src/util/ralloc.c

   1 /*
   2  * Copyright © 2010 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  21  * DEALINGS IN THE SOFTWARE.
  22  */
  23
  24 #include <assert.h>
  25 #include <stdlib.h>
  26 #include <stdarg.h>
  27 #include <stdio.h>
  28 #include <string.h>
  29 #include <stdint.h>
  30
  31 /* Android defines SIZE_MAX in limits.h, instead of the standard stdint.h */
  32 #ifdef ANDROID
  33 #include <limits.h>
  34 #endif
  35
  36 /* Some versions of MinGW are missing _vscprintf's declaration, although they
  37  * still provide the symbol in the import library. */
  38 #ifdef __MINGW32__
  39 _CRTIMP int _vscprintf(const char *format, va_list argptr);
  40 #endif
  41
  42 #include "ralloc.h"
  43
  44 #ifndef va_copy
  45 #ifdef __va_copy
  46 #define va_copy(dest, src) __va_copy((dest), (src))
  47 #else
  48 #define va_copy(dest, src) (dest) = (src)
  49 #endif
  50 #endif
  51
  52 #define CANARY 0x5A1106
  53
  54 struct ralloc_header
  55 {
  56 #ifdef DEBUG
  57    /* A canary value used to determine whether a pointer is ralloc'd. */
  58    unsigned canary;
  59 #endif
  60
  61    struct ralloc_header *parent;
  62
  63    /* The first child (head of a linked list) */
  64    struct ralloc_header *child;
  65
  66    /* Linked list of siblings */
  67    struct ralloc_header *prev;
  68    struct ralloc_header *next;
  69
  70    void (*destructor)(void *);
  71 };
  72
  73 typedef struct ralloc_header ralloc_header;
  74
  75 static void unlink_block(ralloc_header *info);
  76 static void unsafe_free(ralloc_header *info);
  77
  78 static ralloc_header *
  79 get_header(const void *ptr)
  80 {
  81    ralloc_header *info = (ralloc_header *) (((char *) ptr) -
  82                                             sizeof(ralloc_header));
  83 #ifdef DEBUG
  84    assert(info->canary == CANARY);
  85 #endif
  86    return info;
  87 }
  88
  89 #define PTR_FROM_HEADER(info) (((char *) info) + sizeof(ralloc_header))
  90
  91 static void
  92 add_child(ralloc_header *parent, ralloc_header *info)
  93 {
  94    if (parent != NULL) {
  95       info->parent = parent;
  96       info->next = parent->child;
  97       parent->child = info;
  98
  99       if (info->next != NULL)
 100          info->next->prev = info;
 101    }
 102 }
 103
 104 void *
 105 ralloc_context(const void *ctx)
 106 {
 107    return ralloc_size(ctx, 0);
 108 }
 109
 110 void *
 111 ralloc_size(const void *ctx, size_t size)
 112 {
 113    void *block = malloc(size + sizeof(ralloc_header));
 114    ralloc_header *info;
 115    ralloc_header *parent;
 116
 117    if (unlikely(block == NULL))
 118       return NULL;
 119
 120    info = (ralloc_header *) block;
 121    /* measurements have shown that calloc is slower (because of
 122     * the multiplication overflow checking?), so clear things
 123     * manually
 124     */
 125    info->parent = NULL;
 126    info->child = NULL;
 127    info->prev = NULL;
 128    info->next = NULL;
 129    info->destructor = NULL;
 130
 131    parent = ctx != NULL ? get_header(ctx) : NULL;
 132
 133    add_child(parent, info);
 134
 135 #ifdef DEBUG
 136    info->canary = CANARY;
 137 #endif
 138
 139    return PTR_FROM_HEADER(info);
 140 }
 141
 142 void *
 143 rzalloc_size(const void *ctx, size_t size)
 144 {
 145    void *ptr = ralloc_size(ctx, size);
 146
 147    if (likely(ptr))
 148       memset(ptr, 0, size);
 149
 150    return ptr;
 151 }
 152
 153 /* helper function - assumes ptr != NULL */
 154 static void *
 155 resize(void *ptr, size_t size)
 156 {
