATA (ptr
), sizeof (t_guardian
), "guardian");
184 guardian_print (SCM guardian
, SCM port
, scm_print_state
*pstate SCM_UNUSED
)
186 t_guardian
*g
= GUARDIAN_DATA (guardian
);
188 scm_puts ("#<", port
);
191 scm_puts ("destroyed ", port
);
194 scm_puts ("greedy", port
);
196 scm_puts ("sharing", port
);
198 scm_puts (" guardian 0x", port
);
199 scm_intprint ((long) g
, 16, port
);
201 if (! DESTROYED_P (g
))
203 scm_puts (" (reachable: ", port
);
204 scm_display (scm_length (SCM_CDR (g
->live
.head
)), port
);
205 scm_puts (" unreachable: ", port
);
206 scm_display (scm_length (SCM_CDR (g
->zombies
.head
)), port
);
207 scm_puts (")", port
);
210 scm_puts (">", port
);
216 /* This is the Scheme entry point for each guardian: If OBJ is an
217 * object, it's added to the guardian's live list. If OBJ is unbound,
218 * the next available unreachable object (or #f if none) is returned.
220 * If the second optional argument THROW_P is true (the default), then
221 * an error is raised if GUARDIAN is greedy and OBJ is already greedily
222 * guarded. If THROW_P is false, #f is returned instead of raising the
223 * error, and #t is returned if everything is fine.
226 guardian_apply (SCM guardian
, SCM obj
, SCM throw_p
)
228 if (DESTROYED_P (GUARDIAN_DATA (guardian
)))
229 scm_misc_error ("guard", "attempted use of destroyed guardian: ~A",
230 scm_list_1 (guardian
));
232 if (!SCM_UNBNDP (obj
))
233 return scm_guard (guardian
, obj
,
234 (SCM_UNBNDP (throw_p
)
236 : !SCM_FALSEP (throw_p
)));
238 return scm_get_one_zombie (guardian
);
243 scm_guard (SCM guardian
, SCM obj
, int throw_p
)
245 t_guardian
*g
= GUARDIAN_DATA (guardian
);
251 /* This critical section barrier will be replaced by a mutex. */
256 if (!SCM_FALSEP (scm_hashq_get_handle
257 (greedily_guarded_whash
, obj
)))
262 scm_misc_error ("guard",
263 "object is already greedily guarded: ~A",
269 scm_hashq_create_handle_x (greedily_guarded_whash
,
273 z
= scm_cons (SCM_BOOL_F
, SCM_BOOL_F
);
274 TCONC_IN (g
->live
, obj
, z
);
279 return throw_p
? SCM_UNSPECIFIED
: SCM_BOOL_T
;
284 scm_get_one_zombie (SCM guardian
)
286 t_guardian
*g
= GUARDIAN_DATA (guardian
);
287 SCM res
= SCM_BOOL_F
;
289 /* This critical section barrier will be replaced by a mutex. */
292 if (!TCONC_EMPTYP (g
->zombies
))
293 TCONC_OUT (g
->zombies
, res
);
295 if (!SCM_FALSEP (res
) && GREEDY_P (g
))
296 scm_hashq_remove_x (greedily_guarded_whash
, res
);
304 SCM_DEFINE (scm_make_guardian
, "make-guardian", 0, 1, 0,
306 "Create a new guardian.\n"
307 "A guardian protects a set of objects from garbage collection,\n"
308 "allowing a program to apply cleanup or other actions.\n\n"
310 "@code{make-guardian} returns a procedure representing the guardian.\n"
311 "Calling the guardian procedure with an argument adds the\n"
312 "argument to the guardian's set of protected objects.\n"
313 "Calling the guardian procedure without an argument returns\n"
314 "one of the protected objects which are ready for garbage\n"
315 "collection, or @code{#f} if no such object is available.\n"
316 "Objects which are returned in this way are removed from\n"
319 "@code{make-guardian} takes one optional argument that says whether the\n"
320 "new guardian should be greedy or sharing. If there is any chance\n"
321 "that any object protected by the guardian may be resurrected,\n"
322 "then you should make the guardian greedy (this is the default).\n\n"
324 "See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
325 "\"Guardians in a Generation-Based Garbage Collector\".\n"
326 "ACM SIGPLAN Conference on Programming Language Design\n"
327 "and Implementation, June 1993.\n\n"
329 "(the semantics are slightly different at this point, but the\n"
330 "paper still (mostly) accurately describes the interface).")
