summaryrefslogtreecommitdiffstats
path: root/firmware/buflib.c
blob: 0a87a4c4d88b7fd196d154916de3f2b1b1ddf3fd (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
/***************************************************************************
*             __________               __   ___.
*   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
*   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
*   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
*   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
*                     \/            \/     \/    \/            \/
* $Id$
*
* This is a memory allocator designed to provide reasonable management of free
* space and fast access to allocated data. More than one allocator can be used
* at a time by initializing multiple contexts.
*
* Copyright (C) 2009 Andrew Mahone
* Copyright (C) 2011 Thomas Martitz
* 
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
* KIND, either express or implied.
*
****************************************************************************/

#include <stdlib.h> /* for abs() */
#include <stdio.h> /* for snprintf() */
#include <stddef.h> /* for ptrdiff_t */
#include "buflib.h"
#include "string-extra.h" /* strlcpy() */
#include "debug.h"
#include "panic.h"
#include "crc32.h"
#include "system.h" /* for ALIGN_*() */

/* The main goal of this design is fast fetching of the pointer for a handle.
 * For that reason, the handles are stored in a table at the end of the buffer
 * with a fixed address, so that returning the pointer for a handle is a simple
 * table lookup. To reduce the frequency with which allocated blocks will need
 * to be moved to free space, allocations grow up in address from the start of
 * the buffer. The buffer is treated as an array of union buflib_data. Blocks
 * start with a length marker, which is included in their length. Free blocks
 * are marked by negative length. Allocated blocks have a positiv length marker,
 * and additional metadata forllowing that: It follows a pointer
 * (union buflib_data*) to the corresponding handle table entry. so that it can
 * be quickly found and updated during compaction. After that follows
 * the pointer to the struct buflib_callbacks associated with this allocation
 * (may be NULL). That pointer follows a variable length character array
 * containing the nul-terminated string identifier of the allocation. After this
 * array there's a length marker for the length of the character array including
 * this length marker (counted in n*sizeof(union buflib_data)), which allows
 * to find the start of the character array (and therefore the start of the
 * entire block) when only the handle or payload start is known.
 *
 * Example:
 * |<- alloc block #1 ->|<- unalloc block ->|<- alloc block #2      ->|<-handle table->|
 * |L|H|C|cccc|L2|crc|XXXXXX|-L|YYYYYYYYYYYYYYYY|L|H|C|cc|L2|crc|XXXXXXXXXXXXX|AAA|
 *
 * L - length marker (negative if block unallocated)
 * H - handle table enry pointer
 * C - pointer to struct buflib_callbacks
 * c - variable sized string identifier
 * L2 - second length marker for string identifier
 * crc - crc32 protecting buflib cookie integrity
 * X - actual payload
 * Y - unallocated space
 * 
 * A - pointer to start of payload (first X) in the handle table (may be null)
 *
 * The blocks can be walked by jumping the abs() of the L length marker, i.e.
 * union buflib_data* L;
 * for(L = start; L < end; L += abs(L->val)) { .... }
 *
 * 
 * The allocator functions are passed a context struct so that two allocators
 * can be run, for example, one per core may be used, with convenience wrappers
 * for the single-allocator case that use a predefined context.
 */

#define B_ALIGN_DOWN(x) \
    ALIGN_DOWN(x, sizeof(union buflib_data))

#define B_ALIGN_UP(x) \
    ALIGN_UP(x, sizeof(union buflib_data))

#ifdef DEBUG
    #include <stdio.h>
    #define BDEBUGF DEBUGF
#else
    #define BDEBUGF(...) do { } while(0)
#endif

#define IS_MOVABLE(a) (!a[2].ops || a[2].ops->move_callback)
static union buflib_data* find_first_free(struct buflib_context *ctx);
static union buflib_data* find_block_before(struct buflib_context *ctx,
                                            union buflib_data* block,
                                            bool is_free);
/* Initialize buffer manager */
void
buflib_init(struct buflib_context *ctx, void *buf, size_t size)
{
    union buflib_data *bd_buf = buf;

