1 /* Copyright (C) 1998,1999,2000,2001, 2006, 2008, 2009, 2011 Free Software Foundation, Inc.
3 * This library is free software; you can redistribute it and/or
4 * modify it under the terms of the GNU Lesser General Public License
5 * as published by the Free Software Foundation; either version 3 of
6 * the License, or (at your option) any later version.
8 * This library is distributed in the hope that it will be useful, but
9 * WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 * Lesser General Public License for more details.
13 * You should have received a copy of the GNU Lesser General Public
14 * License along with this library; if not, write to the Free Software
15 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
20 /* This is an implementation of guardians as described in
21 * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
22 * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
23 * Programming Language Design and Implementation, June 1993
24 * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
26 * Original design: Mikael Djurfeldt
27 * Original implementation: Michael Livshin
28 * Hacked on since by: everybody
30 * By this point, the semantics are actually quite different from
31 * those described in the abovementioned paper. The semantic changes
32 * are there to improve safety and intuitiveness. The interface is
33 * still (mostly) the one described by the paper, however.
35 * Boiled down again: Marius Vollmer
37 * Now they should again behave like those described in the paper.
38 * Scheme guardians should be simple and friendly, not like the greedy
41 * Rewritten for the Boehm-Demers-Weiser GC by Ludovic Courtès.
42 * FIXME: This is currently not thread-safe.
45 /* Uncomment the following line to debug guardian finalization. */
46 /* #define DEBUG_GUARDIANS 1 */
52 #include "libguile/_scm.h"
53 #include "libguile/async.h"
54 #include "libguile/ports.h"
55 #include "libguile/print.h"
56 #include "libguile/smob.h"
57 #include "libguile/validate.h"
58 #include "libguile/root.h"
59 #include "libguile/hashtab.h"
60 #include "libguile/deprecation.h"
61 #include "libguile/eval.h"
63 #include "libguile/guardians.h"
64 #include "libguile/bdw-gc.h"
69 static scm_t_bits tc16_guardian
;
71 typedef struct t_guardian
75 struct t_guardian
*next
;
78 #define GUARDIAN_P(x) SCM_SMOB_PREDICATE(tc16_guardian, x)
79 #define GUARDIAN_DATA(x) ((t_guardian *) SCM_SMOB_DATA_1 (x))
85 guardian_print (SCM guardian
, SCM port
, scm_print_state
*pstate SCM_UNUSED
)
87 t_guardian
*g
= GUARDIAN_DATA (guardian
);
89 scm_puts_unlocked ("#<guardian ", port
);
90 scm_uintprint ((scm_t_bits
) g
, 16, port
);
92 scm_puts_unlocked (" (reachable: ", port
);
93 scm_display (scm_from_uint (g
->live
), port
);
94 scm_puts_unlocked (" unreachable: ", port
);
95 scm_display (scm_length (g
->zombies
), port
);
96 scm_puts_unlocked (")", port
);
98 scm_puts_unlocked (">", port
);
103 /* Handle finalization of OBJ which is guarded by the guardians listed in
106 finalize_guarded (GC_PTR ptr
, GC_PTR finalizer_data
)
109 SCM obj
, guardian_list
, proxied_finalizer
;
111 obj
= SCM_PACK_POINTER (ptr
);
112 guardian_list
= SCM_CDR (SCM_PACK_POINTER (finalizer_data
));
113 proxied_finalizer
= SCM_CAR (SCM_PACK_POINTER (finalizer_data
));
115 #ifdef DEBUG_GUARDIANS
116 printf ("finalizing guarded %p (%u guardians)\n",
117 ptr
, scm_to_uint (scm_length (guardian_list
)));
120 /* Preallocate a bunch of cells so that we can make sure that no garbage
121 collection (and, thus, nested calls to `finalize_guarded ()') occurs
122 while executing the following loop. This is quite inefficient (call to
123 `scm_length ()') but that shouldn't be a problem in most cases. */
124 cell_pool
= scm_make_list (scm_length (guardian_list
), SCM_UNSPECIFIED
);
126 /* Tell each guardian interested in OBJ that OBJ is no longer
129 !scm_is_null (guardian_list
);
130 guardian_list
= SCM_CDR (guardian_list
))
136 guardian
= scm_c_weak_vector_ref (scm_car (guardian_list
), 0);
