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1 227 jeremybenn
@node Obstacks,Licenses,Functions,Top
2
@chapter Obstacks
3
@cindex obstacks
4
 
5
An @dfn{obstack} is a pool of memory containing a stack of objects.  You
6
can create any number of separate obstacks, and then allocate objects in
7
specified obstacks.  Within each obstack, the last object allocated must
8
always be the first one freed, but distinct obstacks are independent of
9
each other.
10
 
11
Aside from this one constraint of order of freeing, obstacks are totally
12
general: an obstack can contain any number of objects of any size.  They
13
are implemented with macros, so allocation is usually very fast as long as
14
the objects are usually small.  And the only space overhead per object is
15
the padding needed to start each object on a suitable boundary.
16
 
17
@menu
18
* Creating Obstacks::           How to declare an obstack in your program.
19
* Preparing for Obstacks::      Preparations needed before you can
20
                                use obstacks.
21
* Allocation in an Obstack::    Allocating objects in an obstack.
22
* Freeing Obstack Objects::     Freeing objects in an obstack.
23
* Obstack Functions::           The obstack functions are both
24
                                functions and macros.
25
* Growing Objects::             Making an object bigger by stages.
26
* Extra Fast Growing::          Extra-high-efficiency (though more
27
                                complicated) growing objects.
28
* Status of an Obstack::        Inquiries about the status of an obstack.
29
* Obstacks Data Alignment::     Controlling alignment of objects in obstacks.
30
* Obstack Chunks::              How obstacks obtain and release chunks;
31
                                efficiency considerations.
32
* Summary of Obstacks::
33
@end menu
34
 
35
@node Creating Obstacks
36
@section Creating Obstacks
37
 
38
The utilities for manipulating obstacks are declared in the header
39
file @file{obstack.h}.
40
@pindex obstack.h
41
 
42
@comment obstack.h
43
@comment GNU
44
@deftp {Data Type} {struct obstack}
45
An obstack is represented by a data structure of type @code{struct
46
obstack}.  This structure has a small fixed size; it records the status
47
of the obstack and how to find the space in which objects are allocated.
48
It does not contain any of the objects themselves.  You should not try
49
to access the contents of the structure directly; use only the functions
50
described in this chapter.
51
@end deftp
52
 
53
You can declare variables of type @code{struct obstack} and use them as
54
obstacks, or you can allocate obstacks dynamically like any other kind
55
of object.  Dynamic allocation of obstacks allows your program to have a
56
variable number of different stacks.  (You can even allocate an
57
obstack structure in another obstack, but this is rarely useful.)
58
 
59
All the functions that work with obstacks require you to specify which
60
obstack to use.  You do this with a pointer of type @code{struct obstack
61
*}.  In the following, we often say ``an obstack'' when strictly
62
speaking the object at hand is such a pointer.
63
 
64
The objects in the obstack are packed into large blocks called
65
@dfn{chunks}.  The @code{struct obstack} structure points to a chain of
66
the chunks currently in use.
67
 
68
The obstack library obtains a new chunk whenever you allocate an object
69
that won't fit in the previous chunk.  Since the obstack library manages
70
chunks automatically, you don't need to pay much attention to them, but
71
you do need to supply a function which the obstack library should use to
72
get a chunk.  Usually you supply a function which uses @code{malloc}
73
directly or indirectly.  You must also supply a function to free a chunk.
74
These matters are described in the following section.
75
 
76
@node Preparing for Obstacks
77
@section Preparing for Using Obstacks
78
 
79
Each source file in which you plan to use the obstack functions
80
must include the header file @file{obstack.h}, like this:
81
 
82
@smallexample
83
#include <obstack.h>
84
@end smallexample
85
 
86
@findex obstack_chunk_alloc
87
@findex obstack_chunk_free
88
Also, if the source file uses the macro @code{obstack_init}, it must
89
declare or define two functions or macros that will be called by the
90
obstack library.  One, @code{obstack_chunk_alloc}, is used to allocate
91
the chunks of memory into which objects are packed.  The other,
92
@code{obstack_chunk_free}, is used to return chunks when the objects in
93
them are freed.  These macros should appear before any use of obstacks
94
in the source file.
95
 
96
Usually these are defined to use @code{malloc} via the intermediary
97
@code{xmalloc} (@pxref{Unconstrained Allocation, , , libc, The GNU C Library Reference Manual}).  This is done with
98
the following pair of macro definitions:
99
 
