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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Guile Reference Manual. | |
3 | @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004 | |
4 | @c Free Software Foundation, Inc. | |
5 | @c See the file guile.texi for copying conditions. | |
6 | ||
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7 | @c essay \input texinfo |
8 | @c essay @c -*-texinfo-*- | |
9 | @c essay @c %**start of header | |
10 | @c essay @setfilename data-rep.info | |
11 | @c essay @settitle Data Representation in Guile | |
12 | @c essay @c %**end of header | |
13 | ||
14 | @c essay @include version.texi | |
15 | ||
16 | @c essay @dircategory The Algorithmic Language Scheme | |
17 | @c essay @direntry | |
18 | @c essay * data-rep: (data-rep). Data Representation in Guile --- how to use | |
12e5078c | 19 | @c essay Guile objects in your C code. |
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20 | @c essay @end direntry |
21 | ||
22 | @c essay @setchapternewpage off | |
23 | ||
24 | @c essay @ifinfo | |
25 | @c essay Data Representation in Guile | |
26 | ||
3446b6ef | 27 | @c essay Copyright (C) 1998, 1999, 2000, 2003 Free Software Foundation |
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28 | |
29 | @c essay Permission is granted to make and distribute verbatim copies of | |
30 | @c essay this manual provided the copyright notice and this permission notice | |
31 | @c essay are preserved on all copies. | |
32 | ||
33 | @c essay @ignore | |
34 | @c essay Permission is granted to process this file through TeX and print the | |
35 | @c essay results, provided the printed document carries copying permission | |
36 | @c essay notice identical to this one except for the removal of this paragraph | |
37 | @c essay (this paragraph not being relevant to the printed manual). | |
38 | @c essay @end ignore | |
39 | ||
40 | @c essay Permission is granted to copy and distribute modified versions of this | |
41 | @c essay manual under the conditions for verbatim copying, provided that the entire | |
42 | @c essay resulting derived work is distributed under the terms of a permission | |
43 | @c essay notice identical to this one. | |
44 | ||
45 | @c essay Permission is granted to copy and distribute translations of this manual | |
46 | @c essay into another language, under the above conditions for modified versions, | |
47 | @c essay except that this permission notice may be stated in a translation approved | |
48 | @c essay by the Free Software Foundation. | |
49 | @c essay @end ifinfo | |
50 | ||
51 | @c essay @titlepage | |
52 | @c essay @sp 10 | |
53 | @c essay @comment The title is printed in a large font. | |
54 | @c essay @title Data Representation in Guile | |
3229f68b | 55 | @c essay @subtitle $Id: data-rep.texi,v 1.17 2004-04-21 14:32:08 mvo Exp $ |
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56 | @c essay @subtitle For use with Guile @value{VERSION} |
57 | @c essay @author Jim Blandy | |
58 | @c essay @author Free Software Foundation | |
59 | @c essay @author @email{jimb@@red-bean.com} | |
60 | @c essay @c The following two commands start the copyright page. | |
61 | @c essay @page | |
62 | @c essay @vskip 0pt plus 1filll | |
63 | @c essay @vskip 0pt plus 1filll | |
64 | @c essay Copyright @copyright{} 1998 Free Software Foundation | |
65 | ||
66 | @c essay Permission is granted to make and distribute verbatim copies of | |
67 | @c essay this manual provided the copyright notice and this permission notice | |
68 | @c essay are preserved on all copies. | |
69 | ||
70 | @c essay Permission is granted to copy and distribute modified versions of this | |
71 | @c essay manual under the conditions for verbatim copying, provided that the entire | |
72 | @c essay resulting derived work is distributed under the terms of a permission | |
73 | @c essay notice identical to this one. | |
74 | ||
75 | @c essay Permission is granted to copy and distribute translations of this manual | |
76 | @c essay into another language, under the above conditions for modified versions, | |
77 | @c essay except that this permission notice may be stated in a translation approved | |
78 | @c essay by Free Software Foundation. | |
79 | @c essay @end titlepage | |
80 | ||
81 | @c essay @c @smallbook | |
82 | @c essay @c @finalout | |
83 | @c essay @headings double | |
84 | ||
85 | ||
86 | @c essay @node Top, Data Representation in Scheme, (dir), (dir) | |
87 | @c essay @top Data Representation in Guile | |
88 | ||
89 | @c essay @ifinfo | |
90 | @c essay This essay is meant to provide the background necessary to read and | |
91 | @c essay write C code that manipulates Scheme values in a way that conforms to | |
92 | @c essay libguile's interface. If you would like to write or maintain a | |
93 | @c essay Guile-based application in C or C++, this is the first information you | |
94 | @c essay need. | |
95 | ||
96 | @c essay In order to make sense of Guile's @code{SCM_} functions, or read | |
97 | @c essay libguile's source code, it's essential to have a good grasp of how Guile | |
98 | @c essay actually represents Scheme values. Otherwise, a lot of the code, and | |
99 | @c essay the conventions it follows, won't make very much sense. | |
100 | ||
101 | @c essay We assume you know both C and Scheme, but we do not assume you are | |
102 | @c essay familiar with Guile's C interface. | |
103 | @c essay @end ifinfo | |
104 | ||
105 | ||
38a93523 | 106 | @node Data Representation |
3229f68b | 107 | @appendix Data Representation in Guile |
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108 | |
109 | @strong{by Jim Blandy} | |
110 | ||
111 | [Due to the rather non-orthogonal and performance-oriented nature of the | |
112 | SCM interface, you need to understand SCM internals *before* you can use | |
113 | the SCM API. That's why this chapter comes first.] | |
114 | ||
115 | [NOTE: this is Jim Blandy's essay almost entirely unmodified. It has to | |
116 | be adapted to fit this manual smoothly.] | |
117 | ||
118 | In order to make sense of Guile's SCM_ functions, or read libguile's | |
119 | source code, it's essential to have a good grasp of how Guile actually | |
120 | represents Scheme values. Otherwise, a lot of the code, and the | |
121 | conventions it follows, won't make very much sense. This essay is meant | |
122 | to provide the background necessary to read and write C code that | |
123 | manipulates Scheme values in a way that is compatible with libguile. | |
124 | ||
125 | We assume you know both C and Scheme, but we do not assume you are | |
126 | familiar with Guile's implementation. | |
127 | ||
128 | @menu | |
129 | * Data Representation in Scheme:: Why things aren't just totally | |
130 | straightforward, in general terms. | |
131 | * How Guile does it:: How to write C code that manipulates | |
132 | Guile values, with an explanation | |
133 | of Guile's garbage collector. | |
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134 | @end menu |
135 | ||
136 | @node Data Representation in Scheme | |
137 | @section Data Representation in Scheme | |
138 | ||
139 | Scheme is a latently-typed language; this means that the system cannot, | |
140 | in general, determine the type of a given expression at compile time. | |
141 | Types only become apparent at run time. Variables do not have fixed | |
142 | types; a variable may hold a pair at one point, an integer at the next, | |
143 | and a thousand-element vector later. Instead, values, not variables, | |
144 | have fixed types. | |
145 | ||