138 if (scm_is_false (guardian
))
140 /* The guardian itself vanished in the meantime. */
141 #ifdef DEBUG_GUARDIANS
142 printf (" guardian for %p vanished\n", ptr
);
147 g
= GUARDIAN_DATA (guardian
);
151 /* Get a fresh cell from CELL_POOL. */
153 cell_pool
= SCM_CDR (cell_pool
);
155 /* Compute and update G's zombie list. */
156 SCM_SETCAR (zombies
, obj
);
157 SCM_SETCDR (zombies
, g
->zombies
);
158 g
->zombies
= zombies
;
161 g
->zombies
= zombies
;
164 if (scm_is_true (proxied_finalizer
))
166 /* Re-register the finalizer that was in place before we installed this
168 GC_finalization_proc finalizer
, prev_finalizer
;
169 GC_PTR finalizer_data
, prev_finalizer_data
;
171 finalizer
= (GC_finalization_proc
) SCM_UNPACK_POINTER (SCM_CAR (proxied_finalizer
));
172 finalizer_data
= SCM_UNPACK_POINTER (SCM_CDR (proxied_finalizer
));
174 if (finalizer
== NULL
)
177 GC_REGISTER_FINALIZER_NO_ORDER (ptr
, finalizer
, finalizer_data
,
178 &prev_finalizer
, &prev_finalizer_data
);
180 #ifdef DEBUG_GUARDIANS
181 printf (" reinstalled proxied finalizer %p for %p\n", finalizer
, ptr
);
185 #ifdef DEBUG_GUARDIANS
186 printf ("end of finalize (%p)\n", ptr
);
190 /* Add OBJ as a guarded object of GUARDIAN. */
192 scm_i_guard (SCM guardian
, SCM obj
)
194 t_guardian
*g
= GUARDIAN_DATA (guardian
);
196 if (SCM_HEAP_OBJECT_P (obj
))
198 /* Register a finalizer and pass a pair as the ``client data''
199 argument. The pair contains in its car `#f' or a pair describing a
200 ``proxied'' finalizer (see below); its cdr contains a list of
201 guardians interested in OBJ.
203 A ``proxied'' finalizer is a finalizer that was registered for OBJ
204 before OBJ became guarded (e.g., a SMOB `free' function). We are
205 assuming here that finalizers are only used internally, either at
206 the very beginning of an object's lifetime (e.g., see `SCM_NEWSMOB')
207 or by this function. */
208 GC_finalization_proc prev_finalizer
;
210 SCM guardians_for_obj
, finalizer_data
;
214 /* Note: GUARDIANS_FOR_OBJ holds weak references to guardians so
215 that a guardian can be collected before the objects it guards
216 (see `guardians.test'). */
217 guardians_for_obj
= scm_cons (scm_make_weak_vector (SCM_INUM1
, guardian
),
219 finalizer_data
= scm_cons (SCM_BOOL_F
, guardians_for_obj
);
221 /* FIXME: should be SCM_HEAP_OBJECT_BASE, but will the finalizer
222 strip the tag bits of pairs or structs? */
223 GC_REGISTER_FINALIZER_NO_ORDER (SCM_UNPACK_POINTER (obj
), finalize_guarded
,
224 SCM_UNPACK_POINTER (finalizer_data
),
225 &prev_finalizer
, &prev_data
);
227 if (prev_finalizer
== finalize_guarded
)
229 /* OBJ is already guarded by another guardian: add GUARDIAN to its
230 list of guardians. */
231 SCM prev_guardian_list
, prev_finalizer_data
;
233 if (prev_data
== NULL
)
236 prev_finalizer_data
= SCM_PACK_POINTER (prev_data
);
237 if (!scm_is_pair (prev_finalizer_data
))
240 prev_guardian_list
= SCM_CDR (prev_finalizer_data
);
241 SCM_SETCDR (guardians_for_obj
, prev_guardian_list
);
243 /* Also copy information about proxied finalizers. */
244 SCM_SETCAR (finalizer_data
, SCM_CAR (prev_finalizer_data
));
246 else if (prev_finalizer
!= NULL
)
248 /* There was already a finalizer registered for OBJ so we will
249 ``proxy'' it, i.e., record it so that we can re-register it once
250 `finalize_guarded ()' has finished. */
251 SCM proxied_finalizer
;
253 proxied_finalizer
= scm_cons (SCM_PACK_POINTER (prev_finalizer
),
254 SCM_PACK_POINTER (prev_data
));
255 SCM_SETCAR (finalizer_data
, proxied_finalizer
);
261 scm_i_get_one_zombie (SCM guardian
)
263 t_guardian
*g
= GUARDIAN_DATA (guardian
);
264 SCM res
= SCM_BOOL_F
;
266 if (!scm_is_null (g
->zombies
))
268 /* Note: We return zombies in reverse order. */
269 res
= SCM_CAR (g
->zombies
);
270 g
->zombies
= SCM_CDR (g
->zombies
);
276 /* This is the Scheme entry point for each guardian: If OBJ is an
277 * object, it's added to the guardian's live list. If OBJ is unbound,
278 * the next available unreachable object (or #f if none) is returned.