100
@smallexample
101
#define obstack_chunk_alloc xmalloc
102
#define obstack_chunk_free free
103
@end smallexample
104
 
105
@noindent
106
Though the memory you get using obstacks really comes from @code{malloc},
107
using obstacks is faster because @code{malloc} is called less often, for
108
larger blocks of memory.  @xref{Obstack Chunks}, for full details.
109
 
110
At run time, before the program can use a @code{struct obstack} object
111
as an obstack, it must initialize the obstack by calling
112
@code{obstack_init}.
113
 
114
@comment obstack.h
115
@comment GNU
116
@deftypefun int obstack_init (struct obstack *@var{obstack-ptr})
117
Initialize obstack @var{obstack-ptr} for allocation of objects.  This
118
function calls the obstack's @code{obstack_chunk_alloc} function.  If
119
allocation of memory fails, the function pointed to by
120
@code{obstack_alloc_failed_handler} is called.  The @code{obstack_init}
121
function always returns 1 (Compatibility notice: Former versions of
122
obstack returned 0 if allocation failed).
123
@end deftypefun
124
 
125
Here are two examples of how to allocate the space for an obstack and
126
initialize it.  First, an obstack that is a static variable:
127
 
128
@smallexample
129
static struct obstack myobstack;
130
@dots{}
131
obstack_init (&myobstack);
132
@end smallexample
133
 
134
@noindent
135
Second, an obstack that is itself dynamically allocated:
136
 
137
@smallexample
138
struct obstack *myobstack_ptr
139
  = (struct obstack *) xmalloc (sizeof (struct obstack));
140
 
141
obstack_init (myobstack_ptr);
142
@end smallexample
143
 
144
@comment obstack.h
145
@comment GNU
146
@defvar obstack_alloc_failed_handler
147
The value of this variable is a pointer to a function that
148
@code{obstack} uses when @code{obstack_chunk_alloc} fails to allocate
149
memory.  The default action is to print a message and abort.
150
You should supply a function that either calls @code{exit}
151
(@pxref{Program Termination, , , libc, The GNU C Library Reference Manual}) or @code{longjmp} (@pxref{Non-Local
152
Exits, , , libc, The GNU C Library Reference Manual}) and doesn't return.
153
 
154
@smallexample
155
void my_obstack_alloc_failed (void)
156
@dots{}
157
obstack_alloc_failed_handler = &my_obstack_alloc_failed;
158
@end smallexample
159
 
160
@end defvar
161
 
162
@node Allocation in an Obstack
163
@section Allocation in an Obstack
164
@cindex allocation (obstacks)
165
 
166
The most direct way to allocate an object in an obstack is with
167
@code{obstack_alloc}, which is invoked almost like @code{malloc}.
168
 
169
@comment obstack.h
170
@comment GNU
171
@deftypefun {void *} obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
172
This allocates an uninitialized block of @var{size} bytes in an obstack
173
and returns its address.  Here @var{obstack-ptr} specifies which obstack
174
to allocate the block in; it is the address of the @code{struct obstack}
175
object which represents the obstack.  Each obstack function or macro
176
requires you to specify an @var{obstack-ptr} as the first argument.
177
 
178
This function calls the obstack's @code{obstack_chunk_alloc} function if
179
it needs to allocate a new chunk of memory; it calls
180
@code{obstack_alloc_failed_handler} if allocation of memory by
181
@code{obstack_chunk_alloc} failed.
182
@end deftypefun
183
 
184
For example, here is a function that allocates a copy of a string @var{str}
185
in a specific obstack, which is in the variable @code{string_obstack}:
186
 
187
@smallexample
188
struct obstack string_obstack;
189
 
190
char *
191
copystring (char *string)
192
@{
193
  size_t len = strlen (string) + 1;
194
  char *s = (char *) obstack_alloc (&string_obstack, len);
195
  memcpy (s, string, len);
196
  return s;
197
@}
198
@end smallexample
199
 
200
To allocate a block with specified contents, use the function
201
@code{obstack_copy}, declared like this:
202
 
203
@comment obstack.h
204
@comment GNU
205
@deftypefun {void *} obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
206
This allocates a block and initializes it by copying @var{size}
207
bytes of data starting at @var{address}.  It calls
208
@code{obstack_alloc_failed_handler} if allocation of memory by
209
@code{obstack_chunk_alloc} failed.
210
@end deftypefun
211
 