    /* Align on sizeof(buflib_data), to prevent unaligned access */
    ALIGN_BUFFER(bd_buf, size, sizeof(union buflib_data));
    size /= sizeof(union buflib_data);
    /* The handle table is initialized with no entries */
    ctx->handle_table = bd_buf + size;
    ctx->last_handle = bd_buf + size;
    ctx->first_free_handle = bd_buf + size - 1;
    ctx->buf_start = bd_buf;
    /* A marker is needed for the end of allocated data, to make sure that it
     * does not collide with the handle table, and to detect end-of-buffer.
     */
    ctx->alloc_end = bd_buf;
    ctx->compact = true;

    BDEBUGF("buflib initialized with %lu.%2lu kiB",
            (unsigned long)size / 1024, ((unsigned long)size%1000)/10);
}

bool buflib_context_relocate(struct buflib_context *ctx, void *buf)
{
    union buflib_data *handle, *bd_buf = buf;
    ptrdiff_t diff = bd_buf - ctx->buf_start;

    /* cannot continue if the buffer is not aligned, since we would need
     * to reduce the size of the buffer for aligning */
    if ((uintptr_t)buf & 0x3)
        return false;

    /* relocate the handle table entries  */
    for (handle = ctx->last_handle; handle < ctx->handle_table; handle++)
    {
        if (handle->alloc)
            handle->alloc += diff * sizeof(union buflib_data);
    }
    /* relocate the pointers in the context */
    ctx->handle_table       += diff;
    ctx->last_handle        += diff;
    ctx->first_free_handle  += diff;
    ctx->buf_start          += diff;
    ctx->alloc_end          += diff;

    return true;
}

/* Allocate a new handle, returning 0 on failure */
static inline
union buflib_data* handle_alloc(struct buflib_context *ctx)
{
    union buflib_data *handle;
    /* first_free_handle is a lower bound on free handles, work through the
     * table from there until a handle containing NULL is found, or the end
     * of the table is reached.
     */
    for (handle = ctx->first_free_handle; handle >= ctx->last_handle; handle--)
        if (!handle->alloc)
            break;
    /* If the search went past the end of the table, it means we need to extend
     * the table to get a new handle.
     */
    if (handle < ctx->last_handle)
    {
        if (handle >= ctx->alloc_end)
            ctx->last_handle--;
        else
            return NULL;
    }
    handle->val = -1;
    return handle;
}

/* Free one handle, shrinking the handle table if it's the last one */
static inline
void handle_free(struct buflib_context *ctx, union buflib_data *handle)
{
    handle->alloc = 0;
    /* Update free handle lower bound if this handle has a lower index than the
     * old one.
     */
    if (handle > ctx->first_free_handle)
        ctx->first_free_handle = handle;
    if (handle == ctx->last_handle)
        ctx->last_handle++;
    else
        ctx->compact = false;
}

/* Get the start block of an allocation */
static union buflib_data* handle_to_block(struct buflib_context* ctx, int handle)
{
    union buflib_data* name_field =
                (union buflib_data*)buflib_get_name(ctx, handle);

    return name_field ? name_field - 3 : NULL;
}

/* Shrink the handle table, returning true if its size was reduced, false if
 * not
 */
static inline
bool
handle_table_shrink(struct buflib_context *ctx)
{
    bool rv;
    union buflib_data *handle;
    for (handle = ctx->last_handle; !(handle->alloc); handle++);
    if (handle > ctx->first_free_handle)
        ctx->first_free_handle = handle - 1;
    rv = handle != ctx->last_handle;
    ctx->last_handle = handle;
    return rv;
}


/* If shift is non-zero, it represents the number of places to move
 * blocks in memory. Calculate the new address for this block,
 * update its entry in the handle table, and then move its contents.
 *
 * Returns false if moving was unsucessful
 * (NULL callback or BUFLIB_CB_CANNOT_MOVE was returned)
 */
static bool
move_block(struct buflib_context* ctx, union buflib_data* block, int shift)
{
    char* new_start;
    union buflib_data *new_block, *tmp = block[1].handle, *crc_slot;
    struct buflib_callbacks *ops = block[2].ops;
    crc_slot = (union buflib_data*)tmp->alloc - 1;
    int cookie_size = (crc_slot - block)*sizeof(union buflib_data);
    uint32_t crc = crc_32((void *)block, cookie_size, 0xffffffff);