331 #define FUNC_NAME s_scm_make_guardian
333 t_guardian
*g
= scm_gc_malloc (sizeof (t_guardian
), "guardian");
334 SCM z1
= scm_cons (SCM_BOOL_F
, SCM_EOL
);
335 SCM z2
= scm_cons (SCM_BOOL_F
, SCM_EOL
);
338 /* A tconc starts out with one tail pair. */
339 g
->live
.head
= g
->live
.tail
= z1
;
340 g
->zombies
.head
= g
->zombies
.tail
= z2
;
345 /* [cmm] the UNBNDP check below is redundant but I like it. */
346 if (SCM_UNBNDP (greedy_p
) || !SCM_FALSEP (greedy_p
))
349 SCM_NEWSMOB (z
, tc16_guardian
, g
);
356 SCM_DEFINE (scm_guardian_destroyed_p
, "guardian-destroyed?", 1, 0, 0,
358 "Return @code{#t} if @var{guardian} has been destroyed, otherwise @code{#f}.")
359 #define FUNC_NAME s_scm_guardian_destroyed_p
361 SCM res
= SCM_BOOL_F
;
363 /* This critical section barrier will be replaced by a mutex. */
366 res
= SCM_BOOL (DESTROYED_P (GUARDIAN_DATA (guardian
)));
374 SCM_DEFINE (scm_guardian_greedy_p
, "guardian-greedy?", 1, 0, 0,
376 "Return @code{#t} if @var{guardian} is a greedy guardian, otherwise @code{#f}.")
377 #define FUNC_NAME s_scm_guardian_greedy_p
379 return SCM_BOOL (GREEDY_P (GUARDIAN_DATA (guardian
)));
383 SCM_DEFINE (scm_destroy_guardian_x
, "destroy-guardian!", 1, 0, 0,
385 "Destroys @var{guardian}, by making it impossible to put any more\n"
386 "objects in it or get any objects from it. It also unguards any\n"
387 "objects guarded by @var{guardian}.")
388 #define FUNC_NAME s_scm_destroy_guardian_x
390 t_guardian
*g
= GUARDIAN_DATA (guardian
);
392 /* This critical section barrier will be replaced by a mutex. */
398 SCM_MISC_ERROR ("guardian is already destroyed: ~A",
399 scm_list_1 (guardian
));
404 /* clear the "greedily guarded" property of the objects */
406 for (pair
= g
->live
.head
; pair
!= g
->live
.tail
; pair
= SCM_CDR (pair
))
407 scm_hashq_remove_x (greedily_guarded_whash
, SCM_CAR (pair
));
408 for (pair
= g
->zombies
.head
; pair
!= g
->zombies
.tail
; pair
= SCM_CDR (pair
))
409 scm_hashq_remove_x (greedily_guarded_whash
, SCM_CAR (pair
));
412 /* empty the lists */
413 g
->live
.head
= g
->live
.tail
;
414 g
->zombies
.head
= g
->zombies
.tail
;
420 return SCM_UNSPECIFIED
;
424 /* called before gc mark phase begins to initialise the live guardian list. */
426 guardian_gc_init (void *dummy1 SCM_UNUSED
,
427 void *dummy2 SCM_UNUSED
,
428 void *dummy3 SCM_UNUSED
)
430 greedy_guardians
= sharing_guardians
= NULL
;
436 mark_dependencies_in_tconc (t_tconc
*tc
)
441 /* scan the list for unmarked objects, and mark their
443 for (pair
= tc
->head
, prev_ptr
= &tc
->head
;
444 ! SCM_EQ_P (pair
, tc
->tail
);
447 SCM obj
= SCM_CAR (pair
);
448 next_pair
= SCM_CDR (pair
);
450 if (! SCM_MARKEDP (obj
))
452 /* a candidate for finalizing */
453 scm_gc_mark_dependencies (obj
);
455 if (SCM_MARKEDP (obj
))
457 /* uh oh. a cycle. transfer this object (the
458 spine cell, to be exact) to
459 self_centered_zombies, so we'll be able to
460 complain about it later. */
461 *prev_ptr
= next_pair
;
462 SCM_SETGCMARK (pair
);
463 SCM_SETCDR (pair
, self_centered_zombies
);
464 self_centered_zombies
= pair
;
468 /* see if this is a guardian. if yes, list it (but don't
470 if (GUARDIAN_P (obj
))
471 add_to_live_list (GUARDIAN_DATA (obj
));
473 prev_ptr
= SCM_CDRLOC (pair
);
480 mark_dependencies (t_guardian
*g
)
482 mark_dependencies_in_tconc (&g
->zombies
);
483 mark_dependencies_in_tconc (&g
->live
);
487 mark_and_zombify (t_guardian
*g
)
489 SCM tconc_tail
= g
->live
.tail
;
490 SCM
*prev_ptr
= &g
->live
.head
;
491 SCM pair
= g
->live
.head
;
493 while (! SCM_EQ_P (pair
, tconc_tail
))
495 SCM next_pair
= SCM_CDR (pair
);
497 if (!SCM_MARKEDP (SCM_CAR (pair
)))
499 /* got you, zombie! */
501 /* out of the live list! */
502 *prev_ptr