212
@comment obstack.h
213
@comment GNU
214
@deftypefun {void *} obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
215
Like @code{obstack_copy}, but appends an extra byte containing a null
216
character.  This extra byte is not counted in the argument @var{size}.
217
@end deftypefun
218
 
219
The @code{obstack_copy0} function is convenient for copying a sequence
220
of characters into an obstack as a null-terminated string.  Here is an
221
example of its use:
222
 
223
@smallexample
224
char *
225
obstack_savestring (char *addr, int size)
226
@{
227
  return obstack_copy0 (&myobstack, addr, size);
228
@}
229
@end smallexample
230
 
231
@noindent
232
Contrast this with the previous example of @code{savestring} using
233
@code{malloc} (@pxref{Basic Allocation, , , libc, The GNU C Library Reference Manual}).
234
 
235
@node Freeing Obstack Objects
236
@section Freeing Objects in an Obstack
237
@cindex freeing (obstacks)
238
 
239
To free an object allocated in an obstack, use the function
240
@code{obstack_free}.  Since the obstack is a stack of objects, freeing
241
one object automatically frees all other objects allocated more recently
242
in the same obstack.
243
 
244
@comment obstack.h
245
@comment GNU
246
@deftypefun void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
247
If @var{object} is a null pointer, everything allocated in the obstack
248
is freed.  Otherwise, @var{object} must be the address of an object
249
allocated in the obstack.  Then @var{object} is freed, along with
250
everything allocated in @var{obstack} since @var{object}.
251
@end deftypefun
252
 
253
Note that if @var{object} is a null pointer, the result is an
254
uninitialized obstack.  To free all memory in an obstack but leave it
255
valid for further allocation, call @code{obstack_free} with the address
256
of the first object allocated on the obstack:
257
 
258
@smallexample
259
obstack_free (obstack_ptr, first_object_allocated_ptr);
260
@end smallexample
261
 
262
Recall that the objects in an obstack are grouped into chunks.  When all
263
the objects in a chunk become free, the obstack library automatically
264
frees the chunk (@pxref{Preparing for Obstacks}).  Then other
265
obstacks, or non-obstack allocation, can reuse the space of the chunk.
266
 
267
@node Obstack Functions
268
@section Obstack Functions and Macros
269
@cindex macros
270
 
271
The interfaces for using obstacks may be defined either as functions or
272
as macros, depending on the compiler.  The obstack facility works with
273
all C compilers, including both @w{ISO C} and traditional C, but there are
274
precautions you must take if you plan to use compilers other than GNU C.
275
 
276
If you are using an old-fashioned @w{non-ISO C} compiler, all the obstack
277
``functions'' are actually defined only as macros.  You can call these
278
macros like functions, but you cannot use them in any other way (for
279
example, you cannot take their address).
280
 
281
Calling the macros requires a special precaution: namely, the first
282
operand (the obstack pointer) may not contain any side effects, because
283
it may be computed more than once.  For example, if you write this:
284
 
285
@smallexample
286
obstack_alloc (get_obstack (), 4);
287
@end smallexample
288
 
289
@noindent
290
you will find that @code{get_obstack} may be called several times.
291
If you use @code{*obstack_list_ptr++} as the obstack pointer argument,
292
you will get very strange results since the incrementation may occur
293
several times.
294
 
295
In @w{ISO C}, each function has both a macro definition and a function
296
definition.  The function definition is used if you take the address of the
297
function without calling it.  An ordinary call uses the macro definition by
298
default, but you can request the function definition instead by writing the
299
function name in parentheses, as shown here:
300
 
301
@smallexample
302
char *x;
303
void *(*funcp) ();
304
/* @r{Use the macro}.  */
305
x = (char *) obstack_alloc (obptr, size);
306
/* @r{Call the function}.  */
307
x = (char *) (obstack_alloc) (obptr, size);
308
/* @r{Take the address of the function}.  */
309
funcp = obstack_alloc;
310
@end smallexample
311
 
312
@noindent
313
This is the same situation that exists in @w{ISO C} for the standard library
314
functions.  @xref{Macro Definitions, , , libc, The GNU C Library Reference Manual}.
315
 
316
@strong{Warning:} When you do use the macros, you must observe the
317
precaution of avoiding side effects in the first operand, even in @w{ISO C}.
318
 