    /* check for cookie validity */
    if (crc != crc_slot->crc)
        panicf("buflib cookie corrupted, crc: 0x%08x, expected: 0x%08x",
               (unsigned int)crc, (unsigned int)crc_slot->crc);

    if (!IS_MOVABLE(block))
        return false;

    int handle = ctx->handle_table - tmp;
    BDEBUGF("%s(): moving \"%s\"(id=%d) by %d(%d)\n", __func__, block[3].name,
            handle, shift, shift*(int)sizeof(union buflib_data));
    new_block = block + shift;
    new_start = tmp->alloc + shift*sizeof(union buflib_data);

    /* If move must be synchronized with use, user should have specified a
       callback that handles this */
    if (ops && ops->sync_callback)
        ops->sync_callback(handle, true);

    bool retval = false;
    if (!ops || ops->move_callback(handle, tmp->alloc, new_start)
                    != BUFLIB_CB_CANNOT_MOVE)
    {
        tmp->alloc = new_start; /* update handle table */
        memmove(new_block, block, block->val * sizeof(union buflib_data));
        retval = true;
    }

    if (ops && ops->sync_callback)
        ops->sync_callback(handle, false);

    return retval;
}

/* Compact allocations and handle table, adjusting handle pointers as needed.
 * Return true if any space was freed or consolidated, false otherwise.
 */
static bool
buflib_compact(struct buflib_context *ctx)
{
    BDEBUGF("%s(): Compacting!\n", __func__);
    union buflib_data *block,
                      *hole = NULL;
    int shift = 0, len;
    /* Store the results of attempting to shrink the handle table */
    bool ret = handle_table_shrink(ctx);
    /* compaction has basically two modes of operation:
     *  1) the buffer is nicely movable: In this mode, blocks can be simply
     * moved towards the beginning. Free blocks add to a shift value,
     * which is the amount to move.
     *  2) the buffer contains unmovable blocks: unmovable blocks create
     * holes and reset shift. Once a hole is found, we're trying to fill
     * holes first, moving by shift is the fallback. As the shift is reset,
     * this effectively splits the buffer into portions of movable blocks.
     * This mode cannot be used if no holes are found yet as it only works
     * when it moves blocks across the portions. On the other side,
     * moving by shift only works within the same portion
     * For simplicity only 1 hole at a time is considered */
    for(block = find_first_free(ctx); block < ctx->alloc_end; block += len)
    {
        bool movable = true; /* cache result to avoid 2nd call to move_block */
        len = block->val;
        /* This block is free, add its length to the shift value */
        if (len < 0)
        {
            shift += len;
            len = -len;
            continue;
        }
        /* attempt to fill any hole */
        if (hole && -hole->val >= len)
        {
            intptr_t hlen = -hole->val;
            if ((movable = move_block(ctx, block, hole - block)))
            {
                ret = true;
                /* Move was successful. The memory at block is now free */
                block->val = -len;

                /* add its length to shift */
                shift += -len;
                /* Reduce the size of the hole accordingly
                 * but be careful to not overwrite an existing block */
                if (hlen != len)
                {
                    hole += len;
                    hole->val = len - hlen; /* negative */
                }
                else /* hole closed */
                    hole = NULL;
                continue;
            }
        }
        /* attempt move the allocation by shift */
        if (shift)
        {
            union buflib_data* target_block = block + shift;
            if (!movable || !move_block(ctx, block, shift))
            {
                /* free space before an unmovable block becomes a hole,
                 * therefore mark this block free and track the hole */
                target_block->val = shift;
                hole = target_block;
                shift = 0;
            }
            else
                ret = true;
        }
    }
    /* Move the end-of-allocation mark, and return true if any new space has
     * been freed.
     */
    ctx->alloc_end += shift;
    ctx->compact = true;
    return ret || shift;
}