= next_pair
;
505 /* if the guardian is greedy, mark this zombie now. this
506 way it won't be zombified again this time around. */
507 SCM_SETGCMARK (SCM_CAR (pair
));
509 /* into the zombie list! */
510 TCONC_IN (g
->zombies
, SCM_CAR (pair
), pair
);
513 prev_ptr
= SCM_CDRLOC (pair
);
518 /* Mark the cells of the live list (yes, the cells in the list, we
519 don't care about objects pointed to by the list cars, since we
520 know they are already marked). */
521 for (pair
= g
->live
.head
; !SCM_NULLP (pair
); pair
= SCM_CDR (pair
))
522 SCM_SETGCMARK (pair
);
526 /* this is called by the garbage collector between the mark and sweep
527 phases. for each marked guardian, it moves any unmarked object in
528 its live list (tconc) to its zombie list (tconc). */
530 guardian_zombify (void *dummy1 SCM_UNUSED
,
531 void *dummy2 SCM_UNUSED
,
532 void *dummy3 SCM_UNUSED
)
534 t_guardian
*last_greedy_guardian
= NULL
;
535 t_guardian
*last_sharing_guardian
= NULL
;
536 t_guardian
*first_greedy_guardian
= NULL
;
537 t_guardian
*first_sharing_guardian
= NULL
;
540 /* First, find all newly unreachable objects and mark their
543 Note that new guardians may be stuck on the end of the live
544 guardian lists as we run this loop, since guardians might be
545 guarded too. When we mark a guarded guardian, its mark function
546 sticks in the appropriate live guardian list. The loop
547 terminates when no new guardians are found. */
550 first_greedy_guardian
= greedy_guardians
;
551 first_sharing_guardian
= sharing_guardians
;
553 for (g
= greedy_guardians
; g
!= last_greedy_guardian
;
555 mark_dependencies (g
);
556 for (g
= sharing_guardians
; g
!= last_sharing_guardian
;
558 mark_dependencies (g
);
560 last_greedy_guardian
= first_greedy_guardian
;
561 last_sharing_guardian
= first_sharing_guardian
;
562 } while (first_greedy_guardian
!= greedy_guardians
563 || first_sharing_guardian
!= sharing_guardians
);
565 /* now, scan all the guardians that are currently known to be live
566 and move their unmarked objects to zombie lists. */
568 for (g
= greedy_guardians
; g
; g
= g
->next
)
570 mark_and_zombify (g
);
573 for (g
= sharing_guardians
; g
; g
= g
->next
)
575 mark_and_zombify (g
);
579 /* Preserve the zombies in their undead state, by marking to prevent
581 for (g
= greedy_guardians
; g
; g
= g
->next
)
582 scm_gc_mark (g
->zombies
.head
);
583 for (g
= sharing_guardians
; g
; g
= g
->next
)
584 scm_gc_mark (g
->zombies
.head
);
590 whine_about_self_centered_zombies (void *dummy1 SCM_UNUSED
,
591 void *dummy2 SCM_UNUSED
,
592 void *dummy3 SCM_UNUSED
)
594 if (!SCM_NULLP (self_centered_zombies
))
598 scm_puts ("** WARNING: the following guarded objects were unguarded due to cycles:",
600 scm_newline (scm_cur_errp
);
601 for (pair
= self_centered_zombies
;
602 !SCM_NULLP (pair
); pair
= SCM_CDR (pair
))
604 scm_display (SCM_CAR (pair
), scm_cur_errp
);
605 scm_newline (scm_cur_errp
);
608 self_centered_zombies
= SCM_EOL
;
615 scm_init_guardians ()
617 tc16_guardian
= scm_make_smob_type ("guardian", 0);
618 scm_set_smob_mark (tc16_guardian
, guardian_mark
);
619 scm_set_smob_free (tc16_guardian
, guardian_free
);
620 scm_set_smob_print (tc16_guardian
, guardian_print
);
621 scm_set_smob_apply (tc16_guardian
, guardian_apply
, 0, 2, 0);
623 scm_c_hook_add (&scm_before_mark_c_hook
, guardian_gc_init
, 0, 0);
624 scm_c_hook_add (&scm_before_sweep_c_hook
, guardian_zombify
, 0, 0);
626 scm_gc_register_root (&self_centered_zombies
);
627 scm_c_hook_add (&scm_after_gc_c_hook
,
628 whine_about_self_centered_zombies
, 0, 0);
630 greedily_guarded_whash
=
631 scm_permanent_object (scm_make_doubly_weak_hash_table (SCM_MAKINUM (31)));
633 #include "libguile/guardians.x"