libguile/guardians.c

   1 /* Copyright (C) 1998,1999,2000,2001, 2006 Free Software Foundation, Inc.
   2  *
   3  * This library is free software; you can redistribute it and/or
   4  * modify it under the terms of the GNU Lesser General Public
   5  * License as published by the Free Software Foundation; either
   6  * version 2.1 of the License, or (at your option) any later version.
   7  *
   8  * This library 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 GNU
  11  * Lesser General Public License for more details.
  12  *
  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 02110-1301 USA
  16  */
  17
  18 \f
  19
  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
  25  *
  26  * Original design:          Mikael Djurfeldt
  27  * Original implementation:  Michael Livshin
  28  * Hacked on since by:       everybody
  29  *
  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.
  34  *
  35  * Boiled down again:        Marius Vollmer
  36  *
  37  * Now they should again behave like those described in the paper.
  38  * Scheme guardians should be simple and friendly, not like the greedy
  39  * monsters we had...
  40  */
  41
  42
  43 #include "libguile/_scm.h"
  44 #include "libguile/async.h"
  45 #include "libguile/ports.h"
  46 #include "libguile/print.h"
  47 #include "libguile/smob.h"
  48 #include "libguile/validate.h"
  49 #include "libguile/root.h"
  50 #include "libguile/hashtab.h"
  51 #include "libguile/weaks.h"
  52 #include "libguile/deprecation.h"
  53 #include "libguile/eval.h"
  54
  55 #include "libguile/guardians.h"
  56
  57
  58 /* The live and zombies FIFOs are implemented as tconcs as described
  59    in Dybvig's paper.  This decouples addition and removal of elements
  60    so that no synchronization between these needs to take place.
  61 */
  62
  63 typedef struct t_tconc
  64 {
  65   SCM head;
  66   SCM tail;
  67 } t_tconc;
  68
  69 #define TCONC_EMPTYP(tc) (scm_is_eq ((tc).head, (tc).tail))
  70
  71 #define TCONC_IN(tc, obj, pair) \
  72 do { \
  73   SCM_SETCAR ((tc).tail, obj); \
  74   SCM_SET_CELL_OBJECT_1 (pair, SCM_EOL); \
  75   SCM_SET_CELL_OBJECT_0 (pair, SCM_BOOL_F); \
  76   SCM_SETCDR ((tc).tail, pair); \
  77   (tc).tail = pair; \
  78 } while (0)
  79
  80 #define TCONC_OUT(tc, res) \
  81 do { \
  82   (res) = SCM_CAR ((tc).head); \
  83   (tc).head = SCM_CDR ((tc).head); \
  84 } while (0)
  85
  86
  87 static scm_t_bits tc16_guardian;
  88
  89 typedef struct t_guardian
  90 {
  91   t_tconc live;
  92   t_tconc zombies;
  93   struct t_guardian *next;
  94 } t_guardian;
  95
  96 #define GUARDIAN_P(x)    SCM_SMOB_PREDICATE(tc16_guardian, x)
  97 #define GUARDIAN_DATA(x) ((t_guardian *) SCM_CELL_WORD_1 (x))
  98
  99 static t_guardian *guardians;
 100
 101 void
 102 scm_i_init_guardians_for_gc ()
 103 {
 104   guardians = NULL;
 105 }
 106
 107 /* mark a guardian by adding it to the live guardian list.  */
 108 static SCM
 109 guardian_mark (SCM ptr)
 110 {
 111   t_guardian *g = GUARDIAN_DATA (ptr);
 112   g->next = guardians;
 113   guardians = g;
 114
 115   return SCM_BOOL_F;
 116 }
 117
 118 /* Identify inaccessible objects and move them from the live list to
 119    the zombie list.  An object is inaccessible when it is unmarked at
 120    this point.  Therefore, the inaccessible objects are not marked yet
 121    since that would prevent them from being recognized as
 122    inaccessible.