/* Compact the buffer by trying both shrinking and moving.
 *
 * Try to move first. If unsuccesfull, try to shrink. If that was successful
 * try to move once more as there might be more room now.
 */
static bool
buflib_compact_and_shrink(struct buflib_context *ctx, unsigned shrink_hints)
{
    bool result = false;
    /* if something compacted before already there will be no further gain */
    if (!ctx->compact)
        result = buflib_compact(ctx);
    if (!result)
    {
        union buflib_data *this, *before;
        for(this = ctx->buf_start, before = this;
            this < ctx->alloc_end;
            before = this, this += abs(this->val))
        {
            if (this->val > 0 && this[2].ops
                              && this[2].ops->shrink_callback)
            {
                int ret;
                int handle = ctx->handle_table - this[1].handle;
                char* data = this[1].handle->alloc;
                bool last = (this+this->val) == ctx->alloc_end;
                unsigned pos_hints = shrink_hints & BUFLIB_SHRINK_POS_MASK;
                /* adjust what we ask for if there's free space in the front
                 * this isn't too unlikely assuming this block is
                 * shrinkable but not movable */
                if (pos_hints == BUFLIB_SHRINK_POS_FRONT
                    && before != this && before->val < 0)
                {   
                    size_t free_space = (-before->val) * sizeof(union buflib_data);
                    size_t wanted = shrink_hints & BUFLIB_SHRINK_SIZE_MASK;
                    if (wanted < free_space) /* no shrink needed? */
                        continue;
                    wanted -= free_space;
                    shrink_hints = pos_hints | wanted;
                }
                ret = this[2].ops->shrink_callback(handle, shrink_hints,
                                            data, (char*)(this+this->val)-data);
                result |= (ret == BUFLIB_CB_OK);
                /* 'this' might have changed in the callback (if it shrinked
                 * from the top or even freed the handle), get it again */
                this = handle_to_block(ctx, handle);
                /* The handle was possibly be freed in the callback,
                 * re-run the loop with the handle before */
                if (!this)
                    this = before;
                /* could also change with shrinking from back */
                else if (last)
                    ctx->alloc_end = this + this->val;
            }
        }
        /* shrinking was successful at least once, try compaction again */
        if (result)
            result |= buflib_compact(ctx);
    }

    return result;
}

/* Shift buffered items by size units, and update handle pointers. The shift
 * value must be determined to be safe *before* calling.
 */
static void
buflib_buffer_shift(struct buflib_context *ctx, int shift)
{
    memmove(ctx->buf_start + shift, ctx->buf_start,
        (ctx->alloc_end - ctx->buf_start) * sizeof(union buflib_data));
    ctx->buf_start += shift;
    ctx->alloc_end += shift;
    shift *= sizeof(union buflib_data);
    union buflib_data *handle;
    for (handle = ctx->last_handle; handle < ctx->handle_table; handle++)
        if (handle->alloc)
            handle->alloc += shift;
}

/* Shift buffered items up by size bytes, or as many as possible if size == 0.
 * Set size to the number of bytes freed.
 */
void*
buflib_buffer_out(struct buflib_context *ctx, size_t *size)
{
    if (!ctx->compact)
        buflib_compact(ctx);
    size_t avail = ctx->last_handle - ctx->alloc_end;
    size_t avail_b = avail * sizeof(union buflib_data);
    if (*size && *size < avail_b)
    {
        avail = (*size + sizeof(union buflib_data) - 1)
            / sizeof(union buflib_data);
        avail_b = avail * sizeof(union buflib_data);
    }
    *size = avail_b;
    void *ret = ctx->buf_start;
    buflib_buffer_shift(ctx, avail);
    return ret;
}

/* Shift buffered items down by size bytes */
void
buflib_buffer_in(struct buflib_context *ctx, int size)
{
    size /= sizeof(union buflib_data);
    buflib_buffer_shift(ctx, -size);
}

/* Allocate a buffer of size bytes, returning a handle for it */
int
buflib_alloc(struct buflib_context *ctx, size_t size)
{
    return buflib_alloc_ex(ctx, size, "<anonymous>", NULL);
}