 157    ralloc_header *child, *old, *info;
 158
 159    old = get_header(ptr);
 160    info = realloc(old, size + sizeof(ralloc_header));
 161
 162    if (info == NULL)
 163       return NULL;
 164
 165    /* Update parent and sibling's links to the reallocated node. */
 166    if (info != old && info->parent != NULL) {
 167       if (info->parent->child == old)
 168          info->parent->child = info;
 169
 170       if (info->prev != NULL)
 171          info->prev->next = info;
 172
 173       if (info->next != NULL)
 174          info->next->prev = info;
 175    }
 176
 177    /* Update child->parent links for all children */
 178    for (child = info->child; child != NULL; child = child->next)
 179       child->parent = info;
 180
 181    return PTR_FROM_HEADER(info);
 182 }
 183
 184 void *
 185 reralloc_size(const void *ctx, void *ptr, size_t size)
 186 {
 187    if (unlikely(ptr == NULL))
 188       return ralloc_size(ctx, size);
 189
 190    assert(ralloc_parent(ptr) == ctx);
 191    return resize(ptr, size);
 192 }
 193
 194 void *
 195 ralloc_array_size(const void *ctx, size_t size, unsigned count)
 196 {
 197    if (count > SIZE_MAX/size)
 198       return NULL;
 199
 200    return ralloc_size(ctx, size * count);
 201 }
 202
 203 void *
 204 rzalloc_array_size(const void *ctx, size_t size, unsigned count)
 205 {
 206    if (count > SIZE_MAX/size)
 207       return NULL;
 208
 209    return rzalloc_size(ctx, size * count);
 210 }
 211
 212 void *
 213 reralloc_array_size(const void *ctx, void *ptr, size_t size, unsigned count)
 214 {
 215    if (count > SIZE_MAX/size)
 216       return NULL;
 217
 218    return reralloc_size(ctx, ptr, size * count);
 219 }
 220
 221 void
 222 ralloc_free(void *ptr)
 223 {
 224    ralloc_header *info;
 225
 226    if (ptr == NULL)
 227       return;
 228
 229    info = get_header(ptr);
 230    unlink_block(info);
 231    unsafe_free(info);
 232 }
 233
 234 static void
 235 unlink_block(ralloc_header *info)
 236 {
 237    /* Unlink from parent & siblings */
 238    if (info->parent != NULL) {
 239       if (info->parent->child == info)
 240          info->parent->child = info->next;
 241
 242       if (info->prev != NULL)
 243          info->prev->next = info->next;
 244
 245       if (info->next != NULL)
 246          info->next->prev = info->prev;
 247    }
 248    info->parent = NULL;
 249    info->prev = NULL;
 250    info->next = NULL;
 251 }
 252
 253 static void
 254 unsafe_free(ralloc_header *info)
 255 {
 256    /* Recursively free any children...don't waste time unlinking them. */
 257    ralloc_header *temp;
 258    while (info->child != NULL) {
 259       temp = info->child;
 260       info->child = temp->next;
 261       unsafe_free(temp);
 262    }
 263
 264    /* Free the block itself.  Call the destructor first, if any. */
 265    if (info->destructor != NULL)
 266       info->destructor(PTR_FROM_HEADER(info));
 267
 268    free(info);
 269 }
 270
 271 void
 272 ralloc_steal(const void *new_ctx, void *ptr)
 273 {
 274    ralloc_header *info, *parent;
 275
 276    if (unlikely(ptr == NULL))
 277       return;
 278
 279    info = get_header(ptr);
 280    parent = get_header(new_ctx);
 281
 282    unlink_block(info);
 283
 284    add_child(parent, info);
 285 }
 286
 287 void
 288 ralloc_adopt(const void *new_ctx, void *old_ctx)
 289 {
 290    ralloc_header *new_info, *old_info, *child;
 291
 292    if (unlikely(old_ctx == NULL))
 293       return;
 294
 295    old_info = get_header(old_ctx);
 296    new_info = get_header(new_ctx);
