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1 | @c essay \input texinfo |
2 | @c essay @c -*-texinfo-*- | |
3 | @c essay @c %**start of header | |
4 | @c essay @setfilename data-rep.info | |
5 | @c essay @settitle Data Representation in Guile | |
6 | @c essay @c %**end of header | |
7 | ||
8 | @c essay @include version.texi | |
9 | ||
10 | @c essay @dircategory The Algorithmic Language Scheme | |
11 | @c essay @direntry | |
12 | @c essay * data-rep: (data-rep). Data Representation in Guile --- how to use | |
13 | Guile objects in your C code. | |
14 | @c essay @end direntry | |
15 | ||
16 | @c essay @setchapternewpage off | |
17 | ||
18 | @c essay @ifinfo | |
19 | @c essay Data Representation in Guile | |
20 | ||
21 | @c essay Copyright (C) 1998, 1999, 2000 Free Software Foundation | |
22 | ||
23 | @c essay Permission is granted to make and distribute verbatim copies of | |
24 | @c essay this manual provided the copyright notice and this permission notice | |
25 | @c essay are preserved on all copies. | |
26 | ||
27 | @c essay @ignore | |
28 | @c essay Permission is granted to process this file through TeX and print the | |
29 | @c essay results, provided the printed document carries copying permission | |
30 | @c essay notice identical to this one except for the removal of this paragraph | |
31 | @c essay (this paragraph not being relevant to the printed manual). | |
32 | @c essay @end ignore | |
33 | ||
34 | @c essay Permission is granted to copy and distribute modified versions of this | |
35 | @c essay manual under the conditions for verbatim copying, provided that the entire | |
36 | @c essay resulting derived work is distributed under the terms of a permission | |
37 | @c essay notice identical to this one. | |
38 | ||
39 | @c essay Permission is granted to copy and distribute translations of this manual | |
40 | @c essay into another language, under the above conditions for modified versions, | |
41 | @c essay except that this permission notice may be stated in a translation approved | |
42 | @c essay by the Free Software Foundation. | |
43 | @c essay @end ifinfo | |
44 | ||
45 | @c essay @titlepage | |
46 | @c essay @sp 10 | |
47 | @c essay @comment The title is printed in a large font. | |
48 | @c essay @title Data Representation in Guile | |
abaec75d | 49 | @c essay @subtitle $Id: data-rep.texi,v 1.18 2001-04-02 21:53:20 ossau Exp $ |
38a93523 NJ |
50 | @c essay @subtitle For use with Guile @value{VERSION} |
51 | @c essay @author Jim Blandy | |
52 | @c essay @author Free Software Foundation | |
53 | @c essay @author @email{jimb@@red-bean.com} | |
54 | @c essay @c The following two commands start the copyright page. | |
55 | @c essay @page | |
56 | @c essay @vskip 0pt plus 1filll | |
57 | @c essay @vskip 0pt plus 1filll | |
58 | @c essay Copyright @copyright{} 1998 Free Software Foundation | |
59 | ||
60 | @c essay Permission is granted to make and distribute verbatim copies of | |
61 | @c essay this manual provided the copyright notice and this permission notice | |
62 | @c essay are preserved on all copies. | |
63 | ||
64 | @c essay Permission is granted to copy and distribute modified versions of this | |
65 | @c essay manual under the conditions for verbatim copying, provided that the entire | |
66 | @c essay resulting derived work is distributed under the terms of a permission | |
67 | @c essay notice identical to this one. | |
68 | ||
69 | @c essay Permission is granted to copy and distribute translations of this manual | |
70 | @c essay into another language, under the above conditions for modified versions, | |
71 | @c essay except that this permission notice may be stated in a translation approved | |
72 | @c essay by Free Software Foundation. | |
73 | @c essay @end titlepage | |
74 | ||
75 | @c essay @c @smallbook | |
76 | @c essay @c @finalout | |
77 | @c essay @headings double | |
78 | ||
79 | ||
80 | @c essay @node Top, Data Representation in Scheme, (dir), (dir) | |
81 | @c essay @top Data Representation in Guile | |
82 | ||
83 | @c essay @ifinfo | |
84 | @c essay This essay is meant to provide the background necessary to read and | |
85 | @c essay write C code that manipulates Scheme values in a way that conforms to | |
86 | @c essay libguile's interface. If you would like to write or maintain a | |
87 | @c essay Guile-based application in C or C++, this is the first information you | |
88 | @c essay need. | |
89 | ||
90 | @c essay In order to make sense of Guile's @code{SCM_} functions, or read | |
91 | @c essay libguile's source code, it's essential to have a good grasp of how Guile | |
92 | @c essay actually represents Scheme values. Otherwise, a lot of the code, and | |
93 | @c essay the conventions it follows, won't make very much sense. | |
94 | ||
95 | @c essay We assume you know both C and Scheme, but we do not assume you are | |
96 | @c essay familiar with Guile's C interface. | |
97 | @c essay @end ifinfo | |
98 | ||
99 | ||
100 | @page | |
101 | @node Data Representation | |
102 | @chapter Data Representation in Guile | |
103 | ||
104 | @strong{by Jim Blandy} | |
105 | ||
106 | [Due to the rather non-orthogonal and performance-oriented nature of the | |
107 | SCM interface, you need to understand SCM internals *before* you can use | |
108 | the SCM API. That's why this chapter comes first.] | |
109 | ||
110 | [NOTE: this is Jim Blandy's essay almost entirely unmodified. It has to | |
111 | be adapted to fit this manual smoothly.] | |
112 | ||
113 | In order to make sense of Guile's SCM_ functions, or read libguile's | |
114 | source code, it's essential to have a good grasp of how Guile actually | |
115 | represents Scheme values. Otherwise, a lot of the code, and the | |
116 | conventions it follows, won't make very much sense. This essay is meant | |
117 | to provide the background necessary to read and write C code that | |
118 | manipulates Scheme values in a way that is compatible with libguile. | |
119 | ||
120 | We assume you know both C and Scheme, but we do not assume you are | |
121 | familiar with Guile's implementation. | |
122 | ||
123 | @menu | |
124 | * Data Representation in Scheme:: Why things aren't just totally | |
125 | straightforward, in general terms. | |
126 | * How Guile does it:: How to write C code that manipulates | |
127 | Guile values, with an explanation | |
128 | of Guile's garbage collector. | |
129 | * Defining New Types (Smobs):: How to extend Guile with your own | |
130 | application-specific datatypes. | |
131 | @end menu | |
132 | ||
133 | @node Data Representation in Scheme | |
134 | @section Data Representation in Scheme | |
135 | ||
136 | Scheme is a latently-typed language; this means that the system cannot, | |
137 | in general, determine the type of a given expression at compile time. | |
138 | Types only become apparent at run time. Variables do not have fixed | |
139 | types; a variable may hold a pair at one point, an integer at the next, | |
140 | and a thousand-element vector later. Instead, values, not variables, | |
141 | have fixed types. | |
142 | ||
143 | In order to implement standard Scheme functions like @code{pair?} and | |
144 | @code{string?} and provide garbage collection, the representation of | |
145 | every value must contain enough information to accurately determine its | |
146 | type at run time. Often, Scheme systems also use this information to | |
147 | determine whether a program has attempted to apply an operation to an | |
148 | inappropriately typed value (such as taking the @code{car} of a string). | |
149 | ||
150 | Because variables, pairs, and vectors may hold values of any type, | |
151 | Scheme implementations use a uniform representation for values --- a | |
152 | single type large enough to hold either a complete value or a pointer | |
153 | to a complete value, along with the necessary typing information. | |
154 | ||
155 | The following sections will present a simple typing system, and then | |
156 | make some refinements to correct its major weaknesses. However, this is | |
157 | not a description of the system Guile actually uses. It is only an | |
158 | illustration of the issues Guile's system must address. We provide all | |
159 | the information one needs to work with Guile's data in @ref{How Guile | |
160 | does it}. | |
161 | ||
162 | ||
163 | @menu | |
164 | * A Simple Representation:: | |
165 | * Faster Integers:: | |
166 | * Cheaper Pairs:: | |
167 | * Guile Is Hairier:: | |
168 | @end menu | |
169 | ||
170 | @node A Simple Representation | |
171 | @subsection A Simple Representation | |
172 | ||
173 | The simplest way to meet the above requirements in C would be to | |
174 | represent each value as a pointer to a structure containing a type | |
175 | indicator, followed by a union carrying the real value. Assuming that | |
176 | @code{SCM} is the name of our universal type, we can write: | |
177 | ||
178 | @example | |
179 | enum type @{ integer, pair, string, vector, ... @}; | |
180 | ||
181 | typedef struct value *SCM; | |
182 | ||
183 | struct value @{ | |
184 | enum type type; | |
185 | union @{ | |
186 | int integer; | |
187 | struct @{ SCM car, cdr; @} pair; | |
188 | struct @{ int length; char *elts; @} string; | |
189 | struct @{ int length; SCM *elts; @} vector; | |
190 | ... | |
191 | @} value; | |
192 | @}; | |
193 | @end example | |
194 | with the ellipses replaced with code for the remaining Scheme types. | |
195 | ||
196 | This representation is sufficient to implement all of Scheme's | |
197 | semantics. If @var{x} is an @code{SCM} value: | |
198 | @itemize @bullet | |
199 | @item | |
200 | To test if @var{x} is an integer, we can write @code{@var{x}->type == integer}. | |
201 | @item | |
202 | To find its value, we can write @code{@var{x}->value.integer}. | |
203 | @item | |
204 | To test if @var{x} is a vector, we can write @code{@var{x}->type == vector}. | |
205 | @item | |
206 | If we know @var{x} is a vector, we can write | |
207 | @code{@var{x}->value.vector.elts[0]} to refer to its first element. | |
208 | @item | |
209 | If we know @var{x} is a pair, we can write | |
210 | @code{@var{x}->value.pair.car} to extract its car. | |
211 | @end itemize | |
212 | ||
213 | ||
214 | @node Faster Integers | |
215 | @subsection Faster Integers | |
216 | ||
217 | Unfortunately, the above representation has a serious disadvantage. In | |
218 | order to return an integer, an expression must allocate a @code{struct | |
219 | value}, initialize it to represent that integer, and return a pointer to | |
220 | it. Furthermore, fetching an integer's value requires a memory | |