/* Allocate a buffer of size bytes, returning a handle for it.
 *
 * The additional name parameter gives the allocation a human-readable name,
 * the ops parameter points to caller-implemented callbacks for moving and
 * shrinking. NULL for default callbacks (which do nothing but don't
 * prevent moving or shrinking)
 */

int
buflib_alloc_ex(struct buflib_context *ctx, size_t size, const char *name,
                struct buflib_callbacks *ops)
{
    union buflib_data *handle, *block;
    size_t name_len = name ? B_ALIGN_UP(strlen(name)+1) : 0;
    bool last;
    /* This really is assigned a value before use */
    int block_len;
    size += name_len;
    size = (size + sizeof(union buflib_data) - 1) /
           sizeof(union buflib_data)
           /* add 5 objects for alloc len, pointer to handle table entry and
            * name length, the ops pointer and crc */
           + 5;
handle_alloc:
    handle = handle_alloc(ctx);
    if (!handle)
    {
        /* If allocation has failed, and compaction has succeded, it may be
         * possible to get a handle by trying again.
         */
        union buflib_data* last_block = find_block_before(ctx,
                                            ctx->alloc_end, false);
        struct buflib_callbacks* ops = last_block[2].ops;
        unsigned hints = 0;
        if (!ops || !ops->shrink_callback)
        {   /* the last one isn't shrinkable
             * make room in front of a shrinkable and move this alloc */
            hints = BUFLIB_SHRINK_POS_FRONT;
            hints |= last_block->val * sizeof(union buflib_data);
        }
        else if (ops && ops->shrink_callback)
        {   /* the last is shrinkable, make room for handles directly */
            hints = BUFLIB_SHRINK_POS_BACK;
            hints |= 16*sizeof(union buflib_data);
        }
        /* buflib_compact_and_shrink() will compact and move last_block()
         * if possible */
        if (buflib_compact_and_shrink(ctx, hints))
            goto handle_alloc;
        return -1;
    }

buffer_alloc:
    /* need to re-evaluate last before the loop because the last allocation
     * possibly made room in its front to fit this, so last would be wrong */
    last = false;
    for (block = find_first_free(ctx);;block += block_len)
    {
        /* If the last used block extends all the way to the handle table, the
         * block "after" it doesn't have a header. Because of this, it's easier
         * to always find the end of allocation by saving a pointer, and always
         * calculate the free space at the end by comparing it to the
         * last_handle pointer.
         */
        if(block == ctx->alloc_end)
        {
            last = true;
            block_len = ctx->last_handle - block;
            if ((size_t)block_len < size)
                block = NULL;
            break;
        }
        block_len = block->val;
        /* blocks with positive length are already allocated. */
        if(block_len > 0)
            continue;
        block_len = -block_len;
        /* The search is first-fit, any fragmentation this causes will be
         * handled at compaction.
         */
        if ((size_t)block_len >= size)
            break;
    }
    if (!block)
    {
        /* Try compacting if allocation failed */
        unsigned hint = BUFLIB_SHRINK_POS_FRONT |
                    ((size*sizeof(union buflib_data))&BUFLIB_SHRINK_SIZE_MASK);
        if (buflib_compact_and_shrink(ctx, hint))
        {
            goto buffer_alloc;
        } else {
            handle->val=1;
            handle_free(ctx, handle);
            return -2;
        }
    }

    /* Set up the allocated block, by marking the size allocated, and storing
     * a pointer to the handle.
     */
    union buflib_data *name_len_slot, *crc_slot;
    block->val = size;
    block[1].handle = handle;
    block[2].ops = ops;
    strcpy(block[3].name, name);
    name_len_slot = (union buflib_data*)B_ALIGN_UP(block[3].name + name_len);
    name_len_slot->val = 1 + name_len/sizeof(union buflib_data);
    crc_slot = (union buflib_data*)(name_len_slot + 1);
    crc_slot->crc = crc_32((void *)block,
                           (crc_slot - block)*sizeof(union buflib_data),
                           0xffffffff);
    handle->alloc = (char*)(crc_slot + 1);

