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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Guile Reference Manual. | |
0d178803 | 3 | @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2012, 2013 |
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4 | @c Free Software Foundation, Inc. |
5 | @c See the file guile.texi for copying conditions. | |
6 | ||
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7 | @node Memory Management |
8 | @section Memory Management and Garbage Collection | |
9 | ||
10 | Guile uses a @emph{garbage collector} to manage most of its objects. | |
11 | While the garbage collector is designed to be mostly invisible, you | |
877f06c3 | 12 | sometimes need to interact with it explicitly. |
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13 | |
14 | See @ref{Garbage Collection} for a general discussion of how garbage | |
15 | collection relates to using Guile from C. | |
16 | ||
17 | @menu | |
18 | * Garbage Collection Functions:: | |
19 | * Memory Blocks:: | |
20 | * Weak References:: | |
21 | * Guardians:: | |
22 | @end menu | |
23 | ||
24 | ||
25 | @node Garbage Collection Functions | |
26 | @subsection Function related to Garbage Collection | |
27 | ||
28 | @deffn {Scheme Procedure} gc | |
29 | @deffnx {C Function} scm_gc () | |
30 | Scans all of SCM objects and reclaims for further use those that are | |
31 | no longer accessible. You normally don't need to call this function | |
32 | explicitly. It is called automatically when appropriate. | |
33 | @end deffn | |
34 | ||
35 | @deftypefn {C Function} SCM scm_gc_protect_object (SCM @var{obj}) | |
36 | Protects @var{obj} from being freed by the garbage collector, when it | |
37 | otherwise might be. When you are done with the object, call | |
38 | @code{scm_gc_unprotect_object} on the object. Calls to | |
39 | @code{scm_gc_protect}/@code{scm_gc_unprotect_object} can be nested, and | |
40 | the object remains protected until it has been unprotected as many times | |
41 | as it was protected. It is an error to unprotect an object more times | |
42 | than it has been protected. Returns the SCM object it was passed. | |
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43 | |
44 | Note that storing @var{obj} in a C global variable has the same | |
45 | effect@footnote{In Guile up to version 1.8, C global variables were not | |
46 | scanned by the garbage collector; hence, @code{scm_gc_protect_object} | |
47 | was the only way in C to prevent a Scheme object from being freed.}. | |
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48 | @end deftypefn |
49 | ||
50 | @deftypefn {C Function} SCM scm_gc_unprotect_object (SCM @var{obj}) | |
51 | ||
52 | Unprotects an object from the garbage collector which was protected by | |
53 | @code{scm_gc_unprotect_object}. Returns the SCM object it was passed. | |
54 | @end deftypefn | |
55 | ||
56 | @deftypefn {C Function} SCM scm_permanent_object (SCM @var{obj}) | |
57 | ||
58 | Similar to @code{scm_gc_protect_object} in that it causes the | |
59 | collector to always mark the object, except that it should not be | |
60 | nested (only call @code{scm_permanent_object} on an object once), and | |
61 | it has no corresponding unpermanent function. Once an object is | |
62 | declared permanent, it will never be freed. Returns the SCM object it | |
63 | was passed. | |
64 | @end deftypefn | |
65 | ||
66 | @c NOTE: The varargs scm_remember_upto_here is deliberately not | |
67 | @c documented, because we don't think it can be implemented as a nice | |
68 | @c inline compiler directive or asm block. New _3, _4 or whatever | |
69 | @c forms could certainly be added though, if needed. | |
70 | ||
71 | @deftypefn {C Macro} void scm_remember_upto_here_1 (SCM obj) | |
72 | @deftypefnx {C Macro} void scm_remember_upto_here_2 (SCM obj1, SCM obj2) | |
73 | Create a reference to the given object or objects, so they're certain | |
74 | to be present on the stack or in a register and hence will not be | |
75 | freed by the garbage collector before this point. | |
76 | ||
77 | Note that these functions can only be applied to ordinary C local | |
78 | variables (ie.@: ``automatics''). Objects held in global or static | |
79 | variables or some malloced block or the like cannot be protected with | |
80 | this mechanism. | |
81 | @end deftypefn | |
82 | ||
83 | @deffn {Scheme Procedure} gc-stats | |
84 | @deffnx {C Function} scm_gc_stats () | |
85 | Return an association list of statistics about Guile's current | |
86 | use of storage. | |
c93557e7 | 87 | @end deffn |
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89 | @deffn {Scheme Procedure} gc-live-object-stats |
90 | @deffnx {C Function} scm_gc_live_object_stats () | |