280 * If the second optional argument THROW_P is true (the default), then
281 * an error is raised if GUARDIAN is greedy and OBJ is already greedily
282 * guarded. If THROW_P is false, #f is returned instead of raising the
283 * error, and #t is returned if everything is fine.
286 guardian_apply (SCM guardian
, SCM obj
, SCM throw_p
)
288 if (!SCM_UNBNDP (obj
))
290 scm_i_guard (guardian
, obj
);
291 return SCM_UNSPECIFIED
;
294 return scm_i_get_one_zombie (guardian
);
297 SCM_DEFINE (scm_make_guardian
, "make-guardian", 0, 0, 0,
299 "Create a new guardian. A guardian protects a set of objects from\n"
300 "garbage collection, allowing a program to apply cleanup or other\n"
303 "@code{make-guardian} returns a procedure representing the guardian.\n"
304 "Calling the guardian procedure with an argument adds the argument to\n"
305 "the guardian's set of protected objects. Calling the guardian\n"
306 "procedure without an argument returns one of the protected objects\n"
307 "which are ready for garbage collection, or @code{#f} if no such object\n"
308 "is available. Objects which are returned in this way are removed from\n"
311 "You can put a single object into a guardian more than once and you can\n"
312 "put a single object into more than one guardian. The object will then\n"
313 "be returned multiple times by the guardian procedures.\n"
315 "An object is eligible to be returned from a guardian when it is no\n"
316 "longer referenced from outside any guardian.\n"
318 "There is no guarantee about the order in which objects are returned\n"
319 "from a guardian. If you want to impose an order on finalization\n"
320 "actions, for example, you can do that by keeping objects alive in some\n"
321 "global data structure until they are no longer needed for finalizing\n"
324 "Being an element in a weak vector, a key in a hash table with weak\n"
325 "keys, or a value in a hash table with weak value does not prevent an\n"
326 "object from being returned by a guardian. But as long as an object\n"
327 "can be returned from a guardian it will not be removed from such a\n"
328 "weak vector or hash table. In other words, a weak link does not\n"
329 "prevent an object from being considered collectable, but being inside\n"
330 "a guardian prevents a weak link from being broken.\n"
332 "A key in a weak key hash table can be though of as having a strong\n"
333 "reference to its associated value as long as the key is accessible.\n"
334 "Consequently, when the key only accessible from within a guardian, the\n"
335 "reference from the key to the value is also considered to be coming\n"
336 "from within a guardian. Thus, if there is no other reference to the\n"
337 "value, it is eligible to be returned from a guardian.\n")
338 #define FUNC_NAME s_scm_make_guardian
340 t_guardian
*g
= scm_gc_malloc (sizeof (t_guardian
), "guardian");
343 /* A tconc starts out with one tail pair. */
345 g
->zombies
= SCM_EOL
;
349 SCM_NEWSMOB (z
, tc16_guardian
, g
);
356 scm_init_guardians ()
358 /* We use unordered finalization `a la Java. */
359 GC_java_finalization
= 1;
361 tc16_guardian
= scm_make_smob_type ("guardian", 0);
363 scm_set_smob_print (tc16_guardian
, guardian_print
);
364 scm_set_smob_apply (tc16_guardian
, guardian_apply
, 0, 1, 0);
366 #include "libguile/guardians.x"