319
If you use the GNU C compiler, this precaution is not necessary, because
320
various language extensions in GNU C permit defining the macros so as to
321
compute each argument only once.
322
 
323
@node Growing Objects
324
@section Growing Objects
325
@cindex growing objects (in obstacks)
326
@cindex changing the size of a block (obstacks)
327
 
328
Because memory in obstack chunks is used sequentially, it is possible to
329
build up an object step by step, adding one or more bytes at a time to the
330
end of the object.  With this technique, you do not need to know how much
331
data you will put in the object until you come to the end of it.  We call
332
this the technique of @dfn{growing objects}.  The special functions
333
for adding data to the growing object are described in this section.
334
 
335
You don't need to do anything special when you start to grow an object.
336
Using one of the functions to add data to the object automatically
337
starts it.  However, it is necessary to say explicitly when the object is
338
finished.  This is done with the function @code{obstack_finish}.
339
 
340
The actual address of the object thus built up is not known until the
341
object is finished.  Until then, it always remains possible that you will
342
add so much data that the object must be copied into a new chunk.
343
 
344
While the obstack is in use for a growing object, you cannot use it for
345
ordinary allocation of another object.  If you try to do so, the space
346
already added to the growing object will become part of the other object.
347
 
348
@comment obstack.h
349
@comment GNU
350
@deftypefun void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
351
The most basic function for adding to a growing object is
352
@code{obstack_blank}, which adds space without initializing it.
353
@end deftypefun
354
 
355
@comment obstack.h
356
@comment GNU
357
@deftypefun void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
358
To add a block of initialized space, use @code{obstack_grow}, which is
359
the growing-object analogue of @code{obstack_copy}.  It adds @var{size}
360
bytes of data to the growing object, copying the contents from
361
@var{data}.
362
@end deftypefun
363
 
364
@comment obstack.h
365
@comment GNU
366
@deftypefun void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
367
This is the growing-object analogue of @code{obstack_copy0}.  It adds
368
@var{size} bytes copied from @var{data}, followed by an additional null
369
character.
370
@end deftypefun
371
 
372
@comment obstack.h
373
@comment GNU
374
@deftypefun void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{c})
375
To add one character at a time, use the function @code{obstack_1grow}.
376
It adds a single byte containing @var{c} to the growing object.
377
@end deftypefun
378
 
379
@comment obstack.h
380
@comment GNU
381
@deftypefun void obstack_ptr_grow (struct obstack *@var{obstack-ptr}, void *@var{data})
382
Adding the value of a pointer one can use the function
383
@code{obstack_ptr_grow}.  It adds @code{sizeof (void *)} bytes
384
containing the value of @var{data}.
385
@end deftypefun
386
 
387
@comment obstack.h
388
@comment GNU
389
@deftypefun void obstack_int_grow (struct obstack *@var{obstack-ptr}, int @var{data})
390
A single value of type @code{int} can be added by using the
391
@code{obstack_int_grow} function.  It adds @code{sizeof (int)} bytes to
392
the growing object and initializes them with the value of @var{data}.
393
@end deftypefun
394
 
395
@comment obstack.h
396
@comment GNU
397
@deftypefun {void *} obstack_finish (struct obstack *@var{obstack-ptr})
398
When you are finished growing the object, use the function
399
@code{obstack_finish} to close it off and return its final address.
400
 
401
Once you have finished the object, the obstack is available for ordinary
402
allocation or for growing another object.
403
 
404
This function can return a null pointer under the same conditions as
405
@code{obstack_alloc} (@pxref{Allocation in an Obstack}).
406
@end deftypefun
407
 
408
When you build an object by growing it, you will probably need to know
409
afterward how long it became.  You need not keep track of this as you grow
410
the object, because you can find out the length from the obstack just
411
before finishing the object with the function @code{obstack_object_size},
412
declared as follows:
413
 
414
@comment obstack.h
415
@comment GNU
416
@deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
417
This function returns the current size of the growing object, in bytes.
418
Remember to call this function @emph{before} finishing the object.
419
After it is finished, @code{obstack_object_size} will return zero.
420
@end deftypefun
421
 
422
If you have started growing an object and wish to cancel it, you should
423
finish it and then free it, like this:
424
 
425
@smallexample
426
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
427
@end smallexample
428
 