 297
 298    /* If there are no children, bail. */
 299    if (unlikely(old_info->child == NULL))
 300       return;
 301
 302    /* Set all the children's parent to new_ctx; get a pointer to the last child. */
 303    for (child = old_info->child; child->next != NULL; child = child->next) {
 304       child->parent = new_info;
 305    }
 306
 307    /* Connect the two lists together; parent them to new_ctx; make old_ctx empty. */
 308    child->next = new_info->child;
 309    child->parent = new_info;
 310    new_info->child = old_info->child;
 311    old_info->child = NULL;
 312 }
 313
 314 void *
 315 ralloc_parent(const void *ptr)
 316 {
 317    ralloc_header *info;
 318
 319    if (unlikely(ptr == NULL))
 320       return NULL;
 321
 322    info = get_header(ptr);
 323    return info->parent ? PTR_FROM_HEADER(info->parent) : NULL;
 324 }
 325
 326 void
 327 ralloc_set_destructor(const void *ptr, void(*destructor)(void *))
 328 {
 329    ralloc_header *info = get_header(ptr);
 330    info->destructor = destructor;
 331 }
 332
 333 char *
 334 ralloc_strdup(const void *ctx, const char *str)
 335 {
 336    size_t n;
 337    char *ptr;
 338
 339    if (unlikely(str == NULL))
 340       return NULL;
 341
 342    n = strlen(str);
 343    ptr = ralloc_array(ctx, char, n + 1);
 344    memcpy(ptr, str, n);
 345    ptr[n] = '\0';
 346    return ptr;
 347 }
 348
 349 char *
 350 ralloc_strndup(const void *ctx, const char *str, size_t max)
 351 {
 352    size_t n;
 353    char *ptr;
 354
 355    if (unlikely(str == NULL))
 356       return NULL;
 357
 358    n = strnlen(str, max);
 359    ptr = ralloc_array(ctx, char, n + 1);
 360    memcpy(ptr, str, n);
 361    ptr[n] = '\0';
 362    return ptr;
 363 }
 364
 365 /* helper routine for strcat/strncat - n is the exact amount to copy */
 366 static bool
 367 cat(char **dest, const char *str, size_t n)
 368 {
 369    char *both;
 370    size_t existing_length;
 371    assert(dest != NULL && *dest != NULL);
 372
 373    existing_length = strlen(*dest);
 374    both = resize(*dest, existing_length + n + 1);
 375    if (unlikely(both == NULL))
 376       return false;
 377
 378    memcpy(both + existing_length, str, n);
 379    both[existing_length + n] = '\0';
 380
 381    *dest = both;
 382    return true;
 383 }
 384
 385
 386 bool
 387 ralloc_strcat(char **dest, const char *str)
 388 {
 389    return cat(dest, str, strlen(str));
 390 }
 391
 392 bool
 393 ralloc_strncat(char **dest, const char *str, size_t n)
 394 {
 395    /* Clamp n to the string length */
 396    size_t str_length = strlen(str);
 397    if (str_length < n)
 398       n = str_length;
 399
 400    return cat(dest, str, n);
 401 }
 402
 403 char *
 404 ralloc_asprintf(const void *ctx, const char *fmt, ...)
 405 {
 406    char *ptr;
 407    va_list args;
 408    va_start(args, fmt);
 409    ptr = ralloc_vasprintf(ctx, fmt, args);
 410    va_end(args);
 411    return ptr;
 412 }
 413
 414 /* Return the length of the string that would be generated by a printf-style
 415  * format and argument list, not including the \0 byte.
 416  */
 417 static size_t
 418 printf_length(const char *fmt, va_list untouched_args)
 419 {
 420    int size;
 421    char junk;
 422
 423    /* Make a copy of the va_list so the original caller can still use it */
 424    va_list args;
 425    va_copy(args, untouched_args);
 426
 427 #ifdef _WIN32
 428    /* We need to use _vcsprintf to calculate the size as vsnprintf returns -1
 429     * if the number of characters to write is greater than count.
