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* guardians.c (TCONC_IN, scm_make_guardian): set the CDR of the

Browse source 
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.
This commit is contained in:
Gary Houston 2000-04-02 21:52:49 +00:00
parent cf49832683
commit e46f3fa6ba
1 changed files with 159 additions and 117 deletions
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276
libguile/guardians.c
View file

@ -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)
{
@ -148,92 +128,6 @@ guard (SCM cclo, SCM arg)
return scm_get_one_zombie (cclo);
}
static SCM guard1;
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"
"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.")
#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 z;
/* A tconc starts out with one tail pair. */
g->live.head = g->live.tail = z1;
g->zombies.head = g->zombies.tail = z2;
SCM_NEWSMOB (z, scm_tc16_guardian, g);
CCLO_G (cclo) = z;
return cclo;
}
#undef FUNC_NAME
void
scm_guardian_gc_init()
{
current_link_field = &first_live_guardian;
first_live_guardian = NULL;
}
void
scm_guardian_zombify ()
{
guardian_t *g;
/* Note that new guardians may be stuck on the end of the live
guardian list as we run this loop. As we move unmarked objects
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;
while (! SCM_EQ_P (pair, tconc_tail))
{
SCM next_pair = SCM_CDR (pair);
if (SCM_NMARKEDP (SCM_CAR (pair)))
{
/* got you, zombie! */
/* out of the live list! */
*prev_ptr = next_pair;
/* to the zombie list! */
TCONC_IN (g->zombies, SCM_CAR (pair), pair);
}
else
prev_ptr = SCM_CDRLOC (pair);
pair = next_pair;
}
/* Mark the cells of the live list. */
for (pair = g->live.head; SCM_NIMP (pair); pair = SCM_GCCDR (pair))
SCM_SETGCMARK (pair);
/* Bring the zombies back from the dead. */
scm_gc_mark (g->zombies.head);
}
}
void
scm_guard (SCM guardian, SCM obj)
{
@ -263,15 +157,163 @@ scm_get_one_zombie (SCM guardian)
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,
(),
"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.")
#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_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;
SCM_NEWSMOB (z, scm_tc16_guardian, g);
CCLO_G (cclo) = z;
return cclo;
}
#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()
{
current_link_field = &first_live_guardian;
first_live_guardian = NULL;
}
/* 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;
/* Note that new guardians may be stuck on the end of the live
guardian list as we run this loop. As we move unmarked objects
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)
{
SCM tconc_tail = g->live.tail;
SCM *prev_ptr = &g->live.head;
SCM pair = g->live.head;
while (! SCM_EQ_P (pair, tconc_tail))
{
SCM next_pair = SCM_CDR (pair);
if (SCM_NMARKEDP (SCM_CAR (pair)))
{
/* got you, zombie! */
/* out of the live list! */
*prev_ptr = next_pair;
/* into the zombie list! */
TCONC_IN (g->zombies, SCM_CAR (pair), pair);
}
else
prev_ptr = SCM_CDRLOC (pair);
pair = next_pair;
}
/* 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);
/* 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. */
scm_gc_mark (g->zombies.head);
}
}
/* 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)
{
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;
}
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"