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authorGary Houston <ghouston@arglist.com>2000-04-02 21:52:49 +0000
committerGary Houston <ghouston@arglist.com>2000-04-02 21:52:49 +0000
commite46f3fa6ba7d115757ac4c616e556609d7feffc5 (patch)
treea7c79ced5e4bdce13cff1f7aef470b1738f5ffde
parentcf498326837afeb6e45cf9ebda610b9835cc856c (diff)
downloadguile-e46f3fa6ba7d115757ac4c616e556609d7feffc5.tar.gz
* guardians.c (TCONC_IN, scm_make_guardian): set the CDR of the
empty tconc pair to SCM_EOL instead of SCM_BOOL_F, avoiding the use of an improper list (which breaks g_print. g_print isn't used). guardians.c: Added more comments and modified the make-guardian docstring. Reordered a few procedures.
-rw-r--r--libguile/guardians.c194
1 files changed, 118 insertions, 76 deletions
diff --git a/libguile/guardians.c b/libguile/guardians.c
index 5d49b4973..ccf0d284c 100644
--- a/libguile/guardians.c
+++ b/libguile/guardians.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1998, 1999 Free Software Foundation, Inc.
+/* Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
@@ -76,7 +76,7 @@ static long scm_tc16_guardian;
do { \
SCM_SETCAR ((tc).tail, obj); \
SCM_SETCAR (pair, SCM_BOOL_F); \
- SCM_SETCDR (pair, SCM_BOOL_F); \
+ SCM_SETCDR (pair, SCM_EOL); \
SCM_SETCDR ((tc).tail, pair); \
(tc).tail = pair; \
} while (0)
@@ -107,35 +107,15 @@ typedef struct guardian_t
#define GUARDIAN_ZOMBIES(x) (GUARDIAN (x)->zombies)
#define GUARDIAN_NEXT(x) (GUARDIAN (x)->next)
-static guardian_t *first_live_guardian = NULL;
-static guardian_t **current_link_field = NULL;
-
-static SCM
-g_mark (SCM ptr)
-{
- *current_link_field = GUARDIAN (ptr);
- current_link_field = &GUARDIAN_NEXT (ptr);
- GUARDIAN_NEXT (ptr) = NULL;
-
- /* Can't mark zombies here since they can refer to objects which are
- living dead, thereby preventing them to join the zombies. */
- return SCM_BOOL_F;
-}
-
-static int
-g_print (SCM exp, SCM port, scm_print_state *pstate)
-{
- char buf[256];
- sprintf (buf, "#<guardian live objs: %lu zombies: %lu>",
- scm_ilength (SCM_CDR (GUARDIAN_LIVE (exp).head)),
- scm_ilength (SCM_CDR (GUARDIAN_ZOMBIES (exp).head)));
- scm_puts (buf, port);
-
- return 1;
-}
-
#define CCLO_G(cclo) (SCM_VELTS (cclo)[1])
+/* subr constructed from guard below. */
+static SCM guard1;
+
+/* this is wrapped in a compiled closure and is the Scheme entry point
+ for each guardian: if arg is an object, it's added to the
+ guardian's live list. if arg is unbound, the next available
+ zombified object (or #f if none) is returned. */
static SCM
guard (SCM cclo, SCM arg)
{
@@ -148,26 +128,65 @@ guard (SCM cclo, SCM arg)
return scm_get_one_zombie (cclo);
}
-static SCM guard1;
+void
+scm_guard (SCM guardian, SCM obj)
+{
+ SCM g = CCLO_G (guardian);
+
+ if (SCM_NIMP (obj))
+ {
+ SCM z;
+
+ SCM_NEWCELL (z);
+
+ /* This critical section barrier will be replaced by a mutex. */
+ SCM_DEFER_INTS;
+ TCONC_IN (GUARDIAN_LIVE (g), obj, z);
+ SCM_ALLOW_INTS;
+ }
+}
+
+SCM
+scm_get_one_zombie (SCM guardian)
+{
+ SCM g = CCLO_G (guardian);
+ SCM res = SCM_BOOL_F;
+
+ /* This critical section barrier will be replaced by a mutex. */
+ SCM_DEFER_INTS;
+ if (!TCONC_EMPTYP (GUARDIAN_ZOMBIES (g)))
+ TCONC_OUT (GUARDIAN_ZOMBIES (g), res);
+ SCM_ALLOW_INTS;
+ return res;
+}
SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 0, 0,
(),
- "Return a new guardian object.\n"
- "A guardian allows dynamically allocated objects to be\n"
- "saved from deallocation by the garbage collector so that\n"
- "clean up or other actions can be performed using the data\n"
- "stored within the objects.\n"
+ "Create a new guardian.\n"
+ "A guardian protects a set of objects from garbage collection,\n"
+ "allowing a program to apply cleanup or other actions.\n\n"
+
+ "make-guardian returns a procedure representing the guardian.\n"
+ "Calling the guardian procedure with an argument adds the\n"
+ "argument to the guardian's set of protected objects.\n"
+ "Calling the guardian procedure without an argument returns\n"
+ "one of the protected objects which are ready for garbage\n"
+ "collection or @code{#f} if no such object is available.\n"
+ "Objects which are returned in this way are removed from\n"
+ "the guardian.\n\n".
+
"See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
"\"Guardians in a Generation-Based Garbage Collector\".\n"
"ACM SIGPLAN Conference on Programming Language Design\n"
- "and Implementation, June 1993\n.")
+ "and Implementation, June 1993.")