146 | In order to implement standard Scheme functions like @code{pair?} and | |
147 | @code{string?} and provide garbage collection, the representation of | |
148 | every value must contain enough information to accurately determine its | |
149 | type at run time. Often, Scheme systems also use this information to | |
150 | determine whether a program has attempted to apply an operation to an | |
151 | inappropriately typed value (such as taking the @code{car} of a string). | |
152 | ||
153 | Because variables, pairs, and vectors may hold values of any type, | |
154 | Scheme implementations use a uniform representation for values --- a | |
155 | single type large enough to hold either a complete value or a pointer | |
156 | to a complete value, along with the necessary typing information. | |
157 | ||
158 | The following sections will present a simple typing system, and then | |
159 | make some refinements to correct its major weaknesses. However, this is | |
160 | not a description of the system Guile actually uses. It is only an | |
161 | illustration of the issues Guile's system must address. We provide all | |
162 | the information one needs to work with Guile's data in @ref{How Guile | |
163 | does it}. | |
164 | ||
165 | ||
166 | @menu | |
167 | * A Simple Representation:: | |
168 | * Faster Integers:: | |
169 | * Cheaper Pairs:: | |
170 | * Guile Is Hairier:: | |
171 | @end menu | |
172 | ||
173 | @node A Simple Representation | |
174 | @subsection A Simple Representation | |
175 | ||
176 | The simplest way to meet the above requirements in C would be to | |
177 | represent each value as a pointer to a structure containing a type | |
178 | indicator, followed by a union carrying the real value. Assuming that | |
179 | @code{SCM} is the name of our universal type, we can write: | |
180 | ||
181 | @example | |
182 | enum type @{ integer, pair, string, vector, ... @}; | |
183 | ||
184 | typedef struct value *SCM; | |
185 | ||
186 | struct value @{ | |
187 | enum type type; | |
188 | union @{ | |
189 | int integer; | |
190 | struct @{ SCM car, cdr; @} pair; | |
191 | struct @{ int length; char *elts; @} string; | |
192 | struct @{ int length; SCM *elts; @} vector; | |
193 | ... | |
194 | @} value; | |
195 | @}; | |
196 | @end example | |
197 | with the ellipses replaced with code for the remaining Scheme types. | |
198 | ||
199 | This representation is sufficient to implement all of Scheme's | |
200 | semantics. If @var{x} is an @code{SCM} value: | |
201 | @itemize @bullet | |
202 | @item | |
203 | To test if @var{x} is an integer, we can write @code{@var{x}->type == integer}. | |
204 | @item | |
205 | To find its value, we can write @code{@var{x}->value.integer}. | |
206 | @item | |
207 | To test if @var{x} is a vector, we can write @code{@var{x}->type == vector}. | |
208 | @item | |
209 | If we know @var{x} is a vector, we can write | |
210 | @code{@var{x}->value.vector.elts[0]} to refer to its first element. | |
211 | @item | |
212 | If we know @var{x} is a pair, we can write | |
213 | @code{@var{x}->value.pair.car} to extract its car. | |
214 | @end itemize | |
215 | ||
216 | ||
217 | @node Faster Integers | |
218 | @subsection Faster Integers | |
219 | ||
220 | Unfortunately, the above representation has a serious disadvantage. In | |
221 | order to return an integer, an expression must allocate a @code{struct | |
222 | value}, initialize it to represent that integer, and return a pointer to | |
223 | it. Furthermore, fetching an integer's value requires a memory | |
224 | reference, which is much slower than a register reference on most | |
225 | processors. Since integers are extremely common, this representation is | |
226 | too costly, in both time and space. Integers should be very cheap to | |
227 | create and manipulate. | |
228 | ||
229 | One possible solution comes from the observation that, on many | |
230 | architectures, structures must be aligned on a four-byte boundary. | |
231 | (Whether or not the machine actually requires it, we can write our own | |
232 | allocator for @code{struct value} objects that assures this is true.) | |
233 | In this case, the lower two bits of the structure's address are known to | |
234 | be zero. | |
235 | ||
236 | This gives us the room we need to provide an improved representation | |
237 | for integers. We make the following rules: | |
238 | @itemize @bullet | |
239 | @item | |
240 | If the lower two bits of an @code{SCM} value are zero, then the SCM | |
241 | value is a pointer to a @code{struct value}, and everything proceeds as | |
242 | before. | |
243 | @item | |
244 | Otherwise, the @code{SCM} value represents an integer, whose value | |
245 | appears in its upper bits. | |
246 | @end itemize | |
247 | ||
248 | Here is C code implementing this convention: | |
249 | @example | |
250 | enum type @{ pair, string, vector, ... @}; | |
251 | ||
252 | typedef struct value *SCM; | |
253 | ||
254 | struct value @{ | |
255 | enum type type; | |
256 | union @{ | |
257 | struct @{ SCM car, cdr; @} pair; | |
258 | struct @{ int length; char *elts; @} string; | |
259 | struct @{ int length; SCM *elts; @} vector; | |
260 | ... | |
261 | @} value; | |
262 | @}; | |
263 | ||
264 | #define POINTER_P(x) (((int) (x) & 3) == 0) | |
265 | #define INTEGER_P(x) (! POINTER_P (x)) | |
266 | ||
267 | #define GET_INTEGER(x) ((int) (x) >> 2) | |
268 | #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1)) | |
269 | @end example | |
270 | ||
271 | Notice that @code{integer} no longer appears as an element of @code{enum | |
272 | type}, and the union has lost its @code{integer} member. Instead, we | |
273 | use the @code{POINTER_P} and @code{INTEGER_P} macros to make a coarse | |
274 | classification of values into integers and non-integers, and do further | |
275 | type testing as before. | |
276 | ||
277 | Here's how we would answer the questions posed above (again, assume | |
278 | @var{x} is an @code{SCM} value): | |
279 | @itemize @bullet | |
280 | @item | |
281 | To test if @var{x} is an integer, we can write @code{INTEGER_P (@var{x})}. | |
282 | @item | |
283 | To find its value, we can write @code{GET_INTEGER (@var{x})}. | |
284 | @item | |
285 | To test if @var{x} is a vector, we can write: | |
286 | @example | |
287 | @code{POINTER_P (@var{x}) && @var{x}->type == vector} | |
288 | @end example | |
289 | Given the new representation, we must make sure @var{x} is truly a | |
290 | pointer before we dereference it to determine its complete type. | |
291 | @item | |
292 | If we know @var{x} is a vector, we can write | |
293 | @code{@var{x}->value.vector.elts[0]} to refer to its first element, as | |
294 | before. | |
295 | @item | |
296 | If we know @var{x} is a pair, we can write | |
297 | @code{@var{x}->value.pair.car} to extract its car, just as before. | |
298 | @end itemize | |
299 | ||
300 | This representation allows us to operate more efficiently on integers | |
301 | than the first. For example, if @var{x} and @var{y} are known to be | |
302 | integers, we can compute their sum as follows: | |
303 | @example | |
304 | MAKE_INTEGER (GET_INTEGER (@var{x}) + GET_INTEGER (@var{y})) | |
305 | @end example | |
306 | Now, integer math requires no allocation or memory references. Most | |
307 | real Scheme systems actually use an even more efficient representation, | |
308 | but this essay isn't about bit-twiddling. (Hint: what if pointers had | |
309 | @code{01} in their least significant bits, and integers had @code{00}?) | |
310 | ||
311 | ||
312 | @node Cheaper Pairs | |
313 | @subsection Cheaper Pairs | |
314 | ||