221 | reference, which is much slower than a register reference on most | |
222 | processors. Since integers are extremely common, this representation is | |
223 | too costly, in both time and space. Integers should be very cheap to | |
224 | create and manipulate. | |
225 | ||
226 | One possible solution comes from the observation that, on many | |
227 | architectures, structures must be aligned on a four-byte boundary. | |
228 | (Whether or not the machine actually requires it, we can write our own | |
229 | allocator for @code{struct value} objects that assures this is true.) | |
230 | In this case, the lower two bits of the structure's address are known to | |
231 | be zero. | |
232 | ||
233 | This gives us the room we need to provide an improved representation | |
234 | for integers. We make the following rules: | |
235 | @itemize @bullet | |
236 | @item | |
237 | If the lower two bits of an @code{SCM} value are zero, then the SCM | |
238 | value is a pointer to a @code{struct value}, and everything proceeds as | |
239 | before. | |
240 | @item | |
241 | Otherwise, the @code{SCM} value represents an integer, whose value | |
242 | appears in its upper bits. | |
243 | @end itemize | |
244 | ||
245 | Here is C code implementing this convention: | |
246 | @example | |
247 | enum type @{ pair, string, vector, ... @}; | |
248 | ||
249 | typedef struct value *SCM; | |
250 | ||
251 | struct value @{ | |
252 | enum type type; | |
253 | union @{ | |
254 | struct @{ SCM car, cdr; @} pair; | |
255 | struct @{ int length; char *elts; @} string; | |
256 | struct @{ int length; SCM *elts; @} vector; | |
257 | ... | |
258 | @} value; | |
259 | @}; | |
260 | ||
261 | #define POINTER_P(x) (((int) (x) & 3) == 0) | |
262 | #define INTEGER_P(x) (! POINTER_P (x)) | |
263 | ||
264 | #define GET_INTEGER(x) ((int) (x) >> 2) | |
265 | #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1)) | |
266 | @end example | |
267 | ||
268 | Notice that @code{integer} no longer appears as an element of @code{enum | |
269 | type}, and the union has lost its @code{integer} member. Instead, we | |
270 | use the @code{POINTER_P} and @code{INTEGER_P} macros to make a coarse | |
271 | classification of values into integers and non-integers, and do further | |
272 | type testing as before. | |
273 | ||
274 | Here's how we would answer the questions posed above (again, assume | |
275 | @var{x} is an @code{SCM} value): | |
276 | @itemize @bullet | |
277 | @item | |
278 | To test if @var{x} is an integer, we can write @code{INTEGER_P (@var{x})}. | |
279 | @item | |
280 | To find its value, we can write @code{GET_INTEGER (@var{x})}. | |
281 | @item | |
282 | To test if @var{x} is a vector, we can write: | |
283 | @example | |
284 | @code{POINTER_P (@var{x}) && @var{x}->type == vector} | |
285 | @end example | |
286 | Given the new representation, we must make sure @var{x} is truly a | |
287 | pointer before we dereference it to determine its complete type. | |
288 | @item | |
289 | If we know @var{x} is a vector, we can write | |
290 | @code{@var{x}->value.vector.elts[0]} to refer to its first element, as | |
291 | before. | |
292 | @item | |
293 | If we know @var{x} is a pair, we can write | |
294 | @code{@var{x}->value.pair.car} to extract its car, just as before. | |
295 | @end itemize | |
296 | ||
297 | This representation allows us to operate more efficiently on integers | |
298 | than the first. For example, if @var{x} and @var{y} are known to be | |
299 | integers, we can compute their sum as follows: | |
300 | @example | |
301 | MAKE_INTEGER (GET_INTEGER (@var{x}) + GET_INTEGER (@var{y})) | |
302 | @end example | |
303 | Now, integer math requires no allocation or memory references. Most | |
304 | real Scheme systems actually use an even more efficient representation, | |
305 | but this essay isn't about bit-twiddling. (Hint: what if pointers had | |
306 | @code{01} in their least significant bits, and integers had @code{00}?) | |
307 | ||
308 | ||
309 | @node Cheaper Pairs | |
310 | @subsection Cheaper Pairs | |
311 | ||
312 | However, there is yet another issue to confront. Most Scheme heaps | |
313 | contain more pairs than any other type of object; Jonathan Rees says | |
314 | that pairs occupy 45% of the heap in his Scheme implementation, Scheme | |
315 | 48. However, our representation above spends three @code{SCM}-sized | |
316 | words per pair --- one for the type, and two for the @sc{car} and | |
317 | @sc{cdr}. Is there any way to represent pairs using only two words? | |
318 | ||
319 | Let us refine the convention we established earlier. Let us assert | |
320 | that: | |
321 | @itemize @bullet | |
322 | @item | |
323 | If the bottom two bits of an @code{SCM} value are @code{#b00}, then | |
324 | it is a pointer, as before. | |
325 | @item | |
326 | If the bottom two bits are @code{#b01}, then the upper bits are an | |
327 | integer. This is a bit more restrictive than before. | |
328 | @item | |
329 | If the bottom two bits are @code{#b10}, then the value, with the bottom | |
330 | two bits masked out, is the address of a pair. | |
331 | @end itemize | |
332 | ||
333 | Here is the new C code: | |
334 | @example | |
335 | enum type @{ string, vector, ... @}; | |
336 | ||
337 | typedef struct value *SCM; | |
338 | ||
339 | struct value @{ | |
340 | enum type type; | |
341 | union @{ | |
342 | struct @{ int length; char *elts; @} string; | |
343 | struct @{ int length; SCM *elts; @} vector; | |
344 | ... | |
345 | @} value; | |
346 | @}; | |
347 | ||
348 | struct pair @{ | |
349 | SCM car, cdr; | |
350 | @}; | |
351 | ||
352 | #define POINTER_P(x) (((int) (x) & 3) == 0) | |
353 | ||
354 | #define INTEGER_P(x) (((int) (x) & 3) == 1) | |
355 | #define GET_INTEGER(x) ((int) (x) >> 2) | |
356 | #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1)) | |
357 | ||
358 | #define PAIR_P(x) (((int) (x) & 3) == 2) | |
359 | #define GET_PAIR(x) ((struct pair *) ((int) (x) & ~3)) | |
360 | @end example | |
361 | ||
362 | Notice that @code{enum type} and @code{struct value} now only contain | |
363 | provisions for vectors and strings; both integers and pairs have become | |
364 | special cases. The code above also assumes that an @code{int} is large | |
365 | enough to hold a pointer, which isn't generally true. | |
366 | ||
367 | ||
368 | Our list of examples is now as follows: | |
369 | @itemize @bullet | |
370 | @item | |
371 | To test if @var{x} is an integer, we can write @code{INTEGER_P | |
372 | (@var{x})}; this is as before. | |
373 | @item | |
374 | To find its value, we can write @code{GET_INTEGER (@var{x})}, as | |
375 | before. | |
376 | @item | |
377 | To test if @var{x} is a vector, we can write: | |
378 | @example | |
379 | @code{POINTER_P (@var{x}) && @var{x}->type == vector} | |
380 | @end example | |
381 | We must still make sure that @var{x} is a pointer to a @code{struct | |
382 | value} before dereferencing it to find its type. | |
383 | @item | |
384 | If we know @var{x} is a vector, we can write | |
385 | @code{@var{x}->value.vector.elts[0]} to refer to its first element, as | |
386 | before. | |
387 | @item | |
388 | We can write @code{PAIR_P (@var{x})} to determine if @var{x} is a | |
389 | pair, and then write @code{GET_PAIR (@var{x})->car} to refer to its | |
390 | car. | |
391 | @end itemize | |
392 | ||
393 | This change in representation reduces our heap size by 15%. It also | |
394 | makes it cheaper to decide if a value is a pair, because no memory | |
395 | references are necessary; it suffices to check the bottom two bits of | |
396 | the @code{SCM} value. This may be significant when traversing lists, a | |
397 | common activity in a Scheme system. | |
398 | ||
399 | Again, most real Scheme systems use a slighty different implementation; | |
400 | for example, if GET_PAIR subtracts off the low bits of @code{x}, instead | |
401 | of masking them off, the optimizer will often be able to combine that | |
402 | subtraction with the addition of the offset of the structure member we | |
403 | are referencing, making a modified pointer as fast to use as an | |
404 | unmodified pointer. | |
405 | ||
406 | ||
407 | @node Guile Is Hairier | |
408 | @subsection Guile Is Hairier | |
409 | ||
410 | We originally started with a very simple typing system --- each object | |
411 | has a field that indicates its type. Then, for the sake of efficiency | |
412 | in both time and space, we moved some of the typing information directly | |
413 | into the @code{SCM} value, and left the rest in the @code{struct value}. | |
414 | Guile itself employs a more complex hierarchy, storing finer and finer | |
415 | gradations of type information in different places, depending on the | |
416 | object's coarser type. | |
417 | ||
418 | In the author's opinion, Guile could be simplified greatly without | |
419 | significant loss of efficiency, but the simplified system would still be | |
420 | more complex than what we've presented above. | |
421 | ||
422 | ||
423 | @node How Guile does it | |
424 | @section How Guile does it | |
425 | ||
426 | Here we present the specifics of how Guile represents its data. We | |
427 | don't go into complete detail; an exhaustive description of Guile's | |
428 | system would be boring, and we do not wish to encourage people to write | |
429 | code which depends on its details anyway. We do, however, present | |
430 | everything one need know to use Guile's data. | |
431 | ||
432 | ||
433 | @menu | |
434 | * General Rules:: | |
435 | * Conservative GC:: | |
abaec75d | 436 | * Immediates vs Non-immediates:: |
38a93523 NJ |
437 | * Immediate Datatypes:: |
438 | * Non-immediate Datatypes:: | |
439 | * Signalling Type Errors:: | |
440 | @end menu | |
441 | ||
442 | @node General Rules | |
443 | @subsection General Rules | |
444 | ||
445 | Any code which operates on Guile datatypes must @code{#include} the | |
446 | header file @code{<libguile.h>}. This file contains a definition for | |
447 | the @code{SCM} typedef (Guile's universal type, as in the examples | |
448 | above), and definitions and declarations for a host of macros and | |
449 | functions that operate on @code{SCM} values. | |
450 | ||
451 | All identifiers declared by @code{<libguile.h>} begin with @code{scm_} | |
452 | or @code{SCM_}. | |
453 | ||
454 | @c [[I wish this were true, but I don't think it is at the moment. -JimB]] | |
455 | @c Macros do not evaluate their arguments more than once, unless documented | |
456 | @c to do so. | |
457 | ||
458 | The functions described here generally check the types of their | |