#define FUNC_NAME s_scm_make_guardian
{
SCM cclo = scm_makcclo (guard1, 2L);
guardian_t *g = SCM_MUST_MALLOC_TYPE(guardian_t);
- SCM z1 = scm_cons (SCM_BOOL_F, SCM_BOOL_F);
- SCM z2 = scm_cons (SCM_BOOL_F, SCM_BOOL_F);
+ SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
+ SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
SCM z;
+
/* A tconc starts out with one tail pair. */
g->live.head = g->live.tail = z1;
g->zombies.head = g->zombies.tail = z2;
@@ -180,6 +199,13 @@ SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 0, 0,
}
#undef FUNC_NAME
+/* during the gc mark phase, live guardians are linked into a list
+ here. */
+static guardian_t *first_live_guardian = NULL;
+static guardian_t **current_link_field = NULL;
+
+/* called before gc mark phase begins to initialise the live guardian
+ list. */
void
scm_guardian_gc_init()
{
@@ -187,8 +213,24 @@ scm_guardian_gc_init()
first_live_guardian = NULL;
}
-void
-scm_guardian_zombify ()
+/* mark a guardian by adding it to the live guardian list. */
+static SCM
+g_mark (SCM ptr)
+{
+ *current_link_field = GUARDIAN (ptr);
+ current_link_field = &GUARDIAN_NEXT (ptr);
+ GUARDIAN_NEXT (ptr) = NULL;
+
+ /* the objects protected by the guardian are not marked here: that
+ would prevent them from ever getting collected. instead marking
+ is done at the end of the mark phase by scm_guardian_zombify. */
+ return SCM_BOOL_F;
+}
+
+/* this is called by the garbage collector between the mark and sweep
+ phases. for each marked guardian, it moves any unmarked object in
+ its live list (tconc) to its zombie list (tconc). */
+void scm_guardian_zombify (void)
{
guardian_t *g;
@@ -197,10 +239,9 @@ scm_guardian_zombify ()
to the zombie list and mark them, we may find some guarded
guardians. The guardian mark function will stick them on the end
of this list, so they'll be processed properly. */
+
for (g = first_live_guardian; g; g = g->next)
{
- /* Scan the live list for unmarked objects, and move them to the
- zombies tconc. */
SCM tconc_tail = g->live.tail;
SCM *prev_ptr = &g->live.head;
SCM pair = g->live.head;
@@ -216,7 +257,7 @@ scm_guardian_zombify ()
/* out of the live list! */
*prev_ptr = next_pair;
- /* to the zombie list! */
+ /* into the zombie list! */
TCONC_IN (g->zombies, SCM_CAR (pair), pair);
}
else
@@ -225,53 +266,54 @@ scm_guardian_zombify ()
pair = next_pair;
}
- /* Mark the cells of the live list. */
+ /* Mark the cells of the live list (yes, the cells in the list,
+ even though we don't care about objects pointed to by the list
+ cars, since we know they are already marked). */
for (pair = g->live.head; SCM_NIMP (pair); pair = SCM_GCCDR (pair))
- SCM_SETGCMARK (pair);
+ SCM_SETGCMARK (pair);
+
+ /* Preserve the zombies in their undead state, by marking to
+ prevent collection. */
+
+ /* ghouston: possible bug: this may mark objects which are
+ protected by other guardians, but which have no references
+ from outside of the guardian system. section 3 of the paper
+ mentions shared and cyclic objects and it seems that all
+ parts should be made available for collection. Currently the
+ behaviour depends on the order in which guardians are
+ scanned.
+
+ Doesn't it seem a bit disturbing that if a zombie is returned
+ to full life after getting returned from the guardian
+ procedure, it may reference objects which are in a guardian's
+ zombie list? Is it not necessary to move such zombies back
+ to the live list, to avoid allowing the guardian procedure to
+ return an object which is referenced, so not collectable?
+ The paper doesn't give this impression. */
- /* Bring the zombies back from the dead. */
scm_gc_mark (g->zombies.head);
}
}
-void
-scm_guard (SCM guardian, SCM obj)
-{
- SCM g = CCLO_G (guardian);
-
- if (SCM_NIMP (obj))
- {
- SCM z;
-
- SCM_NEWCELL (z);
-
- /* This critical section barrier will be replaced by a mutex. */
- SCM_DEFER_INTS;
- TCONC_IN (GUARDIAN_LIVE (g), obj, z);
- SCM_ALLOW_INTS;
- }
-}
-
-SCM
-scm_get_one_zombie (SCM guardian)
+/* not generally used, since guardian smob is wrapped in a closure.
+ maybe useful for debugging. */
+static int
+g_print (SCM exp, SCM port, scm_print_state *pstate)
{
- SCM g = CCLO_G (guardian);
- SCM res = SCM_BOOL_F;
-
- /* This critical section barrier will be replaced by a mutex. */
- SCM_DEFER_INTS;
- if (!TCONC_EMPTYP (GUARDIAN_ZOMBIES (g)))
- TCONC_OUT (GUARDIAN_ZOMBIES (g), res);
- SCM_ALLOW_INTS;
+ char buf[256];
+ sprintf (buf, "#<guardian live objs: %lu zombies: %lu>",
+ scm_ilength (SCM_CDR (GUARDIAN_LIVE (exp).head)),
+ scm_ilength (SCM_CDR (GUARDIAN_ZOMBIES (exp).head)));
+ scm_puts (buf, port);
- return res;
+ return 1;
}
void
scm_init_guardian()
{
scm_tc16_guardian = scm_make_smob_type_mfpe ("guardian", sizeof (guardian_t),
- g_mark, NULL, g_print, NULL);
+ g_mark, NULL, g_print, NULL);
guard1 = scm_make_subr_opt ("guardian", scm_tc7_subr_2o, guard, 0);
#include "guardians.x"