315 | However, there is yet another issue to confront. Most Scheme heaps | |
316 | contain more pairs than any other type of object; Jonathan Rees says | |
317 | that pairs occupy 45% of the heap in his Scheme implementation, Scheme | |
318 | 48. However, our representation above spends three @code{SCM}-sized | |
319 | words per pair --- one for the type, and two for the @sc{car} and | |
320 | @sc{cdr}. Is there any way to represent pairs using only two words? | |
321 | ||
322 | Let us refine the convention we established earlier. Let us assert | |
323 | that: | |
324 | @itemize @bullet | |
325 | @item | |
326 | If the bottom two bits of an @code{SCM} value are @code{#b00}, then | |
327 | it is a pointer, as before. | |
328 | @item | |
329 | If the bottom two bits are @code{#b01}, then the upper bits are an | |
330 | integer. This is a bit more restrictive than before. | |
331 | @item | |
332 | If the bottom two bits are @code{#b10}, then the value, with the bottom | |
333 | two bits masked out, is the address of a pair. | |
334 | @end itemize | |
335 | ||
336 | Here is the new C code: | |
337 | @example | |
338 | enum type @{ string, vector, ... @}; | |
339 | ||
340 | typedef struct value *SCM; | |
341 | ||
342 | struct value @{ | |
343 | enum type type; | |
344 | union @{ | |
345 | struct @{ int length; char *elts; @} string; | |
346 | struct @{ int length; SCM *elts; @} vector; | |
347 | ... | |
348 | @} value; | |
349 | @}; | |
350 | ||
351 | struct pair @{ | |
352 | SCM car, cdr; | |
353 | @}; | |
354 | ||
355 | #define POINTER_P(x) (((int) (x) & 3) == 0) | |
356 | ||
357 | #define INTEGER_P(x) (((int) (x) & 3) == 1) | |
358 | #define GET_INTEGER(x) ((int) (x) >> 2) | |
359 | #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1)) | |
360 | ||
361 | #define PAIR_P(x) (((int) (x) & 3) == 2) | |
362 | #define GET_PAIR(x) ((struct pair *) ((int) (x) & ~3)) | |
363 | @end example | |
364 | ||
365 | Notice that @code{enum type} and @code{struct value} now only contain | |
366 | provisions for vectors and strings; both integers and pairs have become | |
367 | special cases. The code above also assumes that an @code{int} is large | |
368 | enough to hold a pointer, which isn't generally true. | |
369 | ||
370 | ||
371 | Our list of examples is now as follows: | |
372 | @itemize @bullet | |
373 | @item | |
374 | To test if @var{x} is an integer, we can write @code{INTEGER_P | |
375 | (@var{x})}; this is as before. | |
376 | @item | |
377 | To find its value, we can write @code{GET_INTEGER (@var{x})}, as | |
378 | before. | |
379 | @item | |
380 | To test if @var{x} is a vector, we can write: | |
381 | @example | |
382 | @code{POINTER_P (@var{x}) && @var{x}->type == vector} | |
383 | @end example | |
384 | We must still make sure that @var{x} is a pointer to a @code{struct | |
385 | value} before dereferencing it to find its type. | |
386 | @item | |
387 | If we know @var{x} is a vector, we can write | |
388 | @code{@var{x}->value.vector.elts[0]} to refer to its first element, as | |
389 | before. | |
390 | @item | |
391 | We can write @code{PAIR_P (@var{x})} to determine if @var{x} is a | |
392 | pair, and then write @code{GET_PAIR (@var{x})->car} to refer to its | |
393 | car. | |
394 | @end itemize | |
395 | ||
396 | This change in representation reduces our heap size by 15%. It also | |
397 | makes it cheaper to decide if a value is a pair, because no memory | |
398 | references are necessary; it suffices to check the bottom two bits of | |
399 | the @code{SCM} value. This may be significant when traversing lists, a | |
400 | common activity in a Scheme system. | |
401 | ||
85a9b4ed | 402 | Again, most real Scheme systems use a slightly different implementation; |
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403 | for example, if GET_PAIR subtracts off the low bits of @code{x}, instead |
404 | of masking them off, the optimizer will often be able to combine that | |
405 | subtraction with the addition of the offset of the structure member we | |
406 | are referencing, making a modified pointer as fast to use as an | |
407 | unmodified pointer. | |
408 | ||
409 | ||
410 | @node Guile Is Hairier | |
411 | @subsection Guile Is Hairier | |
412 | ||
413 | We originally started with a very simple typing system --- each object | |
414 | has a field that indicates its type. Then, for the sake of efficiency | |
415 | in both time and space, we moved some of the typing information directly | |
416 | into the @code{SCM} value, and left the rest in the @code{struct value}. | |
417 | Guile itself employs a more complex hierarchy, storing finer and finer | |
418 | gradations of type information in different places, depending on the | |
419 | object's coarser type. | |
420 | ||
421 | In the author's opinion, Guile could be simplified greatly without | |
422 | significant loss of efficiency, but the simplified system would still be | |
423 | more complex than what we've presented above. | |
424 | ||
425 | ||
426 | @node How Guile does it | |
427 | @section How Guile does it | |
428 | ||
429 | Here we present the specifics of how Guile represents its data. We | |
430 | don't go into complete detail; an exhaustive description of Guile's | |
431 | system would be boring, and we do not wish to encourage people to write | |
432 | code which depends on its details anyway. We do, however, present | |
433 | everything one need know to use Guile's data. | |
434 | ||
435 | ||
436 | @menu | |
437 | * General Rules:: | |
438 | * Conservative GC:: | |
abaec75d | 439 | * Immediates vs Non-immediates:: |
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440 | * Immediate Datatypes:: |
441 | * Non-immediate Datatypes:: | |
442 | * Signalling Type Errors:: | |
505392ae | 443 | * Unpacking the SCM type:: |
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444 | @end menu |
445 | ||
446 | @node General Rules | |
447 | @subsection General Rules | |
448 | ||
449 | Any code which operates on Guile datatypes must @code{#include} the | |
450 | header file @code{<libguile.h>}. This file contains a definition for | |
451 | the @code{SCM} typedef (Guile's universal type, as in the examples | |
452 | above), and definitions and declarations for a host of macros and | |
453 | functions that operate on @code{SCM} values. | |
454 | ||
455 | All identifiers declared by @code{<libguile.h>} begin with @code{scm_} | |
456 | or @code{SCM_}. | |
457 | ||
458 | @c [[I wish this were true, but I don't think it is at the moment. -JimB]] | |
459 | @c Macros do not evaluate their arguments more than once, unless documented | |
460 | @c to do so. | |
461 | ||
462 | The functions described here generally check the types of their | |
463 | @code{SCM} arguments, and signal an error if their arguments are of an | |
464 | inappropriate type. Macros generally do not, unless that is their | |
465 | specified purpose. You must verify their argument types beforehand, as | |
466 | necessary. | |
467 | ||
468 | Macros and functions that return a boolean value have names ending in | |
469 | @code{P} or @code{_p} (for ``predicate''). Those that return a negated | |
470 | boolean value have names starting with @code{SCM_N}. For example, | |
471 | @code{SCM_IMP (@var{x})} is a predicate which returns non-zero iff | |
472 | @var{x} is an immediate value (an @code{IM}). @code{SCM_NCONSP | |
473 | (@var{x})} is a predicate which returns non-zero iff @var{x} is | |
474 | @emph{not} a pair object (a @code{CONS}). | |
475 | ||
476 | ||
477 | @node Conservative GC | |
478 | @subsection Conservative Garbage Collection | |
