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 /* This is an implementation of guardians as described in
  20  * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
  21  * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
  22  * Programming Language Design and Implementation, June 1993
  23  * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
  24  *
  25  * Original design:          Mikael Djurfeldt
  26  * Original implementation:  Michael Livshin
  27  * Hacked on since by:       everybody
  28  *
  29  * By this point, the semantics are actually quite different from
  30  * those described in the abovementioned paper.  The semantic changes
  31  * are there to improve safety and intuitiveness.  The interface is
  32  * still (mostly) the one described by the paper, however.
  33  *
  34  * Boiled down again:        Marius Vollmer
  35  *
  36  * Now they should again behave like those described in the paper.
  37  * Scheme guardians should be simple and friendly, not like the greedy
  38  * monsters we had...
  39  *
  40  * Rewritten for the Boehm-Wiser GC by Ludovic Courtès.
  41  * FIXME: This is currently not thread-safe.
  42  */
  43
  44
  45 #include "libguile/_scm.h"
  46 #include "libguile/async.h"
  47 #include "libguile/ports.h"
  48 #include "libguile/print.h"
  49 #include "libguile/smob.h"
  50 #include "libguile/validate.h"
  51 #include "libguile/root.h"
  52 #include "libguile/hashtab.h"
  53 #include "libguile/weaks.h"
  54 #include "libguile/deprecation.h"
  55 #include "libguile/eval.h"
  56
  57 #include "libguile/guardians.h"
  58 #include <gc/gc.h>
  59
  60
  61
  62
  63 static scm_t_bits tc16_guardian;
  64
  65 typedef struct t_guardian
  66 {
  67   unsigned long live;
  68   SCM zombies;
  69   struct t_guardian *next;
  70 } t_guardian;
  71
  72 #define GUARDIAN_P(x)    SCM_SMOB_PREDICATE(tc16_guardian, x)
  73 #define GUARDIAN_DATA(x) ((t_guardian *) SCM_CELL_WORD_1 (x))
  74
  75
  76
  77
  78 static int
  79 guardian_print (SCM guardian, SCM port, scm_print_state *pstate SCM_UNUSED)
  80 {
  81   t_guardian *g = GUARDIAN_DATA (guardian);
  82
  83   scm_puts ("#<guardian ", port);
  84   scm_uintprint ((scm_t_bits) g, 16, port);
  85
  86   scm_puts (" (reachable: ", port);
  87   scm_display (scm_from_uint (g->live), port);
  88   scm_puts (" unreachable: ", port);
  89   scm_display (scm_length (g->zombies), port);
  90   scm_puts (")", port);
  91
  92   scm_puts (">", port);
  93
  94   return 1;
  95 }
  96
  97 /* Handle finalization of OBJ which is guarded by the guardians listed in
  98    GUARDIAN_LIST.  */
  99 static void
 100 finalize_guarded (GC_PTR obj, GC_PTR guardian_list)
 101 {
 102   SCM cell_pool;
 103
 104 #if 0
 105   printf ("finalizing guarded %p (%u guardians)\n",
 106           obj, scm_to_uint (scm_length (guardian_list)));
 107   scm_write (guardian_list, scm_current_output_port ());
 108 #endif
 109
 110   /* Preallocate a bunch of cells so that we can make sure that no garbage
 111      collection (and, thus, nested calls to `finalize_guarded ()') occurs
 112      while executing the following loop.  This is quite inefficient (call to
 113      `scm_length ()') but that shouldn't be a problem in most cases.  */
 114   cell_pool = scm_make_list (scm_length (guardian_list), SCM_UNSPECIFIED);
 115
 116   /* Tell each guardian interested in OBJ that OBJ is no longer
 117      reachable.  */
 118   for (;
 119        guardian_list != SCM_EOL;
 120        guardian_list = SCM_CDR (guardian_list))
 121     {
 122       SCM zombies;
 123       t_guardian *g = GUARDIAN_DATA (SCM_CAR (guardian_list));
 124
 125       if (g->live == 0)
 126         abort ();
 127
 128       /* Get a fresh cell from CELL_POOL.  */
 129       zombies = cell_pool;
 130       cell_pool = SCM_CDR (cell_pool);
 131
 132       /* Compute and update G's zombie list.  */
 133       SCM_SETCAR (zombies, SCM_PACK (obj));
 134       SCM_SETCDR (zombies, g->zombies);
 135       g->zombies = zombies;
 136
 137       g->live--;
 138       g->zombies = zombies;
 139     }
 140
 141 #if 0
 142   printf ("end of finalize (%p)\n", obj);
 143 #endif
 144 }
 145
 146 /* Add OBJ as a guarded object of GUARDIAN.  */
 147 static void
 148 scm_i_guard (SCM guardian, SCM obj)
 149 {
 150   t_guardian *g = GUARDIAN_DATA (guardian);
 151
 152   if (!SCM_IMP (obj))
 153     {
 154       /* Register a finalizer and pass a list of guardians interested in OBJ
 155          as the ``client data'' argument.  */
 156       GC_finalization_proc prev_finalizer;
 157       GC_PTR prev_data;
 158       SCM guardians_for_obj;
 159
 160       g->live++;
 161       guardians_for_obj = scm_cons (guardian, SCM_EOL);
 162
 163       GC_REGISTER_FINALIZER_NO_ORDER ((GC_PTR)obj, finalize_guarded,
 164                                       (GC_PTR)guardians_for_obj,
 165                                       &prev_finalizer, &prev_data);
 166
 167       if ((prev_finalizer == finalize_guarded) && (prev_data != NULL))
 168         {
 169           /* OBJ is already guarded by another guardian: add GUARDIAN to its
 170              list of guardians.  */
 171           SCM prev_guardian_list = SCM_PACK (prev_data);
 172
 173           if (!scm_is_pair (prev_guardian_list))
 174             abort ();
 175
 176           SCM_SETCDR (guardians_for_obj, prev_guardian_list);
 177         }
 178     }
 179 }
 180
 181 static SCM
 182 scm_i_get_one_zombie (SCM guardian)
 183 {
 184   t_guardian *g = GUARDIAN_DATA (guardian);
 185   SCM res = SCM_BOOL_F;
 186
 187   if (g->zombies != SCM_EOL)
 188     {
 189       /* Note: We return zombies in reverse order.  */
 190       res = SCM_CAR (g->zombies);
 191       g->zombies = SCM_CDR (g->zombies);
 192     }
 193
 194   return res;
 195 }
 196
 197 /* This is the Scheme entry point for each guardian: If OBJ is an
 198  * object, it's added to the guardian's live list.  If OBJ is unbound,
 199  * the next available unreachable object (or #f if none) is returned.