429
@noindent
430
This has no effect if no object was growing.
431
 
432
@cindex shrinking objects
433
You can use @code{obstack_blank} with a negative size argument to make
434
the current object smaller.  Just don't try to shrink it beyond zero
435
length---there's no telling what will happen if you do that.
436
 
437
@node Extra Fast Growing
438
@section Extra Fast Growing Objects
439
@cindex efficiency and obstacks
440
 
441
The usual functions for growing objects incur overhead for checking
442
whether there is room for the new growth in the current chunk.  If you
443
are frequently constructing objects in small steps of growth, this
444
overhead can be significant.
445
 
446
You can reduce the overhead by using special ``fast growth''
447
functions that grow the object without checking.  In order to have a
448
robust program, you must do the checking yourself.  If you do this checking
449
in the simplest way each time you are about to add data to the object, you
450
have not saved anything, because that is what the ordinary growth
451
functions do.  But if you can arrange to check less often, or check
452
more efficiently, then you make the program faster.
453
 
454
The function @code{obstack_room} returns the amount of room available
455
in the current chunk.  It is declared as follows:
456
 
457
@comment obstack.h
458
@comment GNU
459
@deftypefun int obstack_room (struct obstack *@var{obstack-ptr})
460
This returns the number of bytes that can be added safely to the current
461
growing object (or to an object about to be started) in obstack
462
@var{obstack} using the fast growth functions.
463
@end deftypefun
464
 
465
While you know there is room, you can use these fast growth functions
466
for adding data to a growing object:
467
 
468
@comment obstack.h
469
@comment GNU
470
@deftypefun void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{c})
471
The function @code{obstack_1grow_fast} adds one byte containing the
472
character @var{c} to the growing object in obstack @var{obstack-ptr}.
473
@end deftypefun
474
 
475
@comment obstack.h
476
@comment GNU
477
@deftypefun void obstack_ptr_grow_fast (struct obstack *@var{obstack-ptr}, void *@var{data})
478
The function @code{obstack_ptr_grow_fast} adds @code{sizeof (void *)}
479
bytes containing the value of @var{data} to the growing object in
480
obstack @var{obstack-ptr}.
481
@end deftypefun
482
 
483
@comment obstack.h
484
@comment GNU
485
@deftypefun void obstack_int_grow_fast (struct obstack *@var{obstack-ptr}, int @var{data})
486
The function @code{obstack_int_grow_fast} adds @code{sizeof (int)} bytes
487
containing the value of @var{data} to the growing object in obstack
488
@var{obstack-ptr}.
489
@end deftypefun
490
 
491
@comment obstack.h
492
@comment GNU
493
@deftypefun void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
494
The function @code{obstack_blank_fast} adds @var{size} bytes to the
495
growing object in obstack @var{obstack-ptr} without initializing them.
496
@end deftypefun
497
 
498
When you check for space using @code{obstack_room} and there is not
499
enough room for what you want to add, the fast growth functions
500
are not safe.  In this case, simply use the corresponding ordinary
501
growth function instead.  Very soon this will copy the object to a
502
new chunk; then there will be lots of room available again.
503
 
504
So, each time you use an ordinary growth function, check afterward for
505
sufficient space using @code{obstack_room}.  Once the object is copied
506
to a new chunk, there will be plenty of space again, so the program will
507
start using the fast growth functions again.
508
 
509
Here is an example:
510
 
511
@smallexample
512
@group
513
void
514
add_string (struct obstack *obstack, const char *ptr, int len)
515
@{
516
  while (len > 0)
517
    @{
518
      int room = obstack_room (obstack);
519
      if (room == 0)
520
        @{
521
          /* @r{Not enough room. Add one character slowly,}
522
             @r{which may copy to a new chunk and make room.}  */
523
          obstack_1grow (obstack, *ptr++);
524
          len--;
525
        @}
526
      else
527
        @{
528
          if (room > len)
529
            room = len;
530
          /* @r{Add fast as much as we have room for.} */
531
          len -= room;
532
          while (room-- > 0)
533
            obstack_1grow_fast (obstack, *ptr++);
534
        @}
535
    @}
536
@}
537
@end group
538
@end smallexample
539
 
540
@node Status of an Obstack
541
@section Status of an Obstack
542
@cindex obstack status
543
@cindex status of obstack
544
 
545
Here are functions that provide information on the current status of
546
allocation in an obstack.  You can use them to learn about an object while
547
still growing it.
548
 