479 | ||
480 | Aside from the latent typing, the major source of constraints on a | |
481 | Scheme implementation's data representation is the garbage collector. | |
482 | The collector must be able to traverse every live object in the heap, to | |
483 | determine which objects are not live. | |
484 | ||
485 | There are many ways to implement this, but Guile uses an algorithm | |
486 | called @dfn{mark and sweep}. The collector scans the system's global | |
487 | variables and the local variables on the stack to determine which | |
488 | objects are immediately accessible by the C code. It then scans those | |
489 | objects to find the objects they point to, @i{et cetera}. The collector | |
490 | sets a @dfn{mark bit} on each object it finds, so each object is | |
491 | traversed only once. This process is called @dfn{tracing}. | |
492 | ||
493 | When the collector can find no unmarked objects pointed to by marked | |
494 | objects, it assumes that any objects that are still unmarked will never | |
495 | be used by the program (since there is no path of dereferences from any | |
496 | global or local variable that reaches them) and deallocates them. | |
497 | ||
498 | In the above paragraphs, we did not specify how the garbage collector | |
499 | finds the global and local variables; as usual, there are many different | |
500 | approaches. Frequently, the programmer must maintain a list of pointers | |
501 | to all global variables that refer to the heap, and another list | |
502 | (adjusted upon entry to and exit from each function) of local variables, | |
503 | for the collector's benefit. | |
504 | ||
505 | The list of global variables is usually not too difficult to maintain, | |
506 | since global variables are relatively rare. However, an explicitly | |
507 | maintained list of local variables (in the author's personal experience) | |
508 | is a nightmare to maintain. Thus, Guile uses a technique called | |
509 | @dfn{conservative garbage collection}, to make the local variable list | |
510 | unnecessary. | |
511 | ||
512 | The trick to conservative collection is to treat the stack as an | |
513 | ordinary range of memory, and assume that @emph{every} word on the stack | |
514 | is a pointer into the heap. Thus, the collector marks all objects whose | |
515 | addresses appear anywhere in the stack, without knowing for sure how | |
516 | that word is meant to be interpreted. | |
517 | ||
518 | Obviously, such a system will occasionally retain objects that are | |
519 | actually garbage, and should be freed. In practice, this is not a | |
520 | problem. The alternative, an explicitly maintained list of local | |
521 | variable addresses, is effectively much less reliable, due to programmer | |
522 | error. | |
523 | ||
524 | To accommodate this technique, data must be represented so that the | |
525 | collector can accurately determine whether a given stack word is a | |
526 | pointer or not. Guile does this as follows: | |
38a93523 | 527 | |
505392ae | 528 | @itemize @bullet |
38a93523 NJ |
529 | @item |
530 | Every heap object has a two-word header, called a @dfn{cell}. Some | |
531 | objects, like pairs, fit entirely in a cell's two words; others may | |
532 | store pointers to additional memory in either of the words. For | |
533 | example, strings and vectors store their length in the first word, and a | |
534 | pointer to their elements in the second. | |
535 | ||
536 | @item | |
537 | Guile allocates whole arrays of cells at a time, called @dfn{heap | |
538 | segments}. These segments are always allocated so that the cells they | |
539 | contain fall on eight-byte boundaries, or whatever is appropriate for | |
540 | the machine's word size. Guile keeps all cells in a heap segment | |
541 | initialized, whether or not they are currently in use. | |
542 | ||
543 | @item | |
544 | Guile maintains a sorted table of heap segments. | |
38a93523 NJ |
545 | @end itemize |
546 | ||
547 | Thus, given any random word @var{w} fetched from the stack, Guile's | |
548 | garbage collector can consult the table to see if @var{w} falls within a | |
549 | known heap segment, and check @var{w}'s alignment. If both tests pass, | |
550 | the collector knows that @var{w} is a valid pointer to a cell, | |
551 | intentional or not, and proceeds to trace the cell. | |
552 | ||
553 | Note that heap segments do not contain all the data Guile uses; cells | |
554 | for objects like vectors and strings contain pointers to other memory | |
555 | areas. However, since those pointers are internal, and not shared among | |
556 | many pieces of code, it is enough for the collector to find the cell, | |
557 | and then use the cell's type to find more pointers to trace. | |
558 | ||
559 | ||
abaec75d NJ |
560 | @node Immediates vs Non-immediates |
561 | @subsection Immediates vs Non-immediates | |
38a93523 NJ |
562 | |
563 | Guile classifies Scheme objects into two kinds: those that fit entirely | |
564 | within an @code{SCM}, and those that require heap storage. | |
565 | ||
566 | The former class are called @dfn{immediates}. The class of immediates | |
567 | includes small integers, characters, boolean values, the empty list, the | |
568 | mysterious end-of-file object, and some others. | |
569 | ||
85a9b4ed | 570 | The remaining types are called, not surprisingly, @dfn{non-immediates}. |
38a93523 NJ |
571 | They include pairs, procedures, strings, vectors, and all other data |
572 | types in Guile. | |
573 | ||
574 | @deftypefn Macro int SCM_IMP (SCM @var{x}) | |
575 | Return non-zero iff @var{x} is an immediate object. | |
576 | @end deftypefn | |
577 | ||
578 | @deftypefn Macro int SCM_NIMP (SCM @var{x}) | |
579 | Return non-zero iff @var{x} is a non-immediate object. This is the | |
580 | exact complement of @code{SCM_IMP}, above. | |
38a93523 NJ |
581 | @end deftypefn |
582 | ||
ffda6093 | 583 | Note that for versions of Guile prior to 1.4 it was necessary to use the |
abaec75d NJ |
584 | @code{SCM_NIMP} macro before calling a finer-grained predicate to |
585 | determine @var{x}'s type, such as @code{SCM_CONSP} or | |
ffda6093 NJ |
586 | @code{SCM_VECTORP}. This is no longer required: the definitions of all |
587 | Guile type predicates now include a call to @code{SCM_NIMP} where | |
588 | necessary. | |
abaec75d | 589 | |
38a93523 NJ |
590 | |
591 | @node Immediate Datatypes | |
592 | @subsection Immediate Datatypes | |
593 | ||
594 | The following datatypes are immediate values; that is, they fit entirely | |
595 | within an @code{SCM} value. The @code{SCM_IMP} and @code{SCM_NIMP} | |
596 | macros will distinguish these from non-immediates; see @ref{Immediates | |
abaec75d | 597 | vs Non-immediates} for an explanation of the distinction. |
38a93523 NJ |
598 | |
599 | Note that the type predicates for immediate values work correctly on any | |
600 | @code{SCM} value; you do not need to call @code{SCM_IMP} first, to | |
505392ae | 601 | establish that a value is immediate. |
38a93523 NJ |
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 | |
cee2ed4f | 736 | specific storage location (in the nomenclature of the Revised^5 Report |
38a93523 NJ |
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 | ||
505392ae NJ |
750 | Note how the type information for a non-immediate object is split |
751 | between the @code{SCM} word and the cell that the @code{SCM} word points | |
752 | to. The @code{SCM} word itself only indicates that the object is | |
753 | non-immediate --- in other words stored in a heap cell. The tag stored | |