 430     */
 431    size = _vscprintf(fmt, args);
 432    (void)junk;
 433 #else
 434    size = vsnprintf(&junk, 1, fmt, args);
 435 #endif
 436    assert(size >= 0);
 437
 438    va_end(args);
 439
 440    return size;
 441 }
 442
 443 char *
 444 ralloc_vasprintf(const void *ctx, const char *fmt, va_list args)
 445 {
 446    size_t size = printf_length(fmt, args) + 1;
 447
 448    char *ptr = ralloc_size(ctx, size);
 449    if (ptr != NULL)
 450       vsnprintf(ptr, size, fmt, args);
 451
 452    return ptr;
 453 }
 454
 455 bool
 456 ralloc_asprintf_append(char **str, const char *fmt, ...)
 457 {
 458    bool success;
 459    va_list args;
 460    va_start(args, fmt);
 461    success = ralloc_vasprintf_append(str, fmt, args);
 462    va_end(args);
 463    return success;
 464 }
 465
 466 bool
 467 ralloc_vasprintf_append(char **str, const char *fmt, va_list args)
 468 {
 469    size_t existing_length;
 470    assert(str != NULL);
 471    existing_length = *str ? strlen(*str) : 0;
 472    return ralloc_vasprintf_rewrite_tail(str, &existing_length, fmt, args);
 473 }
 474
 475 bool
 476 ralloc_asprintf_rewrite_tail(char **str, size_t *start, const char *fmt, ...)
 477 {
 478    bool success;
 479    va_list args;
 480    va_start(args, fmt);
 481    success = ralloc_vasprintf_rewrite_tail(str, start, fmt, args);
 482    va_end(args);
 483    return success;
 484 }
 485
 486 bool
 487 ralloc_vasprintf_rewrite_tail(char **str, size_t *start, const char *fmt,
 488                               va_list args)
 489 {
 490    size_t new_length;
 491    char *ptr;
 492
 493    assert(str != NULL);
 494
 495    if (unlikely(*str == NULL)) {
 496       // Assuming a NULL context is probably bad, but it's expected behavior.
 497       *str = ralloc_vasprintf(NULL, fmt, args);
 498       *start = strlen(*str);
 499       return true;
 500    }
 501
 502    new_length = printf_length(fmt, args);
 503
 504    ptr = resize(*str, *start + new_length + 1);
 505    if (unlikely(ptr == NULL))
 506       return false;
 507
 508    vsnprintf(ptr + *start, new_length + 1, fmt, args);
 509    *str = ptr;
 510    *start += new_length;
 511    return true;
 512 }
 513
 514 /***************************************************************************
 515  * Linear allocator for short-lived allocations.
 516  ***************************************************************************
 517  *
 518  * The allocator consists of a parent node (2K buffer), which requires
 519  * a ralloc parent, and child nodes (allocations). Child nodes can't be freed
 520  * directly, because the parent doesn't track them. You have to release
 521  * the parent node in order to release all its children.
 522  *
 523  * The allocator uses a fixed-sized buffer with a monotonically increasing
 524  * offset after each allocation. If the buffer is all used, another buffer
 525  * is allocated, sharing the same ralloc parent, so all buffers are at
 526  * the same level in the ralloc hierarchy.
 527  *
 528  * The linear parent node is always the first buffer and keeps track of all
 529  * other buffers.
 530  */
 531
 532 #define ALIGN_POT(x, y) (((x) + (y) - 1) & ~((y) - 1))
 533
 534 #define MIN_LINEAR_BUFSIZE 2048
 535 #define SUBALLOC_ALIGNMENT sizeof(uintptr_t)
 536 #define LMAGIC 0x87b9c7d3
 537
 538 struct linear_header {
 539 #ifdef DEBUG
 540    unsigned magic;   /* for debugging */
 541 #endif
 542    unsigned offset;  /* points to the first unused byte in the buffer */
 543    unsigned size;    /* size of the buffer */
 544    void *ralloc_parent;          /* new buffers will use this */
 545    struct linear_header *next;   /* next buffer if we have more */
 546    struct linear_header *latest; /* the only buffer that has free space */
 547
 548    /* After this structure, the buffer begins.
 549     * Each suballocation consists of linear_size_chunk as its header followed
 550     * by the suballocation, so it goes:
 551     *
 552     * - linear_size_chunk
 553     * - allocated space
 554     * - linear_size_chunk
 555     * - allocated space
 556     * etc.