459 | @code{SCM} arguments, and signal an error if their arguments are of an | |
460 | inappropriate type. Macros generally do not, unless that is their | |
461 | specified purpose. You must verify their argument types beforehand, as | |
462 | necessary. | |
463 | ||
464 | Macros and functions that return a boolean value have names ending in | |
465 | @code{P} or @code{_p} (for ``predicate''). Those that return a negated | |
466 | boolean value have names starting with @code{SCM_N}. For example, | |
467 | @code{SCM_IMP (@var{x})} is a predicate which returns non-zero iff | |
468 | @var{x} is an immediate value (an @code{IM}). @code{SCM_NCONSP | |
469 | (@var{x})} is a predicate which returns non-zero iff @var{x} is | |
470 | @emph{not} a pair object (a @code{CONS}). | |
471 | ||
472 | ||
473 | @node Conservative GC | |
474 | @subsection Conservative Garbage Collection | |
475 | ||
476 | Aside from the latent typing, the major source of constraints on a | |
477 | Scheme implementation's data representation is the garbage collector. | |
478 | The collector must be able to traverse every live object in the heap, to | |
479 | determine which objects are not live. | |
480 | ||
481 | There are many ways to implement this, but Guile uses an algorithm | |
482 | called @dfn{mark and sweep}. The collector scans the system's global | |
483 | variables and the local variables on the stack to determine which | |
484 | objects are immediately accessible by the C code. It then scans those | |
485 | objects to find the objects they point to, @i{et cetera}. The collector | |
486 | sets a @dfn{mark bit} on each object it finds, so each object is | |
487 | traversed only once. This process is called @dfn{tracing}. | |
488 | ||
489 | When the collector can find no unmarked objects pointed to by marked | |
490 | objects, it assumes that any objects that are still unmarked will never | |
491 | be used by the program (since there is no path of dereferences from any | |
492 | global or local variable that reaches them) and deallocates them. | |
493 | ||
494 | In the above paragraphs, we did not specify how the garbage collector | |
495 | finds the global and local variables; as usual, there are many different | |
496 | approaches. Frequently, the programmer must maintain a list of pointers | |
497 | to all global variables that refer to the heap, and another list | |
498 | (adjusted upon entry to and exit from each function) of local variables, | |
499 | for the collector's benefit. | |
500 | ||
501 | The list of global variables is usually not too difficult to maintain, | |
502 | since global variables are relatively rare. However, an explicitly | |
503 | maintained list of local variables (in the author's personal experience) | |
504 | is a nightmare to maintain. Thus, Guile uses a technique called | |
505 | @dfn{conservative garbage collection}, to make the local variable list | |
506 | unnecessary. | |
507 | ||
508 | The trick to conservative collection is to treat the stack as an | |
509 | ordinary range of memory, and assume that @emph{every} word on the stack | |
510 | is a pointer into the heap. Thus, the collector marks all objects whose | |
511 | addresses appear anywhere in the stack, without knowing for sure how | |
512 | that word is meant to be interpreted. | |
513 | ||
514 | Obviously, such a system will occasionally retain objects that are | |
515 | actually garbage, and should be freed. In practice, this is not a | |
516 | problem. The alternative, an explicitly maintained list of local | |
517 | variable addresses, is effectively much less reliable, due to programmer | |
518 | error. | |
519 | ||
520 | To accommodate this technique, data must be represented so that the | |
521 | collector can accurately determine whether a given stack word is a | |
522 | pointer or not. Guile does this as follows: | |
523 | @itemize @bullet | |
524 | ||
525 | @item | |
526 | Every heap object has a two-word header, called a @dfn{cell}. Some | |
527 | objects, like pairs, fit entirely in a cell's two words; others may | |
528 | store pointers to additional memory in either of the words. For | |
529 | example, strings and vectors store their length in the first word, and a | |
530 | pointer to their elements in the second. | |
531 | ||
532 | @item | |
533 | Guile allocates whole arrays of cells at a time, called @dfn{heap | |
534 | segments}. These segments are always allocated so that the cells they | |
535 | contain fall on eight-byte boundaries, or whatever is appropriate for | |
536 | the machine's word size. Guile keeps all cells in a heap segment | |
537 | initialized, whether or not they are currently in use. | |
538 | ||
539 | @item | |
540 | Guile maintains a sorted table of heap segments. | |
541 | ||
542 | @end itemize | |
543 | ||
544 | Thus, given any random word @var{w} fetched from the stack, Guile's | |
545 | garbage collector can consult the table to see if @var{w} falls within a | |
546 | known heap segment, and check @var{w}'s alignment. If both tests pass, | |
547 | the collector knows that @var{w} is a valid pointer to a cell, | |
548 | intentional or not, and proceeds to trace the cell. | |
549 | ||
550 | Note that heap segments do not contain all the data Guile uses; cells | |
551 | for objects like vectors and strings contain pointers to other memory | |
552 | areas. However, since those pointers are internal, and not shared among | |
553 | many pieces of code, it is enough for the collector to find the cell, | |
554 | and then use the cell's type to find more pointers to trace. | |
555 | ||
556 | ||
abaec75d NJ |
557 | @node Immediates vs Non-immediates |
558 | @subsection Immediates vs Non-immediates | |
38a93523 NJ |
559 | |
560 | Guile classifies Scheme objects into two kinds: those that fit entirely | |
561 | within an @code{SCM}, and those that require heap storage. | |
562 | ||
563 | The former class are called @dfn{immediates}. The class of immediates | |
564 | includes small integers, characters, boolean values, the empty list, the | |
565 | mysterious end-of-file object, and some others. | |
566 | ||
567 | The remaining types are called, not suprisingly, @dfn{non-immediates}. | |
568 | They include pairs, procedures, strings, vectors, and all other data | |
569 | types in Guile. | |
570 | ||
571 | @deftypefn Macro int SCM_IMP (SCM @var{x}) | |
572 | Return non-zero iff @var{x} is an immediate object. | |
573 | @end deftypefn | |
574 | ||
575 | @deftypefn Macro int SCM_NIMP (SCM @var{x}) | |
576 | Return non-zero iff @var{x} is a non-immediate object. This is the | |
577 | exact complement of @code{SCM_IMP}, above. | |
38a93523 NJ |
578 | @end deftypefn |
579 | ||
abaec75d NJ |
580 | Note that, as of Guile 1.4, it is no longer necessary to use the |
581 | @code{SCM_NIMP} macro before calling a finer-grained predicate to | |
582 | determine @var{x}'s type, such as @code{SCM_CONSP} or | |
583 | @code{SCM_VECTORP}. The definitions of all Guile type predicates | |
584 | now include a call to @code{SCM_NIMP} where necessary. | |
585 | ||
38a93523 NJ |
586 | |
587 | @node Immediate Datatypes | |
588 | @subsection Immediate Datatypes | |
589 | ||
590 | The following datatypes are immediate values; that is, they fit entirely | |
591 | within an @code{SCM} value. The @code{SCM_IMP} and @code{SCM_NIMP} | |
592 | macros will distinguish these from non-immediates; see @ref{Immediates | |
abaec75d | 593 | vs Non-immediates} for an explanation of the distinction. |
38a93523 NJ |
594 | |
595 | Note that the type predicates for immediate values work correctly on any | |
596 | @code{SCM} value; you do not need to call @code{SCM_IMP} first, to | |
597 | establish that a value is immediate. This differs from the | |
598 | non-immediate type predicates, which work correctly only on | |
599 | non-immediate values; you must be sure the value is @code{SCM_NIMP} | |
600 | before applying them. | |
601 | ||
602 | ||
603 | @menu | |
604 | * Integer Data:: | |
605 | * Character Data:: | |
606 | * Boolean Data:: | |
607 | * Unique Values:: | |
608 | @end menu | |
609 | ||
610 | @node Integer Data | |
611 | @subsubsection Integers | |
612 | ||
613 | Here are functions for operating on small integers, that fit within an | |
614 | @code{SCM}. Such integers are called @dfn{immediate numbers}, or | |
615 | @dfn{INUMs}. In general, INUMs occupy all but two bits of an | |
616 | @code{SCM}. | |
617 | ||
618 | Bignums and floating-point numbers are non-immediate objects, and have | |
619 | their own, separate accessors. The functions here will not work on | |
620 | them. This is not as much of a problem as you might think, however, | |
621 | because the system never constructs bignums that could fit in an INUM, | |
622 | and never uses floating point values for exact integers. | |
623 | ||
624 | @deftypefn Macro int SCM_INUMP (SCM @var{x}) | |
625 | Return non-zero iff @var{x} is a small integer value. | |
626 | @end deftypefn | |
627 | ||
628 | @deftypefn Macro int SCM_NINUMP (SCM @var{x}) | |
629 | The complement of SCM_INUMP. | |
630 | @end deftypefn | |
631 | ||
632 | @deftypefn Macro int SCM_INUM (SCM @var{x}) | |
633 | Return the value of @var{x} as an ordinary, C integer. If @var{x} | |
634 | is not an INUM, the result is undefined. | |
635 | @end deftypefn | |
636 | ||
637 | @deftypefn Macro SCM SCM_MAKINUM (int @var{i}) | |
638 | Given a C integer @var{i}, return its representation as an @code{SCM}. | |
639 | This function does not check for overflow. | |
640 | @end deftypefn | |
641 | ||
642 | ||
643 | @node Character Data | |
644 | @subsubsection Characters | |
645 | ||
646 | Here are functions for operating on characters. | |
647 | ||
648 | @deftypefn Macro int SCM_CHARP (SCM @var{x}) | |
649 | Return non-zero iff @var{x} is a character value. | |
650 | @end deftypefn | |
651 | ||
652 | @deftypefn Macro {unsigned int} SCM_CHAR (SCM @var{x}) | |
653 | Return the value of @code{x} as a C character. If @var{x} is not a | |
654 | Scheme character, the result is undefined. | |
655 | @end deftypefn | |
656 | ||
657 | @deftypefn Macro SCM SCM_MAKE_CHAR (int @var{c}) | |
658 | Given a C character @var{c}, return its representation as a Scheme | |
659 | character value. | |
660 | @end deftypefn | |
661 | ||
662 | ||
663 | @node Boolean Data | |
664 | @subsubsection Booleans | |
665 | ||
666 | Here are functions and macros for operating on booleans. | |
667 | ||
668 | @deftypefn Macro SCM SCM_BOOL_T | |
669 | @deftypefnx Macro SCM SCM_BOOL_F | |
670 | The Scheme true and false values. | |
671 | @end deftypefn | |
672 | ||
673 | @deftypefn Macro int SCM_NFALSEP (@var{x}) | |