549
@comment obstack.h
550
@comment GNU
551
@deftypefun {void *} obstack_base (struct obstack *@var{obstack-ptr})
552
This function returns the tentative address of the beginning of the
553
currently growing object in @var{obstack-ptr}.  If you finish the object
554
immediately, it will have that address.  If you make it larger first, it
555
may outgrow the current chunk---then its address will change!
556
 
557
If no object is growing, this value says where the next object you
558
allocate will start (once again assuming it fits in the current
559
chunk).
560
@end deftypefun
561
 
562
@comment obstack.h
563
@comment GNU
564
@deftypefun {void *} obstack_next_free (struct obstack *@var{obstack-ptr})
565
This function returns the address of the first free byte in the current
566
chunk of obstack @var{obstack-ptr}.  This is the end of the currently
567
growing object.  If no object is growing, @code{obstack_next_free}
568
returns the same value as @code{obstack_base}.
569
@end deftypefun
570
 
571
@comment obstack.h
572
@comment GNU
573
@deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
574
This function returns the size in bytes of the currently growing object.
575
This is equivalent to
576
 
577
@smallexample
578
obstack_next_free (@var{obstack-ptr}) - obstack_base (@var{obstack-ptr})
579
@end smallexample
580
@end deftypefun
581
 
582
@node Obstacks Data Alignment
583
@section Alignment of Data in Obstacks
584
@cindex alignment (in obstacks)
585
 
586
Each obstack has an @dfn{alignment boundary}; each object allocated in
587
the obstack automatically starts on an address that is a multiple of the
588
specified boundary.  By default, this boundary is 4 bytes.
589
 
590
To access an obstack's alignment boundary, use the macro
591
@code{obstack_alignment_mask}, whose function prototype looks like
592
this:
593
 
594
@comment obstack.h
595
@comment GNU
596
@deftypefn Macro int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
597
The value is a bit mask; a bit that is 1 indicates that the corresponding
598
bit in the address of an object should be 0.  The mask value should be one
599
less than a power of 2; the effect is that all object addresses are
600
multiples of that power of 2.  The default value of the mask is 3, so that
601
addresses are multiples of 4.  A mask value of 0 means an object can start
602
on any multiple of 1 (that is, no alignment is required).
603
 
604
The expansion of the macro @code{obstack_alignment_mask} is an lvalue,
605
so you can alter the mask by assignment.  For example, this statement:
606
 
607
@smallexample
608
obstack_alignment_mask (obstack_ptr) = 0;
609
@end smallexample
610
 
611
@noindent
612
has the effect of turning off alignment processing in the specified obstack.
613
@end deftypefn
614
 
615
Note that a change in alignment mask does not take effect until
616
@emph{after} the next time an object is allocated or finished in the
617
obstack.  If you are not growing an object, you can make the new
618
alignment mask take effect immediately by calling @code{obstack_finish}.
619
This will finish a zero-length object and then do proper alignment for
620
the next object.
621
 
622
@node Obstack Chunks
623
@section Obstack Chunks
624
@cindex efficiency of chunks
625
@cindex chunks
626
 
627
Obstacks work by allocating space for themselves in large chunks, and
628
then parceling out space in the chunks to satisfy your requests.  Chunks
629
are normally 4096 bytes long unless you specify a different chunk size.
630
The chunk size includes 8 bytes of overhead that are not actually used
631
for storing objects.  Regardless of the specified size, longer chunks
632
will be allocated when necessary for long objects.
633
 
634
The obstack library allocates chunks by calling the function
635
@code{obstack_chunk_alloc}, which you must define.  When a chunk is no
636
longer needed because you have freed all the objects in it, the obstack
637
library frees the chunk by calling @code{obstack_chunk_free}, which you
638
must also define.
639
 
640
These two must be defined (as macros) or declared (as functions) in each
641
source file that uses @code{obstack_init} (@pxref{Creating Obstacks}).
642
Most often they are defined as macros like this:
643
 
644
@smallexample
645
#define obstack_chunk_alloc malloc
646
#define obstack_chunk_free free
647
@end smallexample
648
 
649
Note that these are simple macros (no arguments).  Macro definitions with
650
arguments will not work!  It is necessary that @code{obstack_chunk_alloc}
651
or @code{obstack_chunk_free}, alone, expand into a function name if it is
652
not itself a function name.
653
 