 557     *
 558     * linear_size_chunk is only needed by linear_realloc.
 559     */
 560 };
 561
 562 struct linear_size_chunk {
 563    unsigned size; /* for realloc */
 564    unsigned _padding;
 565 };
 566
 567 typedef struct linear_header linear_header;
 568 typedef struct linear_size_chunk linear_size_chunk;
 569
 570 #define LINEAR_PARENT_TO_HEADER(parent) \
 571    (linear_header*) \
 572    ((char*)(parent) - sizeof(linear_size_chunk) - sizeof(linear_header))
 573
 574 /* Allocate the linear buffer with its header. */
 575 static linear_header *
 576 create_linear_node(void *ralloc_ctx, unsigned min_size)
 577 {
 578    linear_header *node;
 579
 580    min_size += sizeof(linear_size_chunk);
 581
 582    if (likely(min_size < MIN_LINEAR_BUFSIZE))
 583       min_size = MIN_LINEAR_BUFSIZE;
 584
 585    node = ralloc_size(ralloc_ctx, sizeof(linear_header) + min_size);
 586    if (unlikely(!node))
 587       return NULL;
 588
 589 #ifdef DEBUG
 590    node->magic = LMAGIC;
 591 #endif
 592    node->offset = 0;
 593    node->size = min_size;
 594    node->ralloc_parent = ralloc_ctx;
 595    node->next = NULL;
 596    node->latest = node;
 597    return node;
 598 }
 599
 600 void *
 601 linear_alloc_child(void *parent, unsigned size)
 602 {
 603    linear_header *first = LINEAR_PARENT_TO_HEADER(parent);
 604    linear_header *latest = first->latest;
 605    linear_header *new_node;
 606    linear_size_chunk *ptr;
 607    unsigned full_size;
 608
 609    assert(first->magic == LMAGIC);
 610    assert(!latest->next);
 611
 612    size = ALIGN_POT(size, SUBALLOC_ALIGNMENT);
 613    full_size = sizeof(linear_size_chunk) + size;
 614
 615    if (unlikely(latest->offset + full_size > latest->size)) {
 616       /* allocate a new node */
 617       new_node = create_linear_node(latest->ralloc_parent, size);
 618       if (unlikely(!new_node))
 619          return NULL;
 620
 621       first->latest = new_node;
 622       latest->latest = new_node;
 623       latest->next = new_node;
 624       latest = new_node;
 625    }
 626
 627    ptr = (linear_size_chunk *)((char*)&latest[1] + latest->offset);
 628    ptr->size = size;
 629    latest->offset += full_size;
 630    return &ptr[1];
 631 }
 632
 633 void *
 634 linear_alloc_parent(void *ralloc_ctx, unsigned size)
 635 {
 636    linear_header *node;
 637
 638    if (unlikely(!ralloc_ctx))
 639       return NULL;
 640
 641    size = ALIGN_POT(size, SUBALLOC_ALIGNMENT);
 642
 643    node = create_linear_node(ralloc_ctx, size);
 644    if (unlikely(!node))
 645       return NULL;
 646
 647    return linear_alloc_child((char*)node +
 648                              sizeof(linear_header) +
 649                              sizeof(linear_size_chunk), size);
 650 }
 651
 652 void *
 653 linear_zalloc_child(void *parent, unsigned size)
 654 {
 655    void *ptr = linear_alloc_child(parent, size);
 656
 657    if (likely(ptr))
 658       memset(ptr, 0, size);
 659    return ptr;
 660 }
 661
 662 void *
 663 linear_zalloc_parent(void *parent, unsigned size)
 664 {
 665    void *ptr = linear_alloc_parent(parent, size);
 666
 667    if (likely(ptr))
 668       memset(ptr, 0, size);
 669    return ptr;
 670 }
 671
 672 void
 673 linear_free_parent(void *ptr)
 674 {
 675    linear_header *node;
 676
 677    if (unlikely(!ptr))
 678       return;
 679
 680    node = LINEAR_PARENT_TO_HEADER(ptr);
 681    assert(node->magic == LMAGIC);
 682
 683    while (node) {