 123
 124    The pairs that form the life list itself are marked, tho.
 125 */
 126 void
 127 scm_i_identify_inaccessible_guardeds ()
 128 {
 129   t_guardian *g;
 130
 131   for (g = guardians; g; g = g->next)
 132     {
 133       SCM pair, next_pair;
 134       SCM *prev_ptr;
 135
 136       for (pair = g->live.head, prev_ptr = &g->live.head;
 137            !scm_is_eq (pair, g->live.tail);
 138            pair = next_pair)
 139         {
 140           SCM obj = SCM_CAR (pair);
 141           next_pair = SCM_CDR (pair);
 142           if (!SCM_GC_MARK_P (obj))
 143             {
 144               /* Unmarked, move to 'inaccessible' list.
 145                */
 146               *prev_ptr = next_pair;
 147               TCONC_IN (g->zombies, obj, pair);
 148             }
 149           else
 150             {
 151               SCM_SET_GC_MARK (pair);
 152               prev_ptr = SCM_CDRLOC (pair);
 153             }
 154         }
 155       SCM_SET_GC_MARK (pair);
 156     }
 157 }
 158
 159 int
 160 scm_i_mark_inaccessible_guardeds ()
 161 {
 162   t_guardian *g;
 163   int again = 0;
 164
 165   /* We never need to see the guardians again that are processed here,
 166      so we clear the list.  Calling scm_gc_mark below might find new
 167      guardians, however (and other things), and we inform the GC about
 168      this by returning non-zero.  See scm_mark_all in gc-mark.c
 169   */
 170
 171   g = guardians;
 172   guardians = NULL;
 173
 174   for (; g; g = g->next)
 175     {
 176       SCM pair;
 177
 178       for (pair = g->zombies.head;
 179            !scm_is_eq (pair, g->zombies.tail);
 180            pair = SCM_CDR (pair))
 181         {
 182           if (!SCM_GC_MARK_P (pair))
 183             {
 184               scm_gc_mark (SCM_CAR (pair));
 185               SCM_SET_GC_MARK (pair);
 186               again = 1;
 187             }
 188         }
 189       SCM_SET_GC_MARK (pair);
 190     }
 191   return again;
 192 }
 193
 194 static size_t
 195 guardian_free (SCM ptr)
 196 {
 197   scm_gc_free (GUARDIAN_DATA (ptr), sizeof (t_guardian), "guardian");
 198   return 0;
 199 }
 200
 201 static int
 202 guardian_print (SCM guardian, SCM port, scm_print_state *pstate SCM_UNUSED)
 203 {
 204   t_guardian *g = GUARDIAN_DATA (guardian);
 205
 206   scm_puts ("#<guardian ", port);
 207   scm_uintprint ((scm_t_bits) g, 16, port);
 208
 209   scm_puts (" (reachable: ", port);
 210   scm_display (scm_length (SCM_CDR (g->live.head)), port);
 211   scm_puts (" unreachable: ", port);
 212   scm_display (scm_length (SCM_CDR (g->zombies.head)), port);
 213   scm_puts (")", port);
 214
 215   scm_puts (">", port);
 216
 217   return 1;
 218 }
 219
 220 static void
 221 scm_i_guard (SCM guardian, SCM obj)
 222 {
 223   t_guardian *g = GUARDIAN_DATA (guardian);
 224
 225   if (!SCM_IMP (obj))
 226     {
 227       SCM z;
 228       z = scm_cons (SCM_BOOL_F, SCM_BOOL_F);
 229       TCONC_IN (g->live, obj, z);
 230     }
 231 }
 232
 233 static SCM
 234 scm_i_get_one_zombie (SCM guardian)
 235 {
 236   t_guardian *g = GUARDIAN_DATA (guardian);
 237   SCM res = SCM_BOOL_F;
 238
 239   if (!TCONC_EMPTYP (g->zombies))
 240     TCONC_OUT (g->zombies, res);
 241
 242   return res;
 243 }
 244
 245 /* This is the Scheme entry point for each guardian: If OBJ is an
 246  * object, it's added to the guardian's live list.  If OBJ is unbound,
 247  * the next available unreachable object (or #f if none) is returned.
 248  *
 249  * If the second optional argument THROW_P is true (the default), then
 250  * an error is raised if GUARDIAN is greedy and OBJ is already greedily
 251  * guarded.  If THROW_P is false, #f is returned instead of raising the
 252  * error, and #t is returned if everything is fine.