754 | in the first word of the heap cell indicates more precisely the type of | |
755 | that object. | |
756 | ||
ffda6093 NJ |
757 | The type predicates for non-immediate values work correctly on any |
758 | @code{SCM} value; you do not need to call @code{SCM_NIMP} first, to | |
759 | establish that a value is non-immediate. | |
38a93523 NJ |
760 | |
761 | @menu | |
38a93523 NJ |
762 | * Pair Data:: |
763 | * Vector Data:: | |
764 | * Procedures:: | |
765 | * Closures:: | |
766 | * Subrs:: | |
767 | * Port Data:: | |
768 | @end menu | |
769 | ||
38a93523 NJ |
770 | |
771 | @node Pair Data | |
772 | @subsubsection Pairs | |
773 | ||
774 | Pairs are the essential building block of list structure in Scheme. A | |
775 | pair object has two fields, called the @dfn{car} and the @dfn{cdr}. | |
776 | ||
777 | It is conventional for a pair's @sc{car} to contain an element of a | |
778 | list, and the @sc{cdr} to point to the next pair in the list, or to | |
779 | contain @code{SCM_EOL}, indicating the end of the list. Thus, a set of | |
780 | pairs chained through their @sc{cdr}s constitutes a singly-linked list. | |
781 | Scheme and libguile define many functions which operate on lists | |
782 | constructed in this fashion, so although lists chained through the | |
783 | @sc{car}s of pairs will work fine too, they may be less convenient to | |
784 | manipulate, and receive less support from the community. | |
785 | ||
786 | Guile implements pairs by mapping the @sc{car} and @sc{cdr} of a pair | |
787 | directly into the two words of the cell. | |
788 | ||
789 | ||
790 | @deftypefn Macro int SCM_CONSP (SCM @var{x}) | |
791 | Return non-zero iff @var{x} is a Scheme pair object. | |
38a93523 NJ |
792 | @end deftypefn |
793 | ||
794 | @deftypefn Macro int SCM_NCONSP (SCM @var{x}) | |
795 | The complement of SCM_CONSP. | |
796 | @end deftypefn | |
797 | ||
38a93523 NJ |
798 | @deftypefun SCM scm_cons (SCM @var{car}, SCM @var{cdr}) |
799 | Allocate (``CONStruct'') a new pair, with @var{car} and @var{cdr} as its | |
800 | contents. | |
801 | @end deftypefun | |
802 | ||
85a9b4ed | 803 | The macros below perform no type checking. The results are undefined if |
38a93523 NJ |
804 | @var{cell} is an immediate. However, since all non-immediate Guile |
805 | objects are constructed from cells, and these macros simply return the | |
806 | first element of a cell, they actually can be useful on datatypes other | |
807 | than pairs. (Of course, it is not very modular to use them outside of | |
808 | the code which implements that datatype.) | |
809 | ||
810 | @deftypefn Macro SCM SCM_CAR (SCM @var{cell}) | |
811 | Return the @sc{car}, or first field, of @var{cell}. | |
812 | @end deftypefn | |
813 | ||
814 | @deftypefn Macro SCM SCM_CDR (SCM @var{cell}) | |
815 | Return the @sc{cdr}, or second field, of @var{cell}. | |
816 | @end deftypefn | |
817 | ||
818 | @deftypefn Macro void SCM_SETCAR (SCM @var{cell}, SCM @var{x}) | |
819 | Set the @sc{car} of @var{cell} to @var{x}. | |
820 | @end deftypefn | |
821 | ||
822 | @deftypefn Macro void SCM_SETCDR (SCM @var{cell}, SCM @var{x}) | |
823 | Set the @sc{cdr} of @var{cell} to @var{x}. | |
824 | @end deftypefn | |
825 | ||
826 | @deftypefn Macro SCM SCM_CAAR (SCM @var{cell}) | |
827 | @deftypefnx Macro SCM SCM_CADR (SCM @var{cell}) | |
828 | @deftypefnx Macro SCM SCM_CDAR (SCM @var{cell}) @dots{} | |
829 | @deftypefnx Macro SCM SCM_CDDDDR (SCM @var{cell}) | |
830 | Return the @sc{car} of the @sc{car} of @var{cell}, the @sc{car} of the | |
831 | @sc{cdr} of @var{cell}, @i{et cetera}. | |
832 | @end deftypefn | |
833 | ||
834 | ||
835 | @node Vector Data | |
836 | @subsubsection Vectors, Strings, and Symbols | |
837 | ||
838 | Vectors, strings, and symbols have some properties in common. They all | |
839 | have a length, and they all have an array of elements. In the case of a | |
840 | vector, the elements are @code{SCM} values; in the case of a string or | |
841 | symbol, the elements are characters. | |
842 | ||
843 | All these types store their length (along with some tagging bits) in the | |
844 | @sc{car} of their header cell, and store a pointer to the elements in | |
845 | their @sc{cdr}. Thus, the @code{SCM_CAR} and @code{SCM_CDR} macros | |
846 | are (somewhat) meaningful when applied to these datatypes. | |
847 | ||
848 | @deftypefn Macro int SCM_VECTORP (SCM @var{x}) | |
849 | Return non-zero iff @var{x} is a vector. | |
38a93523 NJ |
850 | @end deftypefn |
851 | ||
852 | @deftypefn Macro int SCM_STRINGP (SCM @var{x}) | |
853 | Return non-zero iff @var{x} is a string. | |
38a93523 NJ |
854 | @end deftypefn |
855 | ||
856 | @deftypefn Macro int SCM_SYMBOLP (SCM @var{x}) | |
857 | Return non-zero iff @var{x} is a symbol. | |
38a93523 NJ |
858 | @end deftypefn |
859 | ||
cee2ed4f MG |
860 | @deftypefn Macro int SCM_VECTOR_LENGTH (SCM @var{x}) |
861 | @deftypefnx Macro int SCM_STRING_LENGTH (SCM @var{x}) | |
862 | @deftypefnx Macro int SCM_SYMBOL_LENGTH (SCM @var{x}) | |
863 | Return the length of the object @var{x}. The result is undefined if | |
864 | @var{x} is not a vector, string, or symbol, respectively. | |
38a93523 NJ |
865 | @end deftypefn |
866 | ||
cee2ed4f | 867 | @deftypefn Macro {SCM *} SCM_VECTOR_BASE (SCM @var{x}) |
38a93523 | 868 | Return a pointer to the array of elements of the vector @var{x}. |
505392ae | 869 | The result is undefined if @var{x} is not a vector. |
38a93523 NJ |
870 | @end deftypefn |
871 | ||
cee2ed4f MG |
872 | @deftypefn Macro {char *} SCM_STRING_CHARS (SCM @var{x}) |
873 | @deftypefnx Macro {char *} SCM_SYMBOL_CHARS (SCM @var{x}) | |
874 | Return a pointer to the characters of @var{x}. The result is undefined | |
875 | if @var{x} is not a symbol or string, respectively. | |
38a93523 NJ |
876 | @end deftypefn |
877 | ||
878 | There are also a few magic values stuffed into memory before a symbol's | |
879 | characters, but you don't want to know about those. What cruft! | |
880 | ||
cf4e2dab KR |
881 | Note that @code{SCM_VECTOR_BASE}, @code{SCM_STRING_CHARS} and |
882 | @code{SCM_SYMBOL_CHARS} return pointers to data within the respective | |
883 | object. Care must be taken that the object is not garbage collected | |
884 | while that data is still being accessed. This is the same as for a | |
885 | smob, @xref{Remembering During Operations}. | |
886 | ||
38a93523 NJ |
887 | |
888 | @node Procedures | |
889 | @subsubsection Procedures | |
890 | ||
891 | Guile provides two kinds of procedures: @dfn{closures}, which are the | |
892 | result of evaluating a @code{lambda} expression, and @dfn{subrs}, which | |
893 | are C functions packaged up as Scheme objects, to make them available to | |
894 | Scheme programmers. | |
895 | ||
896 | (There are actually other sorts of procedures: compiled closures, and | |
897 | continuations; see the source code for details about them.) | |
898 | ||
899 | @deftypefun SCM scm_procedure_p (SCM @var{x}) | |
900 | Return @code{SCM_BOOL_T} iff @var{x} is a Scheme procedure object, of | |
901 | any sort. Otherwise, return @code{SCM_BOOL_F}. | |
902 | @end deftypefun | |
903 | ||
904 | ||
905 | @node Closures | |
906 | @subsubsection Closures | |
907 | ||
908 | [FIXME: this needs to be further subbed, but texinfo has no subsubsub] | |
909 | ||
910 | A closure is a procedure object, generated as the value of a | |
911 | @code{lambda} expression in Scheme. The representation of a closure is | |
912 | straightforward --- it contains a pointer to the code of the lambda | |
913 | expression from which it was created, and a pointer to the environment | |