674 | Convert the Scheme boolean value to a C boolean. Since every object in | |
675 | Scheme except @code{#f} is true, this amounts to comparing @var{x} to | |
676 | @code{#f}; hence the name. | |
677 | @c Noel feels a chill here. | |
678 | @end deftypefn | |
679 | ||
680 | @deftypefn Macro SCM SCM_BOOL_NOT (@var{x}) | |
681 | Return the boolean inverse of @var{x}. If @var{x} is not a | |
682 | Scheme boolean, the result is undefined. | |
683 | @end deftypefn | |
684 | ||
685 | ||
686 | @node Unique Values | |
687 | @subsubsection Unique Values | |
688 | ||
689 | The immediate values that are neither small integers, characters, nor | |
690 | booleans are all unique values --- that is, datatypes with only one | |
691 | instance. | |
692 | ||
693 | @deftypefn Macro SCM SCM_EOL | |
694 | The Scheme empty list object, or ``End Of List'' object, usually written | |
695 | in Scheme as @code{'()}. | |
696 | @end deftypefn | |
697 | ||
698 | @deftypefn Macro SCM SCM_EOF_VAL | |
699 | The Scheme end-of-file value. It has no standard written | |
700 | representation, for obvious reasons. | |
701 | @end deftypefn | |
702 | ||
703 | @deftypefn Macro SCM SCM_UNSPECIFIED | |
704 | The value returned by expressions which the Scheme standard says return | |
705 | an ``unspecified'' value. | |
706 | ||
707 | This is sort of a weirdly literal way to take things, but the standard | |
708 | read-eval-print loop prints nothing when the expression returns this | |
709 | value, so it's not a bad idea to return this when you can't think of | |
710 | anything else helpful. | |
711 | @end deftypefn | |
712 | ||
713 | @deftypefn Macro SCM SCM_UNDEFINED | |
714 | The ``undefined'' value. Its most important property is that is not | |
715 | equal to any valid Scheme value. This is put to various internal uses | |
716 | by C code interacting with Guile. | |
717 | ||
718 | For example, when you write a C function that is callable from Scheme | |
719 | and which takes optional arguments, the interpreter passes | |
720 | @code{SCM_UNDEFINED} for any arguments you did not receive. | |
721 | ||
722 | We also use this to mark unbound variables. | |
723 | @end deftypefn | |
724 | ||
725 | @deftypefn Macro int SCM_UNBNDP (SCM @var{x}) | |
726 | Return true if @var{x} is @code{SCM_UNDEFINED}. Apply this to a | |
727 | symbol's value to see if it has a binding as a global variable. | |
728 | @end deftypefn | |
729 | ||
730 | ||
731 | @node Non-immediate Datatypes | |
732 | @subsection Non-immediate Datatypes | |
733 | ||
734 | A non-immediate datatype is one which lives in the heap, either because | |
735 | it cannot fit entirely within a @code{SCM} word, or because it denotes a | |
736 | specific storage location (in the nomenclature of the Revised^4 Report | |
737 | on Scheme). | |
738 | ||
739 | The @code{SCM_IMP} and @code{SCM_NIMP} macros will distinguish these | |
abaec75d | 740 | from immediates; see @ref{Immediates vs Non-immediates}. |
38a93523 NJ |
741 | |
742 | Given a cell, Guile distinguishes between pairs and other non-immediate | |
743 | types by storing special @dfn{tag} values in a non-pair cell's car, that | |
744 | cannot appear in normal pairs. A cell with a non-tag value in its car | |
745 | is an ordinary pair. The type of a cell with a tag in its car depends | |
746 | on the tag; the non-immediate type predicates test this value. If a tag | |
747 | value appears elsewhere (in a vector, for example), the heap may become | |
748 | corrupted. | |
749 | ||
750 | ||
751 | @menu | |
752 | * Non-immediate Type Predicates:: Special rules for using the type | |
753 | predicates described here. | |
754 | * Pair Data:: | |
755 | * Vector Data:: | |
756 | * Procedures:: | |
757 | * Closures:: | |
758 | * Subrs:: | |
759 | * Port Data:: | |
760 | @end menu | |
761 | ||
762 | @node Non-immediate Type Predicates | |
763 | @subsubsection Non-immediate Type Predicates | |
764 | ||
765 | As mentioned in @ref{Conservative GC}, all non-immediate objects | |
766 | start with a @dfn{cell}, or a pair of words. Furthermore, all type | |
767 | information that distinguishes one kind of non-immediate from another is | |
768 | stored in the cell. The type information in the @code{SCM} value | |
769 | indicates only that the object is a non-immediate; all finer | |
770 | distinctions require one to examine the cell itself, usually with the | |
771 | appropriate type predicate macro. | |
772 | ||
773 | The type predicates for non-immediate objects generally assume that | |
774 | their argument is a non-immediate value. Thus, you must be sure that a | |
775 | value is @code{SCM_NIMP} first before passing it to a non-immediate type | |
776 | predicate. Thus, the idiom for testing whether a value is a cell or not | |
777 | is: | |
778 | @example | |
779 | SCM_NIMP (@var{x}) && SCM_CONSP (@var{x}) | |
780 | @end example | |
781 | ||
782 | ||
783 | @node Pair Data | |
784 | @subsubsection Pairs | |
785 | ||
786 | Pairs are the essential building block of list structure in Scheme. A | |
787 | pair object has two fields, called the @dfn{car} and the @dfn{cdr}. | |
788 | ||
789 | It is conventional for a pair's @sc{car} to contain an element of a | |
790 | list, and the @sc{cdr} to point to the next pair in the list, or to | |
791 | contain @code{SCM_EOL}, indicating the end of the list. Thus, a set of | |
792 | pairs chained through their @sc{cdr}s constitutes a singly-linked list. | |
793 | Scheme and libguile define many functions which operate on lists | |
794 | constructed in this fashion, so although lists chained through the | |
795 | @sc{car}s of pairs will work fine too, they may be less convenient to | |
796 | manipulate, and receive less support from the community. | |
797 | ||
798 | Guile implements pairs by mapping the @sc{car} and @sc{cdr} of a pair | |
799 | directly into the two words of the cell. | |
800 | ||
801 | ||
802 | @deftypefn Macro int SCM_CONSP (SCM @var{x}) | |
803 | Return non-zero iff @var{x} is a Scheme pair object. | |
804 | The results are undefined if @var{x} is an immediate value. | |
805 | @end deftypefn | |
806 | ||
807 | @deftypefn Macro int SCM_NCONSP (SCM @var{x}) | |
808 | The complement of SCM_CONSP. | |
809 | @end deftypefn | |
810 | ||
811 | @deftypefn Macro void SCM_NEWCELL (SCM @var{into}) | |
812 | Allocate a new cell, and set @var{into} to point to it. This macro | |
813 | expands to a statement, not an expression, and @var{into} must be an | |
814 | lvalue of type SCM. | |
815 | ||
816 | This is the most primitive way to allocate a cell; it is quite fast. | |
817 | ||
818 | The @sc{car} of the cell initially tags it as a ``free cell''. If the | |
819 | caller intends to use it as an ordinary cons, she must store ordinary | |
820 | SCM values in its @sc{car} and @sc{cdr}. | |
821 | ||
822 | If the caller intends to use it as a header for some other type, she | |
823 | must store an appropriate magic value in the cell's @sc{car}, to mark | |
824 | it as a member of that type, and store whatever value in the @sc{cdr} | |
825 | that type expects. You should generally not do this, unless you are | |
826 | implementing a new datatype, and thoroughly understand the code in | |
827 | @code{<libguile/tags.h>}. | |
828 | @end deftypefn | |
829 | ||
830 | @deftypefun SCM scm_cons (SCM @var{car}, SCM @var{cdr}) | |
831 | Allocate (``CONStruct'') a new pair, with @var{car} and @var{cdr} as its | |
832 | contents. | |
833 | @end deftypefun | |
834 | ||
835 | ||
836 | The macros below perform no typechecking. The results are undefined if | |
837 | @var{cell} is an immediate. However, since all non-immediate Guile | |
838 | objects are constructed from cells, and these macros simply return the | |
839 | first element of a cell, they actually can be useful on datatypes other | |
840 | than pairs. (Of course, it is not very modular to use them outside of | |
841 | the code which implements that datatype.) | |
842 | ||
843 | @deftypefn Macro SCM SCM_CAR (SCM @var{cell}) | |
844 | Return the @sc{car}, or first field, of @var{cell}. | |
845 | @end deftypefn | |
846 | ||
847 | @deftypefn Macro SCM SCM_CDR (SCM @var{cell}) | |
848 | Return the @sc{cdr}, or second field, of @var{cell}. | |
849 | @end deftypefn | |
850 | ||
851 | @deftypefn Macro void SCM_SETCAR (SCM @var{cell}, SCM @var{x}) | |
852 | Set the @sc{car} of @var{cell} to @var{x}. | |
853 | @end deftypefn | |
854 | ||
855 | @deftypefn Macro void SCM_SETCDR (SCM @var{cell}, SCM @var{x}) | |
856 | Set the @sc{cdr} of @var{cell} to @var{x}. | |
857 | @end deftypefn | |
858 | ||
859 | @deftypefn Macro SCM SCM_CAAR (SCM @var{cell}) | |
860 | @deftypefnx Macro SCM SCM_CADR (SCM @var{cell}) | |
861 | @deftypefnx Macro SCM SCM_CDAR (SCM @var{cell}) @dots{} | |
862 | @deftypefnx Macro SCM SCM_CDDDDR (SCM @var{cell}) | |
863 | Return the @sc{car} of the @sc{car} of @var{cell}, the @sc{car} of the | |
864 | @sc{cdr} of @var{cell}, @i{et cetera}. | |
865 | @end deftypefn | |
866 | ||
867 | ||
868 | @node Vector Data | |
869 | @subsubsection Vectors, Strings, and Symbols | |
870 | ||
871 | Vectors, strings, and symbols have some properties in common. They all | |
872 | have a length, and they all have an array of elements. In the case of a | |
873 | vector, the elements are @code{SCM} values; in the case of a string or | |
874 | symbol, the elements are characters. | |
875 | ||
876 | All these types store their length (along with some tagging bits) in the | |
877 | @sc{car} of their header cell, and store a pointer to the elements in | |
878 | their @sc{cdr}. Thus, the @code{SCM_CAR} and @code{SCM_CDR} macros | |
879 | are (somewhat) meaningful when applied to these datatypes. | |
880 | ||
881 | @deftypefn Macro int SCM_VECTORP (SCM @var{x}) | |
882 | Return non-zero iff @var{x} is a vector. | |
883 | The results are undefined if @var{x} is an immediate value. | |
884 | @end deftypefn | |
885 | ||
886 | @deftypefn Macro int SCM_STRINGP (SCM @var{x}) | |
887 | Return non-zero iff @var{x} is a string. | |
888 | The results are undefined if @var{x} is an immediate value. | |
889 | @end deftypefn | |
890 | ||
891 | @deftypefn Macro int SCM_SYMBOLP (SCM @var{x}) | |
892 | Return non-zero iff @var{x} is a symbol. | |
893 | The results are undefined if @var{x} is an immediate value. | |
894 | @end deftypefn | |
895 | ||
896 | @deftypefn Macro int SCM_LENGTH (SCM @var{x}) | |
897 | Return the length of the object @var{x}. | |
898 | The results are undefined if @var{x} is not a vector, string, or symbol. | |
899 | @end deftypefn | |
900 | ||
901 | @deftypefn Macro {SCM *} SCM_VELTS (SCM @var{x}) | |