91 | Return an alist of statistics of the current live objects. | |
92 | @end deffn | |
93 | ||
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94 | @deftypefun void scm_gc_mark (SCM @var{x}) |
95 | Mark the object @var{x}, and recurse on any objects @var{x} refers to. | |
96 | If @var{x}'s mark bit is already set, return immediately. This function | |
97 | must only be called during the mark-phase of garbage collection, | |
98 | typically from a smob @emph{mark} function. | |
99 | @end deftypefun | |
100 | ||
101 | ||
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102 | @node Memory Blocks |
103 | @subsection Memory Blocks | |
104 | ||
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105 | @cindex automatically-managed memory |
106 | @cindex GC-managed memory | |
107 | @cindex conservative garbage collection | |
108 | ||
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109 | In C programs, dynamic management of memory blocks is normally done |
110 | with the functions malloc, realloc, and free. Guile has additional | |
111 | functions for dynamic memory allocation that are integrated into the | |
112 | garbage collector and the error reporting system. | |
113 | ||
114 | Memory blocks that are associated with Scheme objects (for example a | |
c5923112 | 115 | smob) should be allocated with @code{scm_gc_malloc} or |
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116 | @code{scm_gc_malloc_pointerless}. These two functions will either |
117 | return a valid pointer or signal an error. Memory blocks allocated this | |
118 | way can be freed with @code{scm_gc_free}; however, this is not strictly | |
119 | needed: memory allocated with @code{scm_gc_malloc} or | |
120 | @code{scm_gc_malloc_pointerless} is automatically reclaimed when the | |
121 | garbage collector no longer sees any live reference to it@footnote{In | |
122 | Guile up to version 1.8, memory allocated with @code{scm_gc_malloc} | |
123 | @emph{had} to be freed with @code{scm_gc_free}.}. | |
124 | ||
125 | Memory allocated with @code{scm_gc_malloc} is scanned for live pointers. | |
126 | This means that if @code{scm_gc_malloc}-allocated memory contains a | |
127 | pointer to some other part of the memory, the garbage collector notices | |
128 | it and prevents it from being reclaimed@footnote{In Guile up to 1.8, | |
129 | memory allocated with @code{scm_gc_malloc} was @emph{not} scanned. | |
130 | Consequently, the GC had to be told explicitly about pointers to live | |
131 | objects contained in the memory block, e.g., @i{via} SMOB mark functions | |
132 | (@pxref{Smobs, @code{scm_set_smob_mark}})}. Conversely, memory | |
133 | allocated with @code{scm_gc_malloc_pointerless} is assumed to be | |
134 | ``pointer-less'' and is not scanned. | |
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135 | |
136 | For memory that is not associated with a Scheme object, you can use | |
137 | @code{scm_malloc} instead of @code{malloc}. Like | |
138 | @code{scm_gc_malloc}, it will either return a valid pointer or signal | |
139 | an error. However, it will not assume that the new memory block can | |
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140 | be freed by a garbage collection. The memory must be explicitly freed |
141 | with @code{free}. | |
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142 | |
143 | There is also @code{scm_gc_realloc} and @code{scm_realloc}, to be used | |
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144 | in place of @code{realloc} when appropriate, and @code{scm_gc_calloc} |
145 | and @code{scm_calloc}, to be used in place of @code{calloc} when | |
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146 | appropriate. |
147 | ||
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148 | The function @code{scm_dynwind_free} can be useful when memory should be |
149 | freed with libc's @code{free} when leaving a dynwind context, | |
150 | @xref{Dynamic Wind}. | |
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151 | |
152 | @deftypefn {C Function} {void *} scm_malloc (size_t @var{size}) | |
153 | @deftypefnx {C Function} {void *} scm_calloc (size_t @var{size}) | |
154 | Allocate @var{size} bytes of memory and return a pointer to it. When | |
155 | @var{size} is 0, return @code{NULL}. When not enough memory is | |
156 | available, signal an error. This function runs the GC to free up some | |
157 | memory when it deems it appropriate. | |
158 | ||
159 | The memory is allocated by the libc @code{malloc} function and can be | |
160 | freed with @code{free}. There is no @code{scm_free} function to go | |
161 | with @code{scm_malloc} to make it easier to pass memory back and forth | |
a90968fa | 162 | between different modules. |