 200  *
 201  * If the second optional argument THROW_P is true (the default), then
 202  * an error is raised if GUARDIAN is greedy and OBJ is already greedily
 203  * guarded.  If THROW_P is false, #f is returned instead of raising the
 204  * error, and #t is returned if everything is fine.
 205  */
 206 static SCM
 207 guardian_apply (SCM guardian, SCM obj, SCM throw_p)
 208 {
 209 #if ENABLE_DEPRECATED
 210   if (!SCM_UNBNDP (throw_p))
 211     scm_c_issue_deprecation_warning
 212       ("Using the 'throw?' argument of a guardian is deprecated "
 213        "and ineffective.");
 214 #endif
 215
 216   if (!SCM_UNBNDP (obj))
 217     {
 218       scm_i_guard (guardian, obj);
 219       return SCM_UNSPECIFIED;
 220     }
 221   else
 222     return scm_i_get_one_zombie (guardian);
 223 }
 224
 225 SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 0, 0,
 226             (),
 227 "Create a new guardian.  A guardian protects a set of objects from\n"
 228 "garbage collection, allowing a program to apply cleanup or other\n"
 229 "actions.\n"
 230 "\n"
 231 "@code{make-guardian} returns a procedure representing the guardian.\n"
 232 "Calling the guardian procedure with an argument adds the argument to\n"
 233 "the guardian's set of protected objects.  Calling the guardian\n"
 234 "procedure without an argument returns one of the protected objects\n"
 235 "which are ready for garbage collection, or @code{#f} if no such object\n"
 236 "is available.  Objects which are returned in this way are removed from\n"
 237 "the guardian.\n"
 238 "\n"
 239 "You can put a single object into a guardian more than once and you can\n"
 240 "put a single object into more than one guardian.  The object will then\n"
 241 "be returned multiple times by the guardian procedures.\n"
 242 "\n"
 243 "An object is eligible to be returned from a guardian when it is no\n"
 244 "longer referenced from outside any guardian.\n"
 245 "\n"
 246 "There is no guarantee about the order in which objects are returned\n"
 247 "from a guardian.  If you want to impose an order on finalization\n"
 248 "actions, for example, you can do that by keeping objects alive in some\n"
 249 "global data structure until they are no longer needed for finalizing\n"
 250 "other objects.\n"
 251 "\n"
 252 "Being an element in a weak vector, a key in a hash table with weak\n"
 253 "keys, or a value in a hash table with weak value does not prevent an\n"
 254 "object from being returned by a guardian.  But as long as an object\n"
 255 "can be returned from a guardian it will not be removed from such a\n"
 256 "weak vector or hash table.  In other words, a weak link does not\n"
 257 "prevent an object from being considered collectable, but being inside\n"
 258 "a guardian prevents a weak link from being broken.\n"
 259 "\n"
 260 "A key in a weak key hash table can be though of as having a strong\n"
 261 "reference to its associated value as long as the key is accessible.\n"
 262 "Consequently, when the key only accessible from within a guardian, the\n"
 263 "reference from the key to the value is also considered to be coming\n"
 264 "from within a guardian.  Thus, if there is no other reference to the\n"
 265             "value, it is eligible to be returned from a guardian.\n")
 266 #define FUNC_NAME s_scm_make_guardian
 267 {
 268   t_guardian *g = scm_gc_malloc (sizeof (t_guardian), "guardian");
 269   SCM z;
 270
 271   /* A tconc starts out with one tail pair. */
 272   g->live = 0;
 273   g->zombies = SCM_EOL;
 274
 275   g->next = NULL;
 276
 277   SCM_NEWSMOB (z, tc16_guardian, g);
 278
 279   return z;
 280 }
 281 #undef FUNC_NAME
 282
 283 void
 284 scm_init_guardians ()
 285 {
 286   /* We use unordered finalization `a la Java.  */
 287   GC_java_finalization = 1;
 288
 289   tc16_guardian = scm_make_smob_type ("guardian", 0);
 290
 291   scm_set_smob_print (tc16_guardian, guardian_print);
 292 #if ENABLE_DEPRECATED
 293   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 2, 0);
 294 #else
 295   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 1, 0);
 296 #endif
 297
 298 #include "libguile/guardians.x"
 299 }
 300
 301 /*
 302   Local Variables:
 303   c-file-style: "gnu"
 304   End:
 305 */