914 | it closes over. | |
915 | ||
916 | In Guile, each closure also has a property list, allowing the system to | |
917 | store information about the closure. I'm not sure what this is used for | |
918 | at the moment --- the debugger, maybe? | |
919 | ||
920 | @deftypefn Macro int SCM_CLOSUREP (SCM @var{x}) | |
505392ae | 921 | Return non-zero iff @var{x} is a closure. |
38a93523 NJ |
922 | @end deftypefn |
923 | ||
924 | @deftypefn Macro SCM SCM_PROCPROPS (SCM @var{x}) | |
925 | Return the property list of the closure @var{x}. The results are | |
926 | undefined if @var{x} is not a closure. | |
927 | @end deftypefn | |
928 | ||
929 | @deftypefn Macro void SCM_SETPROCPROPS (SCM @var{x}, SCM @var{p}) | |
930 | Set the property list of the closure @var{x} to @var{p}. The results | |
931 | are undefined if @var{x} is not a closure. | |
932 | @end deftypefn | |
933 | ||
934 | @deftypefn Macro SCM SCM_CODE (SCM @var{x}) | |
505392ae | 935 | Return the code of the closure @var{x}. The result is undefined if |
38a93523 NJ |
936 | @var{x} is not a closure. |
937 | ||
938 | This function should probably only be used internally by the | |
939 | interpreter, since the representation of the code is intimately | |
940 | connected with the interpreter's implementation. | |
941 | @end deftypefn | |
942 | ||
943 | @deftypefn Macro SCM SCM_ENV (SCM @var{x}) | |
944 | Return the environment enclosed by @var{x}. | |
505392ae | 945 | The result is undefined if @var{x} is not a closure. |
38a93523 NJ |
946 | |
947 | This function should probably only be used internally by the | |
948 | interpreter, since the representation of the environment is intimately | |
949 | connected with the interpreter's implementation. | |
950 | @end deftypefn | |
951 | ||
952 | ||
953 | @node Subrs | |
954 | @subsubsection Subrs | |
955 | ||
956 | [FIXME: this needs to be further subbed, but texinfo has no subsubsub] | |
957 | ||
958 | A subr is a pointer to a C function, packaged up as a Scheme object to | |
959 | make it callable by Scheme code. In addition to the function pointer, | |
960 | the subr also contains a pointer to the name of the function, and | |
85a9b4ed | 961 | information about the number of arguments accepted by the C function, for |
38a93523 NJ |
962 | the sake of error checking. |
963 | ||
964 | There is no single type predicate macro that recognizes subrs, as | |
965 | distinct from other kinds of procedures. The closest thing is | |
966 | @code{scm_procedure_p}; see @ref{Procedures}. | |
967 | ||
968 | @deftypefn Macro {char *} SCM_SNAME (@var{x}) | |
505392ae | 969 | Return the name of the subr @var{x}. The result is undefined if |
38a93523 NJ |
970 | @var{x} is not a subr. |
971 | @end deftypefn | |
972 | ||
bcf009c3 | 973 | @deftypefun SCM scm_c_define_gsubr (char *@var{name}, int @var{req}, int @var{opt}, int @var{rest}, SCM (*@var{function})()) |
38a93523 NJ |
974 | Create a new subr object named @var{name}, based on the C function |
975 | @var{function}, make it visible to Scheme the value of as a global | |
976 | variable named @var{name}, and return the subr object. | |
977 | ||
978 | The subr object accepts @var{req} required arguments, @var{opt} optional | |
979 | arguments, and a @var{rest} argument iff @var{rest} is non-zero. The C | |
980 | function @var{function} should accept @code{@var{req} + @var{opt}} | |
981 | arguments, or @code{@var{req} + @var{opt} + 1} arguments if @code{rest} | |
982 | is non-zero. | |
983 | ||
984 | When a subr object is applied, it must be applied to at least @var{req} | |
985 | arguments, or else Guile signals an error. @var{function} receives the | |
986 | subr's first @var{req} arguments as its first @var{req} arguments. If | |
987 | there are fewer than @var{opt} arguments remaining, then @var{function} | |
988 | receives the value @code{SCM_UNDEFINED} for any missing optional | |
989 | arguments. If @var{rst} is non-zero, then any arguments after the first | |
990 | @code{@var{req} + @var{opt}} are packaged up as a list as passed as | |
991 | @var{function}'s last argument. | |
992 | ||
993 | Note that subrs can actually only accept a predefined set of | |
994 | combinations of required, optional, and rest arguments. For example, a | |
995 | subr can take one required argument, or one required and one optional | |
996 | argument, but a subr can't take one required and two optional arguments. | |
997 | It's bizarre, but that's the way the interpreter was written. If the | |
bcf009c3 NJ |
998 | arguments to @code{scm_c_define_gsubr} do not fit one of the predefined |
999 | patterns, then @code{scm_c_define_gsubr} will return a compiled closure | |
38a93523 NJ |
1000 | object instead of a subr object. |
1001 | @end deftypefun | |
1002 | ||
1003 | ||
1004 | @node Port Data | |
1005 | @subsubsection Ports | |
1006 | ||
1007 | Haven't written this yet, 'cos I don't understand ports yet. | |
1008 | ||
1009 | ||
1010 | @node Signalling Type Errors | |
1011 | @subsection Signalling Type Errors | |
1012 | ||
1013 | Every function visible at the Scheme level should aggressively check the | |
1014 | types of its arguments, to avoid misinterpreting a value, and perhaps | |
1015 | causing a segmentation fault. Guile provides some macros to make this | |
1016 | easier. | |
1017 | ||
813c57db NJ |
1018 | @deftypefn Macro void SCM_ASSERT (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr}) |
1019 | If @var{test} is zero, signal a ``wrong type argument'' error, | |
1020 | attributed to the subroutine named @var{subr}, operating on the value | |
1021 | @var{obj}, which is the @var{position}'th argument of @var{subr}. | |
38a93523 NJ |
1022 | @end deftypefn |
1023 | ||
1024 | @deftypefn Macro int SCM_ARG1 | |
1025 | @deftypefnx Macro int SCM_ARG2 | |
1026 | @deftypefnx Macro int SCM_ARG3 | |
1027 | @deftypefnx Macro int SCM_ARG4 | |
1028 | @deftypefnx Macro int SCM_ARG5 | |
813c57db NJ |
1029 | @deftypefnx Macro int SCM_ARG6 |
1030 | @deftypefnx Macro int SCM_ARG7 | |
1031 | One of the above values can be used for @var{position} to indicate the | |
1032 | number of the argument of @var{subr} which is being checked. | |
1033 | Alternatively, a positive integer number can be used, which allows to | |
1034 | check arguments after the seventh. However, for parameter numbers up to | |
1035 | seven it is preferable to use @code{SCM_ARGN} instead of the | |
1036 | corresponding raw number, since it will make the code easier to | |
1037 | understand. | |
38a93523 NJ |
1038 | @end deftypefn |
1039 | ||
1040 | @deftypefn Macro int SCM_ARGn | |
813c57db NJ |
1041 | Passing a value of zero or @code{SCM_ARGn} for @var{position} allows to |
1042 | leave it unspecified which argument's type is incorrect. Again, | |
1043 | @code{SCM_ARGn} should be preferred over a raw zero constant. | |
38a93523 NJ |
1044 | @end deftypefn |
1045 | ||
1046 | ||
505392ae NJ |
1047 | @node Unpacking the SCM type |
1048 | @subsection Unpacking the SCM Type | |
1049 | ||
1050 | The previous sections have explained how @code{SCM} values can refer to | |
1051 | immediate and non-immediate Scheme objects. For immediate objects, the | |
1052 | complete object value is stored in the @code{SCM} word itself, while for | |
1053 | non-immediates, the @code{SCM} word contains a pointer to a heap cell, | |
1054 | and further information about the object in question is stored in that | |
1055 | cell. This section describes how the @code{SCM} type is actually | |
1056 | represented and used at the C level. | |
1057 | ||
3229f68b MV |
1058 | In fact, there are two basic C data types to represent objects in |