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163 | |
164 | The function @code{scm_calloc} is similar to @code{scm_malloc}, but | |
165 | initializes the block of memory to zero as well. | |
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166 | |
167 | These functions will (indirectly) call | |
168 | @code{scm_gc_register_allocation}. | |
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169 | @end deftypefn |
170 | ||
171 | @deftypefn {C Function} {void *} scm_realloc (void *@var{mem}, size_t @var{new_size}) | |
172 | Change the size of the memory block at @var{mem} to @var{new_size} and | |
173 | return its new location. When @var{new_size} is 0, this is the same | |
174 | as calling @code{free} on @var{mem} and @code{NULL} is returned. When | |
175 | @var{mem} is @code{NULL}, this function behaves like @code{scm_malloc} | |
176 | and allocates a new block of size @var{new_size}. | |
177 | ||
178 | When not enough memory is available, signal an error. This function | |
179 | runs the GC to free up some memory when it deems it appropriate. | |
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180 | |
181 | This function will call @code{scm_gc_register_allocation}. | |
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182 | @end deftypefn |
183 | ||
184 | ||
185 | ||
186 | ||
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187 | @deftypefn {C Function} {void *} scm_gc_malloc (size_t @var{size}, const char *@var{what}) |
188 | @deftypefnx {C Function} {void *} scm_gc_malloc_pointerless (size_t @var{size}, const char *@var{what}) | |
189 | @deftypefnx {C Function} {void *} scm_gc_realloc (void *@var{mem}, size_t @var{old_size}, size_t @var{new_size}, const char *@var{what}); | |
190 | @deftypefnx {C Function} {void *} scm_gc_calloc (size_t @var{size}, const char *@var{what}) | |
191 | Allocate @var{size} bytes of automatically-managed memory. The memory | |
192 | is automatically freed when no longer referenced from any live memory | |
193 | block. | |
194 | ||
195 | Memory allocated with @code{scm_gc_malloc} or @code{scm_gc_calloc} is | |
196 | scanned for pointers. Memory allocated by | |
197 | @code{scm_gc_malloc_pointerless} is not scanned. | |
198 | ||
199 | The @code{scm_gc_realloc} call preserves the ``pointerlessness'' of the | |
200 | memory area pointed to by @var{mem}. Note that you need to pass the old | |
201 | size of a reallocated memory block as well. See below for a motivation. | |
202 | @end deftypefn | |
203 | ||
204 | ||
205 | @deftypefn {C Function} void scm_gc_free (void *@var{mem}, size_t @var{size}, const char *@var{what}) | |
206 | Explicitly free the memory block pointed to by @var{mem}, which was | |
207 | previously allocated by one of the above @code{scm_gc} functions. | |
208 | ||
209 | Note that you need to explicitly pass the @var{size} parameter. This | |
210 | is done since it should normally be easy to provide this parameter | |
211 | (for memory that is associated with GC controlled objects) and help keep | |
212 | the memory management overhead very low. However, in Guile 2.x, | |
213 | @var{size} is always ignored. | |
214 | @end deftypefn | |
215 | ||
216 | ||
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217 | @deftypefn {C Function} void scm_gc_register_allocation (size_t @var{size}) |
218 | Informs the garbage collector that @var{size} bytes have been allocated, | |
219 | which the collector would otherwise not have known about. | |
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221 | In general, Scheme will decide to collect garbage only after some amount |
222 | of memory has been allocated. Calling this function will make the | |
223 | Scheme garbage collector know about more allocation, and thus run more | |
224 | often (as appropriate). | |
07d83abe | 225 | |
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226 | It is especially important to call this function when large unmanaged |
227 | allocations, like images, may be freed by small Scheme allocations, like | |
228 | SMOBs. | |
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229 | @end deftypefn |
230 | ||
231 | ||
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232 | @deftypefn {C Function} void scm_dynwind_free (void *mem) |
233 | Equivalent to @code{scm_dynwind_unwind_handler (free, @var{mem}, | |
234 | SCM_F_WIND_EXPLICITLY)}. That is, the memory block at @var{mem} will be | |
235 | freed (using @code{free} from the C library) when the current dynwind is | |
236 | left. | |
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237 | @end deftypefn |
238 | ||
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239 | @deffn {Scheme Procedure} malloc-stats |
240 | Return an alist ((@var{what} . @var{n}) ...) describing number | |