902 | Return a pointer to the array of elements of the vector @var{x}. | |
903 | The results are undefined if @var{x} is not a vector. | |
904 | @end deftypefn | |
905 | ||
906 | @deftypefn Macro {char *} SCM_CHARS (SCM @var{x}) | |
907 | Return a pointer to the characters of @var{x}. | |
908 | The results are undefined if @var{x} is not a symbol or a string. | |
909 | @end deftypefn | |
910 | ||
911 | There are also a few magic values stuffed into memory before a symbol's | |
912 | characters, but you don't want to know about those. What cruft! | |
913 | ||
914 | ||
915 | @node Procedures | |
916 | @subsubsection Procedures | |
917 | ||
918 | Guile provides two kinds of procedures: @dfn{closures}, which are the | |
919 | result of evaluating a @code{lambda} expression, and @dfn{subrs}, which | |
920 | are C functions packaged up as Scheme objects, to make them available to | |
921 | Scheme programmers. | |
922 | ||
923 | (There are actually other sorts of procedures: compiled closures, and | |
924 | continuations; see the source code for details about them.) | |
925 | ||
926 | @deftypefun SCM scm_procedure_p (SCM @var{x}) | |
927 | Return @code{SCM_BOOL_T} iff @var{x} is a Scheme procedure object, of | |
928 | any sort. Otherwise, return @code{SCM_BOOL_F}. | |
929 | @end deftypefun | |
930 | ||
931 | ||
932 | @node Closures | |
933 | @subsubsection Closures | |
934 | ||
935 | [FIXME: this needs to be further subbed, but texinfo has no subsubsub] | |
936 | ||
937 | A closure is a procedure object, generated as the value of a | |
938 | @code{lambda} expression in Scheme. The representation of a closure is | |
939 | straightforward --- it contains a pointer to the code of the lambda | |
940 | expression from which it was created, and a pointer to the environment | |
941 | it closes over. | |
942 | ||
943 | In Guile, each closure also has a property list, allowing the system to | |
944 | store information about the closure. I'm not sure what this is used for | |
945 | at the moment --- the debugger, maybe? | |
946 | ||
947 | @deftypefn Macro int SCM_CLOSUREP (SCM @var{x}) | |
948 | Return non-zero iff @var{x} is a closure. The results are | |
949 | undefined if @var{x} is an immediate value. | |
950 | @end deftypefn | |
951 | ||
952 | @deftypefn Macro SCM SCM_PROCPROPS (SCM @var{x}) | |
953 | Return the property list of the closure @var{x}. The results are | |
954 | undefined if @var{x} is not a closure. | |
955 | @end deftypefn | |
956 | ||
957 | @deftypefn Macro void SCM_SETPROCPROPS (SCM @var{x}, SCM @var{p}) | |
958 | Set the property list of the closure @var{x} to @var{p}. The results | |
959 | are undefined if @var{x} is not a closure. | |
960 | @end deftypefn | |
961 | ||
962 | @deftypefn Macro SCM SCM_CODE (SCM @var{x}) | |
963 | Return the code of the closure @var{x}. The results are undefined if | |
964 | @var{x} is not a closure. | |
965 | ||
966 | This function should probably only be used internally by the | |
967 | interpreter, since the representation of the code is intimately | |
968 | connected with the interpreter's implementation. | |
969 | @end deftypefn | |
970 | ||
971 | @deftypefn Macro SCM SCM_ENV (SCM @var{x}) | |
972 | Return the environment enclosed by @var{x}. | |
973 | The results are undefined if @var{x} is not a closure. | |
974 | ||
975 | This function should probably only be used internally by the | |
976 | interpreter, since the representation of the environment is intimately | |
977 | connected with the interpreter's implementation. | |
978 | @end deftypefn | |
979 | ||
980 | ||
981 | @node Subrs | |
982 | @subsubsection Subrs | |
983 | ||
984 | [FIXME: this needs to be further subbed, but texinfo has no subsubsub] | |
985 | ||
986 | A subr is a pointer to a C function, packaged up as a Scheme object to | |
987 | make it callable by Scheme code. In addition to the function pointer, | |
988 | the subr also contains a pointer to the name of the function, and | |
989 | information about the number of arguments accepted by the C fuction, for | |
990 | the sake of error checking. | |
991 | ||
992 | There is no single type predicate macro that recognizes subrs, as | |
993 | distinct from other kinds of procedures. The closest thing is | |
994 | @code{scm_procedure_p}; see @ref{Procedures}. | |
995 | ||
996 | @deftypefn Macro {char *} SCM_SNAME (@var{x}) | |
997 | Return the name of the subr @var{x}. The results are undefined if | |
998 | @var{x} is not a subr. | |
999 | @end deftypefn | |
1000 | ||
1001 | @deftypefun SCM scm_make_gsubr (char *@var{name}, int @var{req}, int @var{opt}, int @var{rest}, SCM (*@var{function})()) | |
1002 | Create a new subr object named @var{name}, based on the C function | |
1003 | @var{function}, make it visible to Scheme the value of as a global | |
1004 | variable named @var{name}, and return the subr object. | |
1005 | ||
1006 | The subr object accepts @var{req} required arguments, @var{opt} optional | |
1007 | arguments, and a @var{rest} argument iff @var{rest} is non-zero. The C | |
1008 | function @var{function} should accept @code{@var{req} + @var{opt}} | |
1009 | arguments, or @code{@var{req} + @var{opt} + 1} arguments if @code{rest} | |
1010 | is non-zero. | |
1011 | ||
1012 | When a subr object is applied, it must be applied to at least @var{req} | |
1013 | arguments, or else Guile signals an error. @var{function} receives the | |
1014 | subr's first @var{req} arguments as its first @var{req} arguments. If | |
1015 | there are fewer than @var{opt} arguments remaining, then @var{function} | |
1016 | receives the value @code{SCM_UNDEFINED} for any missing optional | |
1017 | arguments. If @var{rst} is non-zero, then any arguments after the first | |
1018 | @code{@var{req} + @var{opt}} are packaged up as a list as passed as | |
1019 | @var{function}'s last argument. | |
1020 | ||
1021 | Note that subrs can actually only accept a predefined set of | |
1022 | combinations of required, optional, and rest arguments. For example, a | |
1023 | subr can take one required argument, or one required and one optional | |
1024 | argument, but a subr can't take one required and two optional arguments. | |
1025 | It's bizarre, but that's the way the interpreter was written. If the | |
1026 | arguments to @code{scm_make_gsubr} do not fit one of the predefined | |
1027 | patterns, then @code{scm_make_gsubr} will return a compiled closure | |
1028 | object instead of a subr object. | |
1029 | @end deftypefun | |
1030 | ||
1031 | ||
1032 | @node Port Data | |
1033 | @subsubsection Ports | |
1034 | ||
1035 | Haven't written this yet, 'cos I don't understand ports yet. | |
1036 | ||
1037 | ||
1038 | @node Signalling Type Errors | |
1039 | @subsection Signalling Type Errors | |
1040 | ||
1041 | Every function visible at the Scheme level should aggressively check the | |
1042 | types of its arguments, to avoid misinterpreting a value, and perhaps | |
1043 | causing a segmentation fault. Guile provides some macros to make this | |
1044 | easier. | |
1045 | ||
1046 | @deftypefn Macro void SCM_ASSERT (int @var{test}, SCM @var{obj}, int @var{position}, char *@var{subr}) | |
1047 | If @var{test} is zero, signal an error, attributed to the subroutine | |
1048 | named @var{subr}, operating on the value @var{obj}. The @var{position} | |
1049 | value determines exactly what sort of error to signal. | |
1050 | ||
1051 | If @var{position} is a string, @code{SCM_ASSERT} raises a | |
1052 | ``miscellaneous'' error whose message is that string. | |
1053 | ||
1054 | Otherwise, @var{position} should be one of the values defined below. | |
1055 | @end deftypefn | |
1056 | ||
1057 | @deftypefn Macro int SCM_ARG1 | |
1058 | @deftypefnx Macro int SCM_ARG2 | |
1059 | @deftypefnx Macro int SCM_ARG3 | |
1060 | @deftypefnx Macro int SCM_ARG4 | |
1061 | @deftypefnx Macro int SCM_ARG5 | |
1062 | Signal a ``wrong type argument'' error. When used as the @var{position} | |
1063 | argument of @code{SCM_ASSERT}, @code{SCM_ARG@var{n}} claims that | |
1064 | @var{obj} has the wrong type for the @var{n}'th argument of @var{subr}. | |
1065 | ||
1066 | The only way to complain about the type of an argument after the fifth | |
1067 | is to use @code{SCM_ARGn}, defined below, which doesn't specify which | |
1068 | argument is wrong. You could pass your own error message to | |
1069 | @code{SCM_ASSERT} as the @var{position}, but then the error signalled is | |
1070 | a ``miscellaneous'' error, not a ``wrong type argument'' error. This | |
1071 | seems kludgy to me. | |
1072 | @comment Any function with more than two arguments is wrong --- Perlis | |
1073 | @comment Despite Perlis, I agree. Why not have two Macros, one with | |
1074 | @comment a string error message, and the other with an integer position | |
1075 | @comment that only claims a type error in an argument? | |
1076 | @comment --- Keith Wright | |
1077 | @end deftypefn | |
1078 | ||
1079 | @deftypefn Macro int SCM_ARGn | |
1080 | As above, but does not specify which argument's type is incorrect. | |
1081 | @end deftypefn | |
1082 | ||
1083 | @deftypefn Macro int SCM_WNA | |
1084 | Signal an error complaining that the function received the wrong number | |
1085 | of arguments. | |
1086 | ||
1087 | Interestingly, the message is attributed to the function named by | |
1088 | @var{obj}, not @var{subr}, so @var{obj} must be a Scheme string object | |
1089 | naming the function. Usually, Guile catches these errors before ever | |
1090 | invoking the subr, so we don't run into these problems. | |
1091 | @end deftypefn | |
1092 | ||
1093 | ||
1094 | @node Defining New Types (Smobs) | |
1095 | @section Defining New Types (Smobs) | |
1096 | ||
1097 | @dfn{Smobs} are Guile's mechanism for adding new non-immediate types to | |
1098 | the system.@footnote{The term ``smob'' was coined by Aubrey Jaffer, who | |
1099 | says it comes from ``small object'', referring to the fact that only the | |
1100 | @sc{cdr} and part of the @sc{car} of a smob's cell are available for | |
1101 | use.} To define a new smob type, the programmer provides Guile with | |
1102 | some essential information about the type --- how to print it, how to | |
1103 | garbage collect it, and so on --- and Guile returns a fresh type tag for | |
1104 | use in the @sc{car} of new cells. The programmer can then use | |
1105 | @code{scm_make_gsubr} to make a set of C functions that create and | |
1106 | operate on these objects visible to Scheme code. | |
1107 | ||
1108 | (You can find a complete version of the example code used in this | |
1109 | section in the Guile distribution, in @file{doc/example-smob}. That | |