1059 | Guile: @code{SCM} and @code{scm_t_bits}. | |
505392ae NJ |
1060 | |
1061 | @menu | |
9d5315b6 | 1062 | * Relationship between SCM and scm_t_bits:: |
505392ae NJ |
1063 | * Immediate objects:: |
1064 | * Non-immediate objects:: | |
9d5315b6 | 1065 | * Allocating Cells:: |
505392ae NJ |
1066 | * Heap Cell Type Information:: |
1067 | * Accessing Cell Entries:: | |
1068 | * Basic Rules for Accessing Cell Entries:: | |
1069 | @end menu | |
1070 | ||
1071 | ||
9d5315b6 MV |
1072 | @node Relationship between SCM and scm_t_bits |
1073 | @subsubsection Relationship between @code{SCM} and @code{scm_t_bits} | |
505392ae NJ |
1074 | |
1075 | A variable of type @code{SCM} is guaranteed to hold a valid Scheme | |
9d5315b6 | 1076 | object. A variable of type @code{scm_t_bits}, on the other hand, may |
505392ae NJ |
1077 | hold a representation of a @code{SCM} value as a C integral type, but |
1078 | may also hold any C value, even if it does not correspond to a valid | |
1079 | Scheme object. | |
1080 | ||
1081 | For a variable @var{x} of type @code{SCM}, the Scheme object's type | |
1082 | information is stored in a form that is not directly usable. To be able | |
1083 | to work on the type encoding of the scheme value, the @code{SCM} | |
1084 | variable has to be transformed into the corresponding representation as | |
9d5315b6 | 1085 | a @code{scm_t_bits} variable @var{y} by using the @code{SCM_UNPACK} |
505392ae | 1086 | macro. Once this has been done, the type of the scheme object @var{x} |
9d5315b6 | 1087 | can be derived from the content of the bits of the @code{scm_t_bits} |
505392ae NJ |
1088 | value @var{y}, in the way illustrated by the example earlier in this |
1089 | chapter (@pxref{Cheaper Pairs}). Conversely, a valid bit encoding of a | |
9d5315b6 | 1090 | Scheme value as a @code{scm_t_bits} variable can be transformed into the |
505392ae NJ |
1091 | corresponding @code{SCM} value using the @code{SCM_PACK} macro. |
1092 | ||
505392ae NJ |
1093 | @node Immediate objects |
1094 | @subsubsection Immediate objects | |
1095 | ||
1096 | A Scheme object may either be an immediate, i.e. carrying all necessary | |
1097 | information by itself, or it may contain a reference to a @dfn{cell} | |
1098 | with additional information on the heap. Although in general it should | |
1099 | be irrelevant for user code whether an object is an immediate or not, | |
1100 | within Guile's own code the distinction is sometimes of importance. | |
1101 | Thus, the following low level macro is provided: | |
1102 | ||
1103 | @deftypefn Macro int SCM_IMP (SCM @var{x}) | |
1104 | A Scheme object is an immediate if it fulfills the @code{SCM_IMP} | |
1105 | predicate, otherwise it holds an encoded reference to a heap cell. The | |
1106 | result of the predicate is delivered as a C style boolean value. User | |
1107 | code and code that extends Guile should normally not be required to use | |
1108 | this macro. | |
1109 | @end deftypefn | |
1110 | ||
1111 | @noindent | |
1112 | Summary: | |
1113 | @itemize @bullet | |
1114 | @item | |
1115 | Given a Scheme object @var{x} of unknown type, check first | |
1116 | with @code{SCM_IMP (@var{x})} if it is an immediate object. | |
1117 | @item | |
1118 | If so, all of the type and value information can be determined from the | |
9d5315b6 | 1119 | @code{scm_t_bits} value that is delivered by @code{SCM_UNPACK |
505392ae NJ |
1120 | (@var{x})}. |
1121 | @end itemize | |
1122 | ||
1123 | ||
1124 | @node Non-immediate objects | |
1125 | @subsubsection Non-immediate objects | |
1126 | ||
85a9b4ed | 1127 | A Scheme object of type @code{SCM} that does not fulfill the |
505392ae NJ |
1128 | @code{SCM_IMP} predicate holds an encoded reference to a heap cell. |
1129 | This reference can be decoded to a C pointer to a heap cell using the | |
1130 | @code{SCM2PTR} macro. The encoding of a pointer to a heap cell into a | |
1131 | @code{SCM} value is done using the @code{PTR2SCM} macro. | |
1132 | ||
1133 | @c (FIXME:: this name should be changed) | |
228a24ef | 1134 | @deftypefn Macro (scm_t_cell *) SCM2PTR (SCM @var{x}) |
505392ae NJ |
1135 | Extract and return the heap cell pointer from a non-immediate @code{SCM} |
1136 | object @var{x}. | |
1137 | @end deftypefn | |
1138 | ||
1139 | @c (FIXME:: this name should be changed) | |
228a24ef | 1140 | @deftypefn Macro SCM PTR2SCM (scm_t_cell * @var{x}) |
505392ae NJ |
1141 | Return a @code{SCM} value that encodes a reference to the heap cell |
1142 | pointer @var{x}. | |
1143 | @end deftypefn | |
1144 | ||
1145 | Note that it is also possible to transform a non-immediate @code{SCM} | |
9d5315b6 | 1146 | value by using @code{SCM_UNPACK} into a @code{scm_t_bits} variable. |
505392ae | 1147 | However, the result of @code{SCM_UNPACK} may not be used as a pointer to |
228a24ef | 1148 | a @code{scm_t_cell}: only @code{SCM2PTR} is guaranteed to transform a |
505392ae NJ |
1149 | @code{SCM} object into a valid pointer to a heap cell. Also, it is not |
1150 | allowed to apply @code{PTR2SCM} to anything that is not a valid pointer | |
1151 | to a heap cell. | |
1152 | ||
1153 | @noindent | |
1154 | Summary: | |
1155 | @itemize @bullet | |
1156 | @item | |
1157 | Only use @code{SCM2PTR} on @code{SCM} values for which @code{SCM_IMP} is | |
1158 | false! | |
1159 | @item | |
228a24ef | 1160 | Don't use @code{(scm_t_cell *) SCM_UNPACK (@var{x})}! Use @code{SCM2PTR |
505392ae NJ |
1161 | (@var{x})} instead! |
1162 | @item | |
1163 | Don't use @code{PTR2SCM} for anything but a cell pointer! | |
1164 | @end itemize | |
1165 | ||
9d5315b6 MV |
1166 | @node Allocating Cells |
1167 | @subsubsection Allocating Cells | |
1168 | ||
1169 | Guile provides both ordinary cells with two slots, and double cells | |
1170 | with four slots. The following two function are the most primitive | |
1171 | way to allocate such cells. | |
1172 | ||
1173 | If the caller intends to use it as a header for some other type, she | |
1174 | must pass an appropriate magic value in @var{word_0}, to mark it as a | |
1175 | member of that type, and pass whatever value as @var{word_1}, etc that | |
1176 | the type expects. You should generally not need these functions, | |
1177 | unless you are implementing a new datatype, and thoroughly understand | |
1178 | the code in @code{<libguile/tags.h>}. | |
1179 | ||
1180 | If you just want to allocate pairs, use @code{scm_cons}. | |
1181 | ||
228a24ef | 1182 | @deftypefn Function SCM scm_cell (scm_t_bits word_0, scm_t_bits word_1) |
9d5315b6 MV |
1183 | Allocate a new cell, initialize the two slots with @var{word_0} and |
1184 | @var{word_1}, and return it. | |
1185 | ||
1186 | Note that @var{word_0} and @var{word_1} are of type @code{scm_t_bits}. | |
1187 | If you want to pass a @code{SCM} object, you need to use | |
1188 | @code{SCM_UNPACK}. | |
1189 | @end deftypefn | |
1190 | ||
228a24ef DH |
1191 | @deftypefn Function SCM scm_double_cell (scm_t_bits word_0, scm_t_bits word_1, scm_t_bits word_2, scm_t_bits word_3) |
1192 | Like @code{scm_cell}, but allocates a double cell with four | |
9d5315b6 MV |
1193 | slots. |
1194 | @end deftypefn | |
505392ae NJ |
1195 | |
1196 | @node Heap Cell Type Information | |
1197 | @subsubsection Heap Cell Type Information | |
1198 | ||
1199 | Heap cells contain a number of entries, each of which is either a scheme | |
9d5315b6 | 1200 | object of type @code{SCM} or a raw C value of type @code{scm_t_bits}. |
505392ae NJ |
1201 | Which of the cell entries contain Scheme objects and which contain raw C |