241 | of malloced objects. | |
242 | @var{what} is the second argument to @code{scm_gc_malloc}, | |
243 | @var{n} is the number of objects of that type currently | |
244 | allocated. | |
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245 | |
246 | This function is only available if the @code{GUILE_DEBUG_MALLOC} | |
247 | preprocessor macro was defined when Guile was compiled. | |
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248 | @end deffn |
249 | ||
250 | ||
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251 | @node Weak References |
252 | @subsection Weak References | |
253 | ||
254 | [FIXME: This chapter is based on Mikael Djurfeldt's answer to a | |
255 | question by Michael Livshin. Any mistakes are not theirs, of course. ] | |
256 | ||
257 | Weak references let you attach bookkeeping information to data so that | |
258 | the additional information automatically disappears when the original | |
259 | data is no longer in use and gets garbage collected. In a weak key hash, | |
260 | the hash entry for that key disappears as soon as the key is no longer | |
261 | referenced from anywhere else. For weak value hashes, the same happens | |
262 | as soon as the value is no longer in use. Entries in a doubly weak hash | |
263 | disappear when either the key or the value are not used anywhere else | |
264 | anymore. | |
265 | ||
266 | Object properties offer the same kind of functionality as weak key | |
267 | hashes in many situations. (@pxref{Object Properties}) | |
268 | ||
269 | Here's an example (a little bit strained perhaps, but one of the | |
270 | examples is actually used in Guile): | |
271 | ||
272 | Assume that you're implementing a debugging system where you want to | |
273 | associate information about filename and position of source code | |
274 | expressions with the expressions themselves. | |
275 | ||
276 | Hashtables can be used for that, but if you use ordinary hash tables | |
277 | it will be impossible for the scheme interpreter to "forget" old | |
278 | source when, for example, a file is reloaded. | |
279 | ||
280 | To implement the mapping from source code expressions to positional | |
281 | information it is necessary to use weak-key tables since we don't want | |
282 | the expressions to be remembered just because they are in our table. | |
283 | ||
284 | To implement a mapping from source file line numbers to source code | |
285 | expressions you would use a weak-value table. | |
286 | ||
287 | To implement a mapping from source code expressions to the procedures | |
288 | they constitute a doubly-weak table has to be used. | |
289 | ||
290 | @menu | |
cdf1ad3b | 291 | * Weak hash tables:: |
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292 | * Weak vectors:: |
293 | @end menu | |
294 | ||
295 | ||
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296 | @node Weak hash tables |
297 | @subsubsection Weak hash tables | |
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298 | |
299 | @deffn {Scheme Procedure} make-weak-key-hash-table size | |
300 | @deffnx {Scheme Procedure} make-weak-value-hash-table size | |
301 | @deffnx {Scheme Procedure} make-doubly-weak-hash-table size | |
302 | @deffnx {C Function} scm_make_weak_key_hash_table (size) | |
303 | @deffnx {C Function} scm_make_weak_value_hash_table (size) | |
304 | @deffnx {C Function} scm_make_doubly_weak_hash_table (size) | |
305 | Return a weak hash table with @var{size} buckets. As with any | |
306 | hash table, choosing a good size for the table requires some | |
307 | caution. | |
308 | ||
309 | You can modify weak hash tables in exactly the same way you | |
310 | would modify regular hash tables. (@pxref{Hash Tables}) | |
311 | @end deffn | |
312 | ||
313 | @deffn {Scheme Procedure} weak-key-hash-table? obj | |
314 | @deffnx {Scheme Procedure} weak-value-hash-table? obj | |
315 | @deffnx {Scheme Procedure} doubly-weak-hash-table? obj | |
316 | @deffnx {C Function} scm_weak_key_hash_table_p (obj) | |
317 | @deffnx {C Function} scm_weak_value_hash_table_p (obj) | |
318 | @deffnx {C Function} scm_doubly_weak_hash_table_p (obj) | |
319 | Return @code{#t} if @var{obj} is the specified weak hash | |
320 | table. Note that a doubly weak hash table is neither a weak key | |
321 | nor a weak value hash table. | |
322 | @end deffn | |
323 | ||
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324 | @node Weak vectors |
325 | @subsubsection Weak vectors | |
326 | ||
327 | Weak vectors are mainly useful in Guile's implementation of weak hash | |
328 | tables. | |
329 | ||
330 | @deffn {Scheme Procedure} make-weak-vector size [fill] | |
331 | @deffnx {C Function} scm_make_weak_vector (size, fill) | |