654
If you allocate chunks with @code{malloc}, the chunk size should be a
655
power of 2.  The default chunk size, 4096, was chosen because it is long
656
enough to satisfy many typical requests on the obstack yet short enough
657
not to waste too much memory in the portion of the last chunk not yet used.
658
 
659
@comment obstack.h
660
@comment GNU
661
@deftypefn Macro int obstack_chunk_size (struct obstack *@var{obstack-ptr})
662
This returns the chunk size of the given obstack.
663
@end deftypefn
664
 
665
Since this macro expands to an lvalue, you can specify a new chunk size by
666
assigning it a new value.  Doing so does not affect the chunks already
667
allocated, but will change the size of chunks allocated for that particular
668
obstack in the future.  It is unlikely to be useful to make the chunk size
669
smaller, but making it larger might improve efficiency if you are
670
allocating many objects whose size is comparable to the chunk size.  Here
671
is how to do so cleanly:
672
 
673
@smallexample
674
if (obstack_chunk_size (obstack_ptr) < @var{new-chunk-size})
675
  obstack_chunk_size (obstack_ptr) = @var{new-chunk-size};
676
@end smallexample
677
 
678
@node Summary of Obstacks
679
@section Summary of Obstack Functions
680
 
681
Here is a summary of all the functions associated with obstacks.  Each
682
takes the address of an obstack (@code{struct obstack *}) as its first
683
argument.
684
 
685
@table @code
686
@item void obstack_init (struct obstack *@var{obstack-ptr})
687
Initialize use of an obstack.  @xref{Creating Obstacks}.
688
 
689
@item void *obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
690
Allocate an object of @var{size} uninitialized bytes.
691
@xref{Allocation in an Obstack}.
692
 
693
@item void *obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
694
Allocate an object of @var{size} bytes, with contents copied from
695
@var{address}.  @xref{Allocation in an Obstack}.
696
 
697
@item void *obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
698
Allocate an object of @var{size}+1 bytes, with @var{size} of them copied
699
from @var{address}, followed by a null character at the end.
700
@xref{Allocation in an Obstack}.
701
 
702
@item void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
703
Free @var{object} (and everything allocated in the specified obstack
704
more recently than @var{object}).  @xref{Freeing Obstack Objects}.
705
 
706
@item void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
707
Add @var{size} uninitialized bytes to a growing object.
708
@xref{Growing Objects}.
709
 
710
@item void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
711
Add @var{size} bytes, copied from @var{address}, to a growing object.
712
@xref{Growing Objects}.
713
 
714
@item void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
715
Add @var{size} bytes, copied from @var{address}, to a growing object,
716
and then add another byte containing a null character.  @xref{Growing
717
Objects}.
718
 
719
@item void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{data-char})
720
Add one byte containing @var{data-char} to a growing object.
721
@xref{Growing Objects}.
722
 
723
@item void *obstack_finish (struct obstack *@var{obstack-ptr})
724
Finalize the object that is growing and return its permanent address.
725
@xref{Growing Objects}.
726
 
727
@item int obstack_object_size (struct obstack *@var{obstack-ptr})
728
Get the current size of the currently growing object.  @xref{Growing
729
Objects}.
730
 
731
@item void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
732
Add @var{size} uninitialized bytes to a growing object without checking
733
that there is enough room.  @xref{Extra Fast Growing}.
734
 
735
@item void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{data-char})
736
Add one byte containing @var{data-char} to a growing object without
737
checking that there is enough room.  @xref{Extra Fast Growing}.
738
 
739
@item int obstack_room (struct obstack *@var{obstack-ptr})
740
Get the amount of room now available for growing the current object.
741
@xref{Extra Fast Growing}.
742
 
743
@item int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
744
The mask used for aligning the beginning of an object.  This is an
745
lvalue.  @xref{Obstacks Data Alignment}.
746
 
747
@item int obstack_chunk_size (struct obstack *@var{obstack-ptr})
748
The size for allocating chunks.  This is an lvalue.  @xref{Obstack Chunks}.
749
 
750
@item void *obstack_base (struct obstack *@var{obstack-ptr})
751
Tentative starting address of the currently growing object.
752
@xref{Status of an Obstack}.
753
 
754
@item void *obstack_next_free (struct obstack *@var{obstack-ptr})
755
Address just after the end of the currently growing object.
756
@xref{Status of an Obstack}.
757
@end table
758
 

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