 253  */
 254 static SCM
 255 guardian_apply (SCM guardian, SCM obj, SCM throw_p)
 256 {
 257 #if ENABLE_DEPRECATED
 258   if (!SCM_UNBNDP (throw_p))
 259     scm_c_issue_deprecation_warning
 260       ("Using the 'throw?' argument of a guardian is deprecated "
 261        "and ineffective.");
 262 #endif
 263
 264   if (!SCM_UNBNDP (obj))
 265     {
 266       scm_i_guard (guardian, obj);
 267       return SCM_UNSPECIFIED;
 268     }
 269   else
 270     return scm_i_get_one_zombie (guardian);
 271 }
 272
 273 SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 0, 0,
 274             (),
 275 "Create a new guardian.  A guardian protects a set of objects from\n"
 276 "garbage collection, allowing a program to apply cleanup or other\n"
 277 "actions.\n"
 278 "\n"
 279 "@code{make-guardian} returns a procedure representing the guardian.\n"
 280 "Calling the guardian procedure with an argument adds the argument to\n"
 281 "the guardian's set of protected objects.  Calling the guardian\n"
 282 "procedure without an argument returns one of the protected objects\n"
 283 "which are ready for garbage collection, or @code{#f} if no such object\n"
 284 "is available.  Objects which are returned in this way are removed from\n"
 285 "the guardian.\n"
 286 "\n"
 287 "You can put a single object into a guardian more than once and you can\n"
 288 "put a single object into more than one guardian.  The object will then\n"
 289 "be returned multiple times by the guardian procedures.\n"
 290 "\n"
 291 "An object is eligible to be returned from a guardian when it is no\n"
 292 "longer referenced from outside any guardian.\n"
 293 "\n"
 294 "There is no guarantee about the order in which objects are returned\n"
 295 "from a guardian.  If you want to impose an order on finalization\n"
 296 "actions, for example, you can do that by keeping objects alive in some\n"
 297 "global data structure until they are no longer needed for finalizing\n"
 298 "other objects.\n"
 299 "\n"
 300 "Being an element in a weak vector, a key in a hash table with weak\n"
 301 "keys, or a value in a hash table with weak value does not prevent an\n"
 302 "object from being returned by a guardian.  But as long as an object\n"
 303 "can be returned from a guardian it will not be removed from such a\n"
 304 "weak vector or hash table.  In other words, a weak link does not\n"
 305 "prevent an object from being considered collectable, but being inside\n"
 306 "a guardian prevents a weak link from being broken.\n"
 307 "\n"
 308 "A key in a weak key hash table can be though of as having a strong\n"
 309 "reference to its associated value as long as the key is accessible.\n"
 310 "Consequently, when the key only accessible from within a guardian, the\n"
 311 "reference from the key to the value is also considered to be coming\n"
 312 "from within a guardian.  Thus, if there is no other reference to the\n"
 313             "value, it is eligible to be returned from a guardian.\n")
 314 #define FUNC_NAME s_scm_make_guardian
 315 {
 316   t_guardian *g = scm_gc_malloc (sizeof (t_guardian), "guardian");
 317   SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
 318   SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
 319   SCM z;
 320
 321   /* A tconc starts out with one tail pair. */
 322   g->live.head = g->live.tail = z1;
 323   g->zombies.head = g->zombies.tail = z2;
 324
 325   g->next = NULL;
 326
 327   SCM_NEWSMOB (z, tc16_guardian, g);
 328
 329   return z;
 330 }
 331 #undef FUNC_NAME
 332
 333 void
 334 scm_init_guardians ()
 335 {
 336   tc16_guardian = scm_make_smob_type ("guardian", 0);
 337   scm_set_smob_mark (tc16_guardian, guardian_mark);
 338   scm_set_smob_free (tc16_guardian, guardian_free);
 339   scm_set_smob_print (tc16_guardian, guardian_print);
 340 #if ENABLE_DEPRECATED
 341   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 2, 0);
 342 #else
 343   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 1, 0);
 344 #endif
 345
 346 #include "libguile/guardians.x"
 347 }
 348
 349 /*
 350   Local Variables:
 351   c-file-style: "gnu"
 352   End:
 353 */