    BDEBUGF("buflib_alloc_ex: size=%d handle=%p clb=%p crc=0x%0x name=\"%s\"\n",
            (unsigned int)size, (void *)handle, (void *)ops,
            (unsigned int)crc_slot->crc, block[3].name);

    block += size;
    /* alloc_end must be kept current if we're taking the last block. */
    if (last)
        ctx->alloc_end = block;
    /* Only free blocks *before* alloc_end have tagged length. */
    else if ((size_t)block_len > size)
        block->val = size - block_len;
    /* Return the handle index as a positive integer. */
    return ctx->handle_table - handle;
}

static union buflib_data*
find_first_free(struct buflib_context *ctx)
{
    union buflib_data* ret = ctx->buf_start;
    while(ret < ctx->alloc_end)
    {
        if (ret->val < 0)
            break;
        ret += ret->val;
    }
    /* ret is now either a free block or the same as alloc_end, both is fine */
    return ret;
}

/* Finds the free block before block, and returns NULL if it's not free */
static union buflib_data*
find_block_before(struct buflib_context *ctx, union buflib_data* block,
                  bool is_free)
{
    union buflib_data *ret        = ctx->buf_start,
                      *next_block = ret;

    /* find the block that's before the current one */
    while (next_block < block)
    {
        ret = next_block;
        next_block += abs(ret->val);
    }

    /* If next_block == block, the above loop didn't go anywhere. If it did,
     * and the block before this one is empty, that is the wanted one
     */
    if (next_block == block && ret < block)
    {
        if (is_free && ret->val >= 0) /* NULL if found block isn't free */
            return NULL;
        return ret;
    }
    return NULL;
}

/* Free the buffer associated with handle_num. */
int
buflib_free(struct buflib_context *ctx, int handle_num)
{
    union buflib_data *handle = ctx->handle_table - handle_num,
                      *freed_block = handle_to_block(ctx, handle_num),
                      *block, *next_block;
    /* We need to find the block before the current one, to see if it is free
     * and can be merged with this one.
     */
    block = find_block_before(ctx, freed_block, true);
    if (block)
    {
        block->val -= freed_block->val;
    }
    else
    {
    /* Otherwise, set block to the newly-freed block, and mark it free, before
     * continuing on, since the code below exects block to point to a free
     * block which may have free space after it.
     */
        block = freed_block;
        block->val = -block->val;
    }
    next_block = block - block->val;
    /* Check if we are merging with the free space at alloc_end. */
    if (next_block == ctx->alloc_end)
        ctx->alloc_end = block;
    /* Otherwise, the next block might still be a "normal" free block, and the
     * mid-allocation free means that the buffer is no longer compact.
     */
    else {
        ctx->compact = false;
        if (next_block->val < 0)
            block->val += next_block->val;
    }
    handle_free(ctx, handle);
    handle->alloc = NULL;

    return 0; /* unconditionally */
}

static size_t
free_space_at_end(struct buflib_context* ctx)
{
    /* subtract 5 elements for
     * val, handle, name_len, ops and the handle table entry*/
    ptrdiff_t diff = (ctx->last_handle - ctx->alloc_end - 5);
    diff -= 16; /* space for future handles */
    diff *= sizeof(union buflib_data); /* make it bytes */
    diff -= 16; /* reserve 16 for the name */

    if (diff > 0)
        return diff;
    else
        return 0;
}

/* Return the maximum allocatable contiguous memory in bytes */
size_t
buflib_allocatable(struct buflib_context* ctx)
{
    union buflib_data *this;
    size_t free_space = 0, max_free_space = 0;

    /* make sure buffer is as contiguous as possible  */
    if (!ctx->compact)
        buflib_compact(ctx);

    /* now look if there's free in holes */
    for(this = find_first_free(ctx); this < ctx->alloc_end; this += abs(this->val))
    {
        if (this->val < 0)
        {
            free_space += -this->val;
            continue;
        }
        /* an unmovable section resets the count as free space
         * can't be contigous */
        if (!IS_MOVABLE(this))
        {
            if (max_free_space < free_space)
                max_free_space = free_space;
            free_space = 0;
        }
    }

    /* select the best */
    max_free_space = MAX(max_free_space, free_space);
    max_free_space *= sizeof(union buflib_data);
    max_free_space = MAX(max_free_space, free_space_at_end(ctx));

    if (max_free_space > 0)
        return max_free_space;
    else
        return 0;
}

/* Return the amount of unallocated memory in bytes (even if not contiguous) */
size_t
buflib_available(struct buflib_context* ctx)
{
    union buflib_data *this;
    size_t free_space = 0;