1110 | directory includes a makefile and a suitable @code{main} function, so | |
1111 | you can build a complete interactive Guile shell, extended with the | |
1112 | datatypes described here.) | |
1113 | ||
1114 | @menu | |
1115 | * Describing a New Type:: | |
1116 | * Creating Instances:: | |
1117 | * Typechecking:: | |
1118 | * Garbage Collecting Smobs:: | |
1119 | * A Common Mistake In Allocating Smobs:: | |
1120 | * Garbage Collecting Simple Smobs:: | |
1121 | * A Complete Example:: | |
1122 | @end menu | |
1123 | ||
1124 | @node Describing a New Type | |
1125 | @subsection Describing a New Type | |
1126 | ||
1127 | To define a new type, the programmer must write four functions to | |
1128 | manage instances of the type: | |
1129 | ||
1130 | @table @code | |
1131 | @item mark | |
1132 | Guile will apply this function to each instance of the new type it | |
1133 | encounters during garbage collection. This function is responsible for | |
1134 | telling the collector about any other non-immediate objects the object | |
1135 | refers to. The default smob mark function is to not mark any data. | |
1136 | @xref{Garbage Collecting Smobs}, for more details. | |
1137 | ||
1138 | @item free | |
1139 | Guile will apply this function to each instance of the new type it could | |
1140 | not find any live pointers to. The function should release all | |
1141 | resources held by the object and return the number of bytes released. | |
1142 | This is analagous to the Java finalization method-- it is invoked at | |
1143 | an unspecified time (when garbage collection occurs) after the object | |
1144 | is dead. | |
1145 | The default free function frees the smob data (if the size of the struct | |
1146 | passed to @code{scm_make_smob_type} or @code{scm_make_smob_type_mfpe} is | |
1147 | non-zero) using @code{scm_must_free} and returns the size of that | |
1148 | struct. @xref{Garbage Collecting Smobs}, for more details. | |
1149 | ||
1150 | @item print | |
1151 | @c GJB:FIXME:: @var{exp} and @var{port} need to refer to a prototype of | |
1152 | @c the print function.... where is that, or where should it go? | |
1153 | Guile will apply this function to each instance of the new type to print | |
1154 | the value, as for @code{display} or @code{write}. The function should | |
1155 | write a printed representation of @var{exp} on @var{port}, in accordance | |
1156 | with the parameters in @var{pstate}. (For more information on print | |
1157 | states, see @ref{Port Data}.) The default print function prints @code{#<NAME ADDRESS>} | |
1158 | where @code{NAME} is the first argument passed to @code{scm_make_smob_type} or | |
1159 | @code{scm_make_smob_type_mfpe}. | |
1160 | ||
1161 | @item equalp | |
1162 | If Scheme code asks the @code{equal?} function to compare two instances | |
1163 | of the same smob type, Guile calls this function. It should return | |
1164 | @code{SCM_BOOL_T} if @var{a} and @var{b} should be considered | |
1165 | @code{equal?}, or @code{SCM_BOOL_F} otherwise. If @code{equalp} is | |
1166 | @code{NULL}, @code{equal?} will assume that two instances of this type are | |
1167 | never @code{equal?} unless they are @code{eq?}. | |
1168 | ||
1169 | @end table | |
1170 | ||
1171 | To actually register the new smob type, call @code{scm_make_smob_type}: | |
1172 | ||
1173 | @deftypefun long scm_make_smob_type (const char *name, scm_sizet size) | |
1174 | This function implements the standard way of adding a new smob type, | |
1175 | named @var{name}, with instance size @var{size}, to the system. The | |
1176 | return value is a tag that is used in creating instances of the type. | |
1177 | If @var{size} is 0, then no memory will be allocated when instances of | |
1178 | the smob are created, and nothing will be freed by the default free | |
1179 | function. Default values are provided for mark, free, print, and, | |
1180 | equalp, as described above. If you want to customize any of these | |
1181 | functions, the call to @code{scm_make_smob_type} should be immediately | |
1182 | followed by calls to one or several of @code{scm_set_smob_mark}, | |
1183 | @code{scm_set_smob_free}, @code{scm_set_smob_print}, and/or | |
1184 | @code{scm_set_smob_equalp}. | |
1185 | @end deftypefun | |
1186 | ||
1187 | Each of the below @code{scm_set_smob_XXX} functions registers a smob | |
1188 | special function for a given type. Each function is intended to be used | |
1189 | only zero or one time per type, and the call should be placed | |
1190 | immediately following the call to @code{scm_make_smob_type}. | |
1191 | ||
1192 | @deftypefun void scm_set_smob_mark (long tc, SCM (*mark) (SCM)) | |
1193 | This function sets the smob marking procedure for the smob type specified by | |
1194 | the tag @var{tc}. @var{tc} is the tag returned by @code{scm_make_smob_type}. | |
1195 | @end deftypefun | |
1196 | ||
1197 | @deftypefun void scm_set_smob_free (long tc, scm_sizet (*free) (SCM)) | |
1198 | This function sets the smob freeing procedure for the smob type specified by | |
1199 | the tag @var{tc}. @var{tc} is the tag returned by @code{scm_make_smob_type}. | |
1200 | @end deftypefun | |
1201 | ||
1202 | @deftypefun void scm_set_smob_print (long tc, int (*print) (SCM,SCM,scm_print_state*)) | |
1203 | This function sets the smob printing procedure for the smob type specified by | |
1204 | the tag @var{tc}. @var{tc} is the tag returned by @code{scm_make_smob_type}. | |
1205 | @end deftypefun | |
1206 | ||
1207 | @deftypefun void scm_set_smob_equalp (long tc, SCM (*equalp) (SCM,SCM)) | |
1208 | This function sets the smob equality-testing predicate for the smob type specified by | |
1209 | the tag @var{tc}. @var{tc} is the tag returned by @code{scm_make_smob_type}. | |
1210 | @end deftypefun | |
1211 | ||
1212 | Instead of using @code{scm_make_smob_type} and calling each of the | |
1213 | individual @code{scm_set_smob_XXX} functions to register each special | |
1214 | function independently, you can use @code{scm_make_smob_type_mfpe} to | |
1215 | register all of the special functions at once as you create the smob | |
1216 | type@footnote{Warning: There is an ongoing discussion among the developers which | |
1217 | may result in deprecating @code{scm_make_smob_type_mfpe} in next release | |
1218 | of Guile.}: | |
1219 | ||
1220 | @deftypefun long scm_make_smob_type_mfpe(const char *name, scm_sizet size, SCM (*mark) (SCM), scm_sizet (*free) (SCM), int (*print) (SCM, SCM, scm_print_state*), SCM (*equalp) (SCM, SCM)) | |
1221 | This function invokes @code{scm_make_smob_type} on its first two arguments | |
1222 | to add a new smob type named @var{name}, with instance size @var{size} to the system. | |
1223 | It also registers the @var{mark}, @var{free}, @var{print}, @var{equalp} smob | |
1224 | special functions for that new type. Any of these parameters can be @code{NULL} | |
1225 | to have that special function use the default behaviour for guile. | |
1226 | The return value is a tag that is used in creating instances of the type. If @var{size} | |
1227 | is 0, then no memory will be allocated when instances of the smob are created, and | |
1228 | nothing will be freed by the default free function. | |
1229 | @end deftypefun | |
1230 | ||
1231 | For example, here is how one might declare and register a new type | |
1232 | representing eight-bit grayscale images: | |
1233 | @example | |
1234 | #include <libguile.h> | |
1235 | ||
1236 | long image_tag; | |
1237 | ||
1238 | void | |
1239 | init_image_type () | |
1240 | @{ | |
1241 | image_tag = scm_make_smob_type_mfpe ("image",sizeof(struct image), | |
1242 | mark_image, free_image, print_image, NULL); | |
1243 | @} | |
1244 | @end example | |
1245 | ||
1246 | ||
1247 | @node Creating Instances | |
1248 | @subsection Creating Instances | |
1249 | ||
1250 | Like other non-immediate types, smobs start with a cell whose @sc{car} | |
1251 | contains typing information, and whose @code{cdr} is free for any use. For smobs, | |
1252 | the @code{cdr} stores a pointer to the internal C structure holding the | |
1253 | smob-specific data. | |
1254 | To create an instance of a smob type following these standards, you should | |
1255 | use @code{SCM_NEWSMOB}: | |
1256 | ||
1257 | @deftypefn Macro void SCM_NEWSMOB(SCM value,long tag,void *data) | |
1258 | Make @var{value} contain a smob instance of the type with tag @var{tag} | |
1259 | and smob data @var{data}. @var{value} must be previously declared | |
1260 | as C type @code{SCM}. | |
1261 | @end deftypefn | |
1262 | ||
1263 | Since it is often the case (e.g., in smob constructors) that you will | |
1264 | create a smob instance and return it, there is also a slightly specialized | |
1265 | macro for this situation: | |
1266 | ||
1267 | @deftypefn Macro fn_returns SCM_RETURN_NEWSMOB(long tab, void *data) | |
1268 | This macro expands to a block of code that creates a smob instance of | |
1269 | the type with tag @var{tag} and smob data @var{data}, and returns | |
1270 | that @code{SCM} value. It should be the last piece of code in | |
1271 | a block. | |
1272 | @end deftypefn | |
1273 | ||
1274 | Guile provides the following functions for managing memory, which are | |
1275 | often helpful when implementing smobs: | |
1276 | ||
1277 | @deftypefun {char *} scm_must_malloc (long @var{len}, char *@var{what}) | |
1278 | Allocate @var{len} bytes of memory, using @code{malloc}, and return a | |
1279 | pointer to them. | |
1280 | ||
1281 | If there is not enough memory available, invoke the garbage collector, | |
1282 | and try once more. If there is still not enough, signal an error, | |
1283 | reporting that we could not allocate @var{what}. | |
1284 | ||
1285 | This function also helps maintain statistics about the size of the heap. | |
1286 | @end deftypefun | |
1287 | ||
1288 | @deftypefun {char *} scm_must_realloc (char *@var{addr}, long @var{olen}, long @var{len}, char *@var{what}) | |
1289 | Resize (and possibly relocate) the block of memory at @var{addr}, to | |
1290 | have a size of @var{len} bytes, by calling @code{realloc}. Return a | |
1291 | pointer to the new block. | |
1292 | ||
1293 | If there is not enough memory available, invoke the garbage collector, | |
1294 | and try once more. If there is still not enough, signal an error, | |
1295 | reporting that we could not allocate @var{what}. | |
1296 | ||
1297 | The value @var{olen} should be the old size of the block of memory at | |
1298 | @var{addr}; it is only used for keeping statistics on the size of the | |
1299 | heap. | |
1300 | @end deftypefun | |
1301 | ||
1302 | @deftypefun void scm_must_free (char *@var{addr}) | |