332 | Return a weak vector with @var{size} elements. If the optional | |
333 | argument @var{fill} is given, all entries in the vector will be | |
334 | set to @var{fill}. The default value for @var{fill} is the | |
335 | empty list. | |
336 | @end deffn | |
337 | ||
df0a1002 | 338 | @deffn {Scheme Procedure} weak-vector elem @dots{} |
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339 | @deffnx {Scheme Procedure} list->weak-vector l |
340 | @deffnx {C Function} scm_weak_vector (l) | |
341 | Construct a weak vector from a list: @code{weak-vector} uses | |
342 | the list of its arguments while @code{list->weak-vector} uses | |
343 | its only argument @var{l} (a list) to construct a weak vector | |
344 | the same way @code{list->vector} would. | |
345 | @end deffn | |
346 | ||
347 | @deffn {Scheme Procedure} weak-vector? obj | |
348 | @deffnx {C Function} scm_weak_vector_p (obj) | |
349 | Return @code{#t} if @var{obj} is a weak vector. Note that all | |
350 | weak hashes are also weak vectors. | |
351 | @end deffn | |
352 | ||
353 | ||
354 | @node Guardians | |
355 | @subsection Guardians | |
356 | ||
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357 | Guardians provide a way to be notified about objects that would |
358 | otherwise be collected as garbage. Guarding them prevents the objects | |
359 | from being collected and cleanup actions can be performed on them, for | |
360 | example. | |
07d83abe | 361 | |
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362 | See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in |
363 | a Generation-Based Garbage Collector". ACM SIGPLAN Conference on | |
364 | Programming Language Design and Implementation, June 1993. | |
07d83abe | 365 | |
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366 | @deffn {Scheme Procedure} make-guardian |
367 | @deffnx {C Function} scm_make_guardian () | |
368 | Create a new guardian. A guardian protects a set of objects from | |
369 | garbage collection, allowing a program to apply cleanup or other | |
370 | actions. | |
07d83abe | 371 | |
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372 | @code{make-guardian} returns a procedure representing the guardian. |
373 | Calling the guardian procedure with an argument adds the argument to | |
374 | the guardian's set of protected objects. Calling the guardian | |
375 | procedure without an argument returns one of the protected objects | |
376 | which are ready for garbage collection, or @code{#f} if no such object | |
377 | is available. Objects which are returned in this way are removed from | |
378 | the guardian. | |
07d83abe | 379 | |
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380 | You can put a single object into a guardian more than once and you can |
381 | put a single object into more than one guardian. The object will then | |
382 | be returned multiple times by the guardian procedures. | |
383 | ||
384 | An object is eligible to be returned from a guardian when it is no | |
385 | longer referenced from outside any guardian. | |
386 | ||
387 | There is no guarantee about the order in which objects are returned | |
388 | from a guardian. If you want to impose an order on finalization | |
389 | actions, for example, you can do that by keeping objects alive in some | |
390 | global data structure until they are no longer needed for finalizing | |
391 | other objects. | |
392 | ||
393 | Being an element in a weak vector, a key in a hash table with weak | |
e5547d5f | 394 | keys, or a value in a hash table with weak values does not prevent an |
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395 | object from being returned by a guardian. But as long as an object |
396 | can be returned from a guardian it will not be removed from such a | |
397 | weak vector or hash table. In other words, a weak link does not | |
398 | prevent an object from being considered collectable, but being inside | |
399 | a guardian prevents a weak link from being broken. | |
400 | ||
e5547d5f | 401 | A key in a weak key hash table can be thought of as having a strong |
930888e8 | 402 | reference to its associated value as long as the key is accessible. |
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403 | Consequently, when the key is only accessible from within a guardian, |
404 | the reference from the key to the value is also considered to be | |
405 | coming from within a guardian. Thus, if there is no other reference | |
406 | to the value, it is eligible to be returned from a guardian. | |
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407 | @end deffn |
408 | ||
409 | ||
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410 | @c Local Variables: |
411 | @c TeX-master: "guile.texi" | |
412 | @c End: |