    /* now look if there's free in holes */
    for(this = find_first_free(ctx); this < ctx->alloc_end; this += abs(this->val))
    {
        if (this->val < 0)
        {
            free_space += -this->val;
            continue;
        }
    }

    free_space *= sizeof(union buflib_data); /* make it bytes */
    free_space += free_space_at_end(ctx);

    return free_space;
}

/*
 * Allocate all available (as returned by buflib_available()) memory and return
 * a handle to it
 *
 * This grabs a lock which can only be unlocked by buflib_free() or
 * buflib_shrink(), to protect from further allocations (which couldn't be
 * serviced anyway).
 */
int
buflib_alloc_maximum(struct buflib_context* ctx, const char* name, size_t *size, struct buflib_callbacks *ops)
{
    /* limit name to 16 since that's what buflib_available() accounts for it */
    char buf[16];

    /* ignore ctx->compact because it's true if all movable blocks are contiguous
     * even if the buffer has holes due to unmovable allocations */
    unsigned hints;
    size_t bufsize = ctx->handle_table - ctx->buf_start;
    bufsize = MIN(BUFLIB_SHRINK_SIZE_MASK, bufsize*sizeof(union buflib_data)); /* make it bytes */
    /* try as hard as possible to free up space. allocations are
     * welcome to give up some or all of their memory */
    hints = BUFLIB_SHRINK_POS_BACK | BUFLIB_SHRINK_POS_FRONT | bufsize;
    /* compact until no space can be gained anymore */
    while (buflib_compact_and_shrink(ctx, hints));

    *size = buflib_allocatable(ctx);
    if (*size <= 0) /* OOM */
        return -1;

    strlcpy(buf, name, sizeof(buf));

    return buflib_alloc_ex(ctx, *size, buf, ops);
}

/* Shrink the allocation indicated by the handle according to new_start and
 * new_size. Grow is not possible, therefore new_start and new_start + new_size
 * must be within the original allocation
 */
bool
buflib_shrink(struct buflib_context* ctx, int handle, void* new_start, size_t new_size)
{
    union buflib_data *crc_slot;
    int cookie_size;
    char* oldstart = buflib_get_data(ctx, handle);
    char* newstart = new_start;
    char* newend = newstart + new_size;

    /* newstart must be higher and new_size not "negative" */
    if (newstart < oldstart || newend < newstart)
        return false;
    union buflib_data *block = handle_to_block(ctx, handle),
                      *old_next_block = block + block->val,
                /* newstart isn't necessarily properly aligned but it
                 * needn't be since it's only dereferenced by the user code */
                      *aligned_newstart = (union buflib_data*)B_ALIGN_DOWN(newstart),
                      *aligned_oldstart = (union buflib_data*)B_ALIGN_DOWN(oldstart),
                      *new_next_block =   (union buflib_data*)B_ALIGN_UP(newend),
                      *new_block, metadata_size;

    /* growing is not supported */
    if (new_next_block > old_next_block)
        return false;

    metadata_size.val = aligned_oldstart - block;
    /* update val and the handle table entry */
    new_block = aligned_newstart - metadata_size.val;
    block[0].val = new_next_block - new_block;

    block[1].handle->alloc = newstart;
    if (block != new_block)
    {
        /* move metadata over, i.e. pointer to handle table entry and name
         * This is actually the point of no return. Data in the allocation is
         * being modified, and therefore we must successfully finish the shrink
         * operation */
        memmove(new_block, block, metadata_size.val*sizeof(metadata_size));
        /* mark the old block unallocated */
        block->val = block - new_block;
        /* find the block before in order to merge with the new free space */
        union buflib_data *free_before = find_block_before(ctx, block, true);
        if (free_before)
            free_before->val += block->val;

        /* We didn't handle size changes yet, assign block to the new one
         * the code below the wants block whether it changed or not */
        block = new_block;
    }

    /* update crc of the cookie */
    crc_slot = (union buflib_data*)new_block[1].handle->alloc - 1;
    cookie_size = (crc_slot - new_block)*sizeof(union buflib_data);
    crc_slot->crc = crc_32((void *)new_block, cookie_size, 0xffffffff);