1303 | Free the block of memory at @var{addr}, using @code{free}. If | |
1304 | @var{addr} is zero, signal an error, complaining of an attempt to free | |
1305 | something that is already free. | |
1306 | ||
1307 | This does no record-keeping; instead, the smob's @code{free} function | |
1308 | must take care of that. | |
1309 | ||
1310 | This function isn't usually sufficiently different from the usual | |
1311 | @code{free} function to be worth using. | |
1312 | @end deftypefun | |
1313 | ||
1314 | ||
1315 | Continuing the above example, if the global variable @code{image_tag} | |
1316 | contains a tag returned by @code{scm_newsmob}, here is how we could | |
1317 | construct a smob whose @sc{cdr} contains a pointer to a freshly | |
1318 | allocated @code{struct image}: | |
1319 | ||
1320 | @example | |
1321 | struct image @{ | |
1322 | int width, height; | |
1323 | char *pixels; | |
1324 | ||
1325 | /* The name of this image */ | |
1326 | SCM name; | |
1327 | ||
1328 | /* A function to call when this image is | |
1329 | modified, e.g., to update the screen, | |
1330 | or SCM_BOOL_F if no action necessary */ | |
1331 | SCM update_func; | |
1332 | @}; | |
1333 | ||
1334 | SCM | |
1335 | make_image (SCM name, SCM s_width, SCM s_height) | |
1336 | @{ | |
1337 | struct image *image; | |
1338 | int width, height; | |
1339 | ||
1340 | SCM_ASSERT (SCM_NIMP (name) && SCM_STRINGP (name), name, | |
1341 | SCM_ARG1, "make-image"); | |
1342 | SCM_ASSERT (SCM_INUMP (s_width), s_width, SCM_ARG2, "make-image"); | |
1343 | SCM_ASSERT (SCM_INUMP (s_height), s_height, SCM_ARG3, "make-image"); | |
1344 | ||
1345 | width = SCM_INUM (s_width); | |
1346 | height = SCM_INUM (s_height); | |
1347 | ||
1348 | image = (struct image *) scm_must_malloc (sizeof (struct image), "image"); | |
1349 | image->width = width; | |
1350 | image->height = height; | |
1351 | image->pixels = scm_must_malloc (width * height, "image pixels"); | |
1352 | image->name = name; | |
1353 | image->update_func = SCM_BOOL_F; | |
1354 | ||
1355 | SCM_RETURN_NEWSMOB (image_tag, image); | |
1356 | @} | |
1357 | @end example | |
1358 | ||
1359 | ||
1360 | @node Typechecking | |
1361 | @subsection Typechecking | |
1362 | ||
1363 | Functions that operate on smobs should aggressively check the types of | |
1364 | their arguments, to avoid misinterpreting some other datatype as a smob, | |
1365 | and perhaps causing a segmentation fault. Fortunately, this is pretty | |
1366 | simple to do. The function need only verify that its argument is a | |
1367 | non-immediate, whose @sc{car} is the type tag returned by | |
1368 | @code{scm_newsmob}. | |
1369 | ||
1370 | For example, here is a simple function that operates on an image smob, | |
1371 | and checks the type of its argument. We also present an expanded | |
1372 | version of the @code{init_image_type} function, to make | |
1373 | @code{clear_image} and the image constructor function @code{make_image} | |
1374 | visible to Scheme code. | |
1375 | @example | |
1376 | SCM | |
1377 | clear_image (SCM image_smob) | |
1378 | @{ | |
1379 | int area; | |
1380 | struct image *image; | |
1381 | ||
1382 | SCM_ASSERT (SCM_SMOB_PREDICATE (image_tag, image_smob), | |
1383 | image_smob, SCM_ARG1, "clear-image"); | |
1384 | ||
1385 | image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1386 | area = image->width * image->height; | |
1387 | memset (image->pixels, 0, area); | |
1388 | ||
1389 | /* Invoke the image's update function. */ | |
1390 | if (image->update_func != SCM_BOOL_F) | |
1391 | scm_apply (image->update_func, SCM_EOL, SCM_EOL); | |
1392 | ||
1393 | return SCM_UNSPECIFIED; | |
1394 | @} | |
1395 | ||
1396 | ||
1397 | void | |
1398 | init_image_type () | |
1399 | @{ | |
1400 | image_tag = scm_newsmob (&image_funs); | |
1401 | ||
1402 | scm_make_gsubr ("make-image", 3, 0, 0, make_image); | |
1403 | scm_make_gsubr ("clear-image", 1, 0, 0, clear_image); | |
1404 | @} | |
1405 | @end example | |
1406 | ||
1407 | Note that checking types is a little more complicated during garbage | |
1408 | collection; see the description of @code{SCM_GCTYP16} in @ref{Garbage | |
1409 | Collecting Smobs}. | |
1410 | ||
1411 | @c GJB:FIXME:: should talk about guile-snarf somewhere! | |
1412 | ||
1413 | @node Garbage Collecting Smobs | |
1414 | @subsection Garbage Collecting Smobs | |
1415 | ||
1416 | Once a smob has been released to the tender mercies of the Scheme | |
1417 | system, it must be prepared to survive garbage collection. Guile calls | |
1418 | the @code{mark} and @code{free} functions of the @code{scm_smobfuns} | |
1419 | structure to manage this. | |
1420 | ||
1421 | As described before (@pxref{Conservative GC}), every object in the | |
1422 | Scheme system has a @dfn{mark bit}, which the garbage collector uses to | |
1423 | tell live objects from dead ones. When collection starts, every | |
1424 | object's mark bit is clear. The collector traces pointers through the | |
1425 | heap, starting from objects known to be live, and sets the mark bit on | |
1426 | each object it encounters. When it can find no more unmarked objects, | |
1427 | the collector walks all objects, live and dead, frees those whose mark | |
1428 | bits are still clear, and clears the mark bit on the others. | |
1429 | ||
1430 | The two main portions of the collection are called the @dfn{mark phase}, | |
1431 | during which the collector marks live objects, and the @dfn{sweep | |
1432 | phase}, during which the collector frees all unmarked objects. | |
1433 | ||
1434 | The mark bit of a smob lives in its @sc{car}, along with the smob's type | |
1435 | tag. When the collector encounters a smob, it sets the smob's mark bit, | |
1436 | and uses the smob's type tag to find the appropriate @code{mark} | |
1437 | function for that smob: the one listed in that smob's | |
1438 | @code{scm_smobfuns} structure. It then calls the @code{mark} function, | |
1439 | passing it the smob as its only argument. | |
1440 | ||
1441 | The @code{mark} function is responsible for marking any other Scheme | |
1442 | objects the smob refers to. If it does not do so, the objects' mark | |
1443 | bits will still be clear when the collector begins to sweep, and the | |
1444 | collector will free them. If this occurs, it will probably break, or at | |
1445 | least confuse, any code operating on the smob; the smob's @code{SCM} | |
1446 | values will have become dangling references. | |
1447 | ||
1448 | To mark an arbitrary Scheme object, the @code{mark} function may call | |
1449 | this function: | |
1450 | ||
1451 | @deftypefun void scm_gc_mark (SCM @var{x}) | |
1452 | Mark the object @var{x}, and recurse on any objects @var{x} refers to. | |
1453 | If @var{x}'s mark bit is already set, return immediately. | |
1454 | @end deftypefun | |
1455 | ||
1456 | Thus, here is how we might write the @code{mark} function for the image | |
1457 | smob type discussed above: | |
1458 | @example | |
1459 | @group | |
1460 | SCM | |
1461 | mark_image (SCM image_smob) | |
1462 | @{ | |
1463 | /* Mark the image's name and update function. */ | |
1464 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1465 | ||
1466 | scm_gc_mark (image->name); | |
1467 | scm_gc_mark (image->update_func); | |
1468 | ||
1469 | return SCM_BOOL_F; | |
1470 | @} | |
1471 | @end group | |
1472 | @end example | |
1473 | ||
1474 | Note that, even though the image's @code{update_func} could be an | |
1475 | arbitrarily complex structure (representing a procedure and any values | |
1476 | enclosed in its environment), @code{scm_gc_mark} will recurse as | |
1477 | necessary to mark all its components. Because @code{scm_gc_mark} sets | |
1478 | an object's mark bit before it recurses, it is not confused by | |
1479 | circular structures. | |
1480 | ||
1481 | As an optimization, the collector will mark whatever value is returned | |
1482 | by the @code{mark} function; this helps limit depth of recursion during | |
1483 | the mark phase. Thus, the code above could also be written as: | |
1484 | @example | |
1485 | @group | |
1486 | SCM | |
1487 | mark_image (SCM image_smob) | |
1488 | @{ | |
1489 | /* Mark the image's name and update function. */ | |
1490 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1491 | ||
1492 | scm_gc_mark (image->name); | |
1493 | return image->update_func; | |
1494 | @} | |
1495 | @end group | |
1496 | @end example | |
1497 | ||
1498 | ||
1499 | Finally, when the collector encounters an unmarked smob during the sweep | |
1500 | phase, it uses the smob's tag to find the appropriate @code{free} | |
1501 | function for the smob. It then calls the function, passing it the smob | |
1502 | as its only argument. | |
1503 | ||
1504 | The @code{free} function must release any resources used by the smob. | |
1505 | However, it need not free objects managed by the collector; the | |
1506 | collector will take care of them. The return type of the @code{free} | |
1507 | function should be @code{scm_sizet}, an unsigned integral type; the | |
1508 | @code{free} function should return the number of bytes released, to help | |
1509 | the collector maintain statistics on the size of the heap. | |
1510 | ||
1511 | Here is how we might write the @code{free} function for the image smob | |
1512 | type: | |
1513 | @example | |
1514 | scm_sizet | |
1515 | free_image (SCM image_smob) | |
1516 | @{ | |
1517 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1518 | scm_sizet size = image->width * image->height + sizeof (*image); | |
1519 | ||
1520 | free (image->pixels); | |
1521 | free (image); | |
1522 | ||
1523 | return size; | |
1524 | @} | |
1525 | @end example | |
1526 | ||
1527 | During the sweep phase, the garbage collector will clear the mark bits | |
1528 | on all live objects. The code which implements a smob need not do this | |
1529 | itself. | |
1530 | ||
1531 | There is no way for smob code to be notified when collection is | |
1532 | complete. | |
1533 | ||
1534 | Note that, since a smob's mark bit lives in its @sc{car}, along with the | |
1535 | smob's type tag, the technique for checking the type of a smob described | |
1536 | in @ref{Typechecking} will not necessarily work during GC. If you need | |
1537 | to find out whether a given object is a particular smob type during GC, | |
1538 | use the following macro: | |
1539 | ||
1540 | @deftypefn Macro void SCM_GCTYP16 (SCM @var{x}) | |
1541 | Return the type bits of the smob @var{x}, with the mark bit clear. | |
1542 | ||
1543 | Use this macro instead of @code{SCM_CAR} to check the type of a smob | |