1202 | values is determined by the first entry of the cell, which holds the | |
1203 | cell type information. | |
1204 | ||
9d5315b6 | 1205 | @deftypefn Macro scm_t_bits SCM_CELL_TYPE (SCM @var{x}) |
505392ae NJ |
1206 | For a non-immediate Scheme object @var{x}, deliver the content of the |
1207 | first entry of the heap cell referenced by @var{x}. This value holds | |
1208 | the information about the cell type. | |
1209 | @end deftypefn | |
1210 | ||
9d5315b6 | 1211 | @deftypefn Macro void SCM_SET_CELL_TYPE (SCM @var{x}, scm_t_bits @var{t}) |
505392ae NJ |
1212 | For a non-immediate Scheme object @var{x}, write the value @var{t} into |
1213 | the first entry of the heap cell referenced by @var{x}. The value | |
1214 | @var{t} must hold a valid cell type. | |
1215 | @end deftypefn | |
1216 | ||
1217 | ||
1218 | @node Accessing Cell Entries | |
1219 | @subsubsection Accessing Cell Entries | |
1220 | ||
1221 | For a non-immediate Scheme object @var{x}, the object type can be | |
1222 | determined by reading the cell type entry using the @code{SCM_CELL_TYPE} | |
1223 | macro. For each different type of cell it is known which cell entries | |
1224 | hold Scheme objects and which cell entries hold raw C data. To access | |
1225 | the different cell entries appropriately, the following macros are | |
1226 | provided. | |
1227 | ||
9d5315b6 | 1228 | @deftypefn Macro scm_t_bits SCM_CELL_WORD (SCM @var{x}, unsigned int @var{n}) |
505392ae NJ |
1229 | Deliver the cell entry @var{n} of the heap cell referenced by the |
1230 | non-immediate Scheme object @var{x} as raw data. It is illegal, to | |
1231 | access cell entries that hold Scheme objects by using these macros. For | |
1232 | convenience, the following macros are also provided. | |
230712c9 | 1233 | @itemize @bullet |
505392ae NJ |
1234 | @item |
1235 | SCM_CELL_WORD_0 (@var{x}) @result{} SCM_CELL_WORD (@var{x}, 0) | |
1236 | @item | |
1237 | SCM_CELL_WORD_1 (@var{x}) @result{} SCM_CELL_WORD (@var{x}, 1) | |
1238 | @item | |
1239 | @dots{} | |
1240 | @item | |
1241 | SCM_CELL_WORD_@var{n} (@var{x}) @result{} SCM_CELL_WORD (@var{x}, @var{n}) | |
1242 | @end itemize | |
1243 | @end deftypefn | |
1244 | ||
1245 | @deftypefn Macro SCM SCM_CELL_OBJECT (SCM @var{x}, unsigned int @var{n}) | |
1246 | Deliver the cell entry @var{n} of the heap cell referenced by the | |
1247 | non-immediate Scheme object @var{x} as a Scheme object. It is illegal, | |
1248 | to access cell entries that do not hold Scheme objects by using these | |
1249 | macros. For convenience, the following macros are also provided. | |
230712c9 | 1250 | @itemize @bullet |
505392ae NJ |
1251 | @item |
1252 | SCM_CELL_OBJECT_0 (@var{x}) @result{} SCM_CELL_OBJECT (@var{x}, 0) | |
1253 | @item | |
1254 | SCM_CELL_OBJECT_1 (@var{x}) @result{} SCM_CELL_OBJECT (@var{x}, 1) | |
1255 | @item | |
1256 | @dots{} | |
1257 | @item | |
1258 | SCM_CELL_OBJECT_@var{n} (@var{x}) @result{} SCM_CELL_OBJECT (@var{x}, | |
1259 | @var{n}) | |
1260 | @end itemize | |
1261 | @end deftypefn | |
1262 | ||
9d5315b6 | 1263 | @deftypefn Macro void SCM_SET_CELL_WORD (SCM @var{x}, unsigned int @var{n}, scm_t_bits @var{w}) |
505392ae NJ |
1264 | Write the raw C value @var{w} into entry number @var{n} of the heap cell |
1265 | referenced by the non-immediate Scheme value @var{x}. Values that are | |
1266 | written into cells this way may only be read from the cells using the | |
1267 | @code{SCM_CELL_WORD} macros or, in case cell entry 0 is written, using | |
1268 | the @code{SCM_CELL_TYPE} macro. For the special case of cell entry 0 it | |
1269 | has to be made sure that @var{w} contains a cell type information which | |
1270 | does not describe a Scheme object. For convenience, the following | |
1271 | macros are also provided. | |
230712c9 | 1272 | @itemize @bullet |
505392ae NJ |
1273 | @item |
1274 | SCM_SET_CELL_WORD_0 (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD | |
1275 | (@var{x}, 0, @var{w}) | |
1276 | @item | |
1277 | SCM_SET_CELL_WORD_1 (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD | |
1278 | (@var{x}, 1, @var{w}) | |
1279 | @item | |
1280 | @dots{} | |
1281 | @item | |
1282 | SCM_SET_CELL_WORD_@var{n} (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD | |
1283 | (@var{x}, @var{n}, @var{w}) | |
1284 | @end itemize | |
1285 | @end deftypefn | |
1286 | ||
1287 | @deftypefn Macro void SCM_SET_CELL_OBJECT (SCM @var{x}, unsigned int @var{n}, SCM @var{o}) | |
1288 | Write the Scheme object @var{o} into entry number @var{n} of the heap | |
1289 | cell referenced by the non-immediate Scheme value @var{x}. Values that | |
1290 | are written into cells this way may only be read from the cells using | |
1291 | the @code{SCM_CELL_OBJECT} macros or, in case cell entry 0 is written, | |
1292 | using the @code{SCM_CELL_TYPE} macro. For the special case of cell | |
1293 | entry 0 the writing of a Scheme object into this cell is only allowed | |
1294 | if the cell forms a Scheme pair. For convenience, the following macros | |
1295 | are also provided. | |
230712c9 | 1296 | @itemize @bullet |
505392ae NJ |
1297 | @item |
1298 | SCM_SET_CELL_OBJECT_0 (@var{x}, @var{o}) @result{} SCM_SET_CELL_OBJECT | |
1299 | (@var{x}, 0, @var{o}) | |
1300 | @item | |
1301 | SCM_SET_CELL_OBJECT_1 (@var{x}, @var{o}) @result{} SCM_SET_CELL_OBJECT | |
1302 | (@var{x}, 1, @var{o}) | |
1303 | @item | |
1304 | @dots{} | |
1305 | @item | |
1306 | SCM_SET_CELL_OBJECT_@var{n} (@var{x}, @var{o}) @result{} | |
1307 | SCM_SET_CELL_OBJECT (@var{x}, @var{n}, @var{o}) | |
1308 | @end itemize | |
1309 | @end deftypefn | |
1310 | ||
1311 | @noindent | |
1312 | Summary: | |
1313 | @itemize @bullet | |
1314 | @item | |
1315 | For a non-immediate Scheme object @var{x} of unknown type, get the type | |
1316 | information by using @code{SCM_CELL_TYPE (@var{x})}. | |
1317 | @item | |
1318 | As soon as the cell type information is available, only use the | |
1319 | appropriate access methods to read and write data to the different cell | |
1320 | entries. | |
1321 | @end itemize | |
1322 | ||
1323 | ||
1324 | @node Basic Rules for Accessing Cell Entries | |
1325 | @subsubsection Basic Rules for Accessing Cell Entries | |
1326 | ||
1327 | For each cell type it is generally up to the implementation of that type | |
1328 | which of the corresponding cell entries hold Scheme objects and which | |
1329 | hold raw C values. However, there is one basic rule that has to be | |
1330 | followed: Scheme pairs consist of exactly two cell entries, which both | |
1331 | contain Scheme objects. Further, a cell which contains a Scheme object | |
1332 | in it first entry has to be a Scheme pair. In other words, it is not | |
1333 | allowed to store a Scheme object in the first cell entry and a non | |
1334 | Scheme object in the second cell entry. | |
1335 | ||
1336 | @c Fixme:shouldn't this rather be SCM_PAIRP / SCM_PAIR_P ? | |
1337 | @deftypefn Macro int SCM_CONSP (SCM @var{x}) | |
1338 | Determine, whether the Scheme object @var{x} is a Scheme pair, | |
1339 | i.e. whether @var{x} references a heap cell consisting of exactly two | |
1340 | entries, where both entries contain a Scheme object. In this case, both | |
1341 | entries will have to be accessed using the @code{SCM_CELL_OBJECT} | |
c4d0cddd NJ |
1342 | macros. On the contrary, if the @code{SCM_CONSP} predicate is not |
1343 | fulfilled, the first entry of the Scheme cell is guaranteed not to be a | |
1344 | Scheme value and thus the first cell entry must be accessed using the | |
505392ae NJ |
1345 | @code{SCM_CELL_WORD_0} macro. |
1346 | @end deftypefn | |
1347 | ||
1348 |