    /* Now deal with size changes that create free blocks after the allocation */
    if (old_next_block != new_next_block)
    {
        if (ctx->alloc_end == old_next_block)
            ctx->alloc_end = new_next_block;
        else if (old_next_block->val < 0)
        {   /* enlarge next block by moving it up */
            new_next_block->val = old_next_block->val - (old_next_block - new_next_block);
        }
        else if (old_next_block != new_next_block)
        {   /* creating a hole */
            /* must be negative to indicate being unallocated */
            new_next_block->val = new_next_block - old_next_block;
        }
    }

    return true;
}

const char* buflib_get_name(struct buflib_context *ctx, int handle)
{
    union buflib_data *data = ALIGN_DOWN(buflib_get_data(ctx, handle), sizeof (*data));
    if (!data)
        return NULL;
    size_t len = data[-2].val;
    if (len <= 1)
        return NULL;
    return data[-len-1].name;
}

#ifdef DEBUG
void buflib_check_valid(struct buflib_context *ctx)
{
    union buflib_data *crc_slot;
    int cookie_size;
    uint32_t crc;

    for(union buflib_data* this = ctx->buf_start;
                           this < ctx->alloc_end;
                           this += abs(this->val))
    {
        if (this->val < 0)
            continue;

        crc_slot = (union buflib_data*)
                       ((union buflib_data*)this[1].handle)->alloc - 1;
        cookie_size = (crc_slot - this)*sizeof(union buflib_data);
        crc = crc_32((void *)this, cookie_size, 0xffffffff);

        if (crc != crc_slot->crc)
            panicf("buflib check crc: 0x%08x, expected: 0x%08x",
                   (unsigned int)crc, (unsigned int)crc_slot->crc);
    }
}
#endif

#ifdef BUFLIB_DEBUG_BLOCKS
void buflib_print_allocs(struct buflib_context *ctx,
                                        void (*print)(int, const char*))
{
    union buflib_data *this, *end = ctx->handle_table;
    char buf[128];
    for(this = end - 1; this >= ctx->last_handle; this--)
    {
        if (!this->alloc) continue;

        int handle_num;
        const char *name;
        union buflib_data *block_start, *alloc_start;
        intptr_t alloc_len;

        handle_num = end - this;
        alloc_start = buflib_get_data(ctx, handle_num);
        name = buflib_get_name(ctx, handle_num);
        block_start = (union buflib_data*)name - 3;
        alloc_len = block_start->val * sizeof(union buflib_data);

        snprintf(buf, sizeof(buf),
                "%s(%d):\t%p\n"
                "   \t%p\n"
                "   \t%ld\n",
                name?:"(null)", handle_num, block_start, alloc_start, alloc_len);
        /* handle_num is 1-based */
        print(handle_num - 1, buf);
    }
}

void buflib_print_blocks(struct buflib_context *ctx,
                                        void (*print)(int, const char*))
{
    char buf[128];
    int i = 0;
    for(union buflib_data* this = ctx->buf_start;
                           this < ctx->alloc_end;
                           this += abs(this->val))
    {
        snprintf(buf, sizeof(buf), "%8p: val: %4ld (%s)",
                                this, this->val,
                                this->val > 0? this[3].name:"<unallocated>");
        print(i++, buf);
    }
}
#endif

#ifdef BUFLIB_DEBUG_BLOCK_SINGLE
int buflib_get_num_blocks(struct buflib_context *ctx)
{
    int i = 0;
    for(union buflib_data* this = ctx->buf_start;
                           this < ctx->alloc_end;
                           this += abs(this->val))
    {
        i++;
    }
    return i;
}

void buflib_print_block_at(struct buflib_context *ctx, int block_num,
                                char* buf, size_t bufsize)
{
    union buflib_data* this = ctx->buf_start;
    while(block_num > 0 && this < ctx->alloc_end)
    {
        this += abs(this->val);
        block_num -= 1;
    }
    snprintf(buf, bufsize, "%8p: val: %4ld (%s)",
                            this, (long)this->val,
                            this->val > 0? this[3].name:"<unallocated>");
}

#endif