1544 | during GC. Usually, only code called by the smob's @code{mark} function | |
1545 | need worry about this. | |
1546 | @end deftypefn | |
1547 | ||
1548 | It is usually a good idea to minimize the amount of processing done | |
1549 | during garbage collection; keep @code{mark} and @code{free} functions | |
1550 | very simple. Since collections occur at unpredictable times, it is easy | |
1551 | for any unusual activity to interfere with normal code. | |
1552 | ||
1553 | ||
1554 | @node A Common Mistake In Allocating Smobs, Garbage Collecting Simple Smobs, Garbage Collecting Smobs, Defining New Types (Smobs) | |
1555 | @subsection A Common Mistake In Allocating Smobs | |
1556 | ||
1557 | When constructing new objects, you must be careful that the garbage | |
1558 | collector can always find any new objects you allocate. For example, | |
1559 | suppose we wrote the @code{make_image} function this way: | |
1560 | ||
1561 | @example | |
1562 | SCM | |
1563 | make_image (SCM name, SCM s_width, SCM s_height) | |
1564 | @{ | |
1565 | struct image *image; | |
1566 | SCM image_smob; | |
1567 | int width, height; | |
1568 | ||
1569 | SCM_ASSERT (SCM_NIMP (name) && SCM_STRINGP (name), name, | |
1570 | SCM_ARG1, "make-image"); | |
1571 | SCM_ASSERT (SCM_INUMP (s_width), s_width, SCM_ARG2, "make-image"); | |
1572 | SCM_ASSERT (SCM_INUMP (s_height), s_height, SCM_ARG3, "make-image"); | |
1573 | ||
1574 | width = SCM_INUM (s_width); | |
1575 | height = SCM_INUM (s_height); | |
1576 | ||
1577 | image = (struct image *) scm_must_malloc (sizeof (struct image), "image"); | |
1578 | image->width = width; | |
1579 | image->height = height; | |
1580 | image->pixels = scm_must_malloc (width * height, "image pixels"); | |
1581 | ||
1582 | /* THESE TWO LINES HAVE CHANGED: */ | |
1583 | image->name = scm_string_copy (name); | |
1584 | image->update_func = scm_make_gsubr (@dots{}); | |
1585 | ||
1586 | SCM_NEWCELL (image_smob); | |
1587 | SCM_SETCDR (image_smob, image); | |
1588 | SCM_SETCAR (image_smob, image_tag); | |
1589 | ||
1590 | return image_smob; | |
1591 | @} | |
1592 | @end example | |
1593 | ||
1594 | This code is incorrect. The calls to @code{scm_string_copy} and | |
1595 | @code{scm_make_gsubr} allocate fresh objects. Allocating any new object | |
1596 | may cause the garbage collector to run. If @code{scm_make_gsubr} | |
1597 | invokes a collection, the garbage collector has no way to discover that | |
1598 | @code{image->name} points to the new string object; the @code{image} | |
1599 | structure is not yet part of any Scheme object, so the garbage collector | |
1600 | will not traverse it. Since the garbage collector cannot find any | |
1601 | references to the new string object, it will free it, leaving | |
1602 | @code{image} pointing to a dead object. | |
1603 | ||
1604 | A correct implementation might say, instead: | |
1605 | @example | |
1606 | image->name = SCM_BOOL_F; | |
1607 | image->update_func = SCM_BOOL_F; | |
1608 | ||
1609 | SCM_NEWCELL (image_smob); | |
1610 | SCM_SETCDR (image_smob, image); | |
1611 | SCM_SETCAR (image_smob, image_tag); | |
1612 | ||
1613 | image->name = scm_string_copy (name); | |
1614 | image->update_func = scm_make_gsubr (@dots{}); | |
1615 | ||
1616 | return image_smob; | |
1617 | @end example | |
1618 | ||
1619 | Now, by the time we allocate the new string and function objects, | |
1620 | @code{image_smob} points to @code{image}. If the garbage collector | |
1621 | scans the stack, it will find a reference to @code{image_smob} and | |
1622 | traverse @code{image}, so any objects @code{image} points to will be | |
1623 | preserved. | |
1624 | ||
1625 | ||
1626 | @node Garbage Collecting Simple Smobs, A Complete Example, A Common Mistake In Allocating Smobs, Defining New Types (Smobs) | |
1627 | @subsection Garbage Collecting Simple Smobs | |
1628 | ||
1629 | It is often useful to define very simple smob types --- smobs which have | |
1630 | no data to mark, other than the cell itself, or smobs whose @sc{cdr} is | |
1631 | simply an ordinary Scheme object, to be marked recursively. Guile | |
1632 | provides some functions to handle these common cases; you can use these | |
1633 | functions as your smob type's @code{mark} function, if your smob's | |
1634 | structure is simple enough. | |
1635 | ||
1636 | If the smob refers to no other Scheme objects, then no action is | |
1637 | necessary; the garbage collector has already marked the smob cell | |
1638 | itself. In that case, you can use zero as your mark function. | |
1639 | ||
1640 | @deftypefun SCM scm_markcdr (SCM @var{x}) | |
1641 | Mark the references in the smob @var{x}, assuming that @var{x}'s | |
1642 | @sc{cdr} contains an ordinary Scheme object, and @var{x} refers to no | |
1643 | other objects. This function simply returns @var{x}'s @sc{cdr}. | |
1644 | @end deftypefun | |
1645 | ||
1646 | @deftypefun scm_sizet scm_free0 (SCM @var{x}) | |
1647 | Do nothing; return zero. This function is appropriate for smobs that | |
1648 | use either zero or @code{scm_markcdr} as their marking functions, and | |
1649 | refer to no heap storage, including memory managed by @code{malloc}, | |
1650 | other than the smob's header cell. | |
1651 | @end deftypefun | |
1652 | ||
1653 | ||
1654 | @node A Complete Example | |
1655 | @subsection A Complete Example | |
1656 | ||
1657 | Here is the complete text of the implementation of the image datatype, | |
1658 | as presented in the sections above. We also provide a definition for | |
1659 | the smob's @code{print} function, and make some objects and functions | |
1660 | static, to clarify exactly what the surrounding code is using. | |
1661 | ||
1662 | As mentioned above, you can find this code in the Guile distribution, in | |
1663 | @file{doc/example-smob}. That directory includes a makefile and a | |
1664 | suitable @code{main} function, so you can build a complete interactive | |
1665 | Guile shell, extended with the datatypes described here.) | |
1666 | ||
1667 | @example | |
1668 | /* file "image-type.c" */ | |
1669 | ||
1670 | #include <stdlib.h> | |
1671 | #include <libguile.h> | |
1672 | ||
1673 | static long image_tag; | |
1674 | ||
1675 | struct image @{ | |
1676 | int width, height; | |
1677 | char *pixels; | |
1678 | ||
1679 | /* The name of this image */ | |
1680 | SCM name; | |
1681 | ||
1682 | /* A function to call when this image is | |
1683 | modified, e.g., to update the screen, | |
1684 | or SCM_BOOL_F if no action necessary */ | |
1685 | SCM update_func; | |
1686 | @}; | |
1687 | ||
1688 | static SCM | |
1689 | make_image (SCM name, SCM s_width, SCM s_height) | |
1690 | @{ | |
1691 | struct image *image; | |
1692 | SCM image_smob; | |
1693 | int width, height; | |
1694 | ||
1695 | SCM_ASSERT (SCM_NIMP (name) && SCM_STRINGP (name), name, | |
1696 | SCM_ARG1, "make-image"); | |
1697 | SCM_ASSERT (SCM_INUMP (s_width), s_width, SCM_ARG2, "make-image"); | |
1698 | SCM_ASSERT (SCM_INUMP (s_height), s_height, SCM_ARG3, "make-image"); | |
1699 | ||
1700 | width = SCM_INUM (s_width); | |
1701 | height = SCM_INUM (s_height); | |
1702 | ||
1703 | image = (struct image *) scm_must_malloc (sizeof (struct image), "image"); | |
1704 | image->width = width; | |
1705 | image->height = height; | |
1706 | image->pixels = scm_must_malloc (width * height, "image pixels"); | |
1707 | image->name = name; | |
1708 | image->update_func = SCM_BOOL_F; | |
1709 | ||
1710 | SCM_NEWSMOB (image_smob, image_tag, image); | |
1711 | ||
1712 | return image_smob; | |
1713 | @} | |
1714 | ||
1715 | static SCM | |
1716 | clear_image (SCM image_smob) | |
1717 | @{ | |
1718 | int area; | |
1719 | struct image *image; | |
1720 | ||
1721 | SCM_ASSERT (SCM_SMOB_PREDICATE (image_tag, image_smob), | |
1722 | image_smob, SCM_ARG1, "clear-image"); | |
1723 | ||
1724 | image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1725 | area = image->width * image->height; | |
1726 | memset (image->pixels, 0, area); | |
1727 | ||
1728 | /* Invoke the image's update function. */ | |
1729 | if (image->update_func != SCM_BOOL_F) | |
1730 | scm_apply (image->update_func, SCM_EOL, SCM_EOL); | |
1731 | ||
1732 | return SCM_UNSPECIFIED; | |
1733 | @} | |
1734 | ||
1735 | static SCM | |
1736 | mark_image (SCM image_smob) | |
1737 | @{ | |
1738 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1739 | ||
1740 | scm_gc_mark (image->name); | |
1741 | return image->update_func; | |
1742 | @} | |
1743 | ||
1744 | static scm_sizet | |
1745 | free_image (SCM image_smob) | |
1746 | @{ | |
1747 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1748 | scm_sizet size = image->width * image->height + sizeof (struct image); | |
1749 | ||
1750 | free (image->pixels); | |
1751 | free (image); | |
1752 | ||
1753 | return size; | |
1754 | @} | |
1755 | ||
1756 | static int | |
1757 | print_image (SCM image_smob, SCM port, scm_print_state *pstate) | |
1758 | @{ | |
1759 | struct image *image = (struct image *) SCM_SMOB_DATA (image_smob); | |
1760 | ||
1761 | scm_puts ("#<image ", port); | |
1762 | scm_display (image->name, port); | |
1763 | scm_puts (">", port); | |
1764 | ||
1765 | /* non-zero means success */ | |
1766 | return 1; | |
1767 | @} | |
1768 | ||
1769 | static scm_smobfuns image_funs = @{ | |
1770 | mark_image, free_image, print_image, 0 | |
1771 | @}; | |
1772 | ||
1773 | void | |
1774 | init_image_type () | |
1775 | @{ | |
1776 | image_tag = scm_newsmob (&image_funs); | |
1777 | ||
1778 | scm_make_gsubr ("clear-image", 1, 0, 0, clear_image); | |
1779 | scm_make_gsubr ("make-image", 3, 0, 0, make_image); | |
1780 | @} | |
1781 | @end example | |
1782 | ||
1783 | Here is a sample build and interaction with the code from the | |
1784 | @file{example-smob} directory, on the author's machine: | |
1785 | ||
1786 | @example | |
1787 | zwingli:example-smob$ make CC=gcc | |
1788 | gcc `guile-config compile` -c image-type.c -o image-type.o | |
1789 | gcc `guile-config compile` -c myguile.c -o myguile.o | |
1790 | gcc image-type.o myguile.o `guile-config link` -o myguile | |
1791 | zwingli:example-smob$ ./myguile | |
1792 | guile> make-image | |
1793 | #<primitive-procedure make-image> | |
1794 | guile> (define i (make-image "Whistler's Mother" 100 100)) | |
1795 | guile> i | |
1796 | #<image Whistler's Mother> | |
1797 | guile> (clear-image i) | |
1798 | guile> (clear-image 4) | |
1799 | ERROR: In procedure clear-image in expression (clear-image 4): | |
1800 | ERROR: Wrong type argument in position 1: 4 | |
1801 | ABORT: (wrong-type-arg) | |
1802 | ||
1803 | Type "(backtrace)" to get more information. | |
1804 | guile> | |
1805 | @end example | |
1806 | ||
1807 | @c essay @bye |