 684       void *ptr = node;
 685
 686       node = node->next;
 687       ralloc_free(ptr);
 688    }
 689 }
 690
 691 void
 692 ralloc_steal_linear_parent(void *new_ralloc_ctx, void *ptr)
 693 {
 694    linear_header *node;
 695
 696    if (unlikely(!ptr))
 697       return;
 698
 699    node = LINEAR_PARENT_TO_HEADER(ptr);
 700    assert(node->magic == LMAGIC);
 701
 702    while (node) {
 703       ralloc_steal(new_ralloc_ctx, node);
 704       node->ralloc_parent = new_ralloc_ctx;
 705       node = node->next;
 706    }
 707 }
 708
 709 void *
 710 ralloc_parent_of_linear_parent(void *ptr)
 711 {
 712    linear_header *node = LINEAR_PARENT_TO_HEADER(ptr);
 713    assert(node->magic == LMAGIC);
 714    return node->ralloc_parent;
 715 }
 716
 717 void *
 718 linear_realloc(void *parent, void *old, unsigned new_size)
 719 {
 720    unsigned old_size = 0;
 721    ralloc_header *new_ptr;
 722
 723    new_ptr = linear_alloc_child(parent, new_size);
 724
 725    if (unlikely(!old))
 726       return new_ptr;
 727
 728    old_size = ((linear_size_chunk*)old)[-1].size;
 729
 730    if (likely(new_ptr && old_size))
 731       memcpy(new_ptr, old, MIN2(old_size, new_size));
 732
 733    return new_ptr;
 734 }
 735
 736 /* All code below is pretty much copied from ralloc and only the alloc
 737  * calls are different.
 738  */
 739
 740 char *
 741 linear_strdup(void *parent, const char *str)
 742 {
 743    unsigned n;
 744    char *ptr;
 745
 746    if (unlikely(!str))
 747       return NULL;
 748
 749    n = strlen(str);
 750    ptr = linear_alloc_child(parent, n + 1);
 751    if (unlikely(!ptr))
 752       return NULL;
 753
 754    memcpy(ptr, str, n);
 755    ptr[n] = '\0';
 756    return ptr;
 757 }
 758
 759 char *
 760 linear_asprintf(void *parent, const char *fmt, ...)
 761 {
 762    char *ptr;
 763    va_list args;
 764    va_start(args, fmt);
 765    ptr = linear_vasprintf(parent, fmt, args);
 766    va_end(args);
 767    return ptr;
 768 }
 769
 770 char *
 771 linear_vasprintf(void *parent, const char *fmt, va_list args)
 772 {
 773    unsigned size = printf_length(fmt, args) + 1;
 774
 775    char *ptr = linear_alloc_child(parent, size);
 776    if (ptr != NULL)
 777       vsnprintf(ptr, size, fmt, args);
 778
 779    return ptr;
 780 }
 781
 782 bool
 783 linear_asprintf_append(void *parent, char **str, const char *fmt, ...)
 784 {
 785    bool success;
 786    va_list args;
 787    va_start(args, fmt);
 788    success = linear_vasprintf_append(parent, str, fmt, args);
 789    va_end(args);
 790    return success;
 791 }
 792
 793 bool
 794 linear_vasprintf_append(void *parent, char **str, const char *fmt, va_list args)
 795 {
 796    size_t existing_length;
 797    assert(str != NULL);
 798    existing_length = *str ? strlen(*str) : 0;
 799    return linear_vasprintf_rewrite_tail(parent, str, &existing_length, fmt, args);
 800 }
 801
 802 bool
 803 linear_asprintf_rewrite_tail(void *parent, char **str, size_t *start,
 804                              const char *fmt, ...)
 805 {
 806    bool success;
 807    va_list args;
 808    va_start(args, fmt);
 809    success = linear_vasprintf_rewrite_tail(parent, str, start, fmt, args);
 810    va_end(args);
 811    return success;
 812 }
 813
 814 bool
 815 linear_vasprintf_rewrite_tail(void *parent, char **str, size_t *start,
 816                               const char *fmt, va_list args)
 817 {
 818    size_t new_length;
 819    char *ptr;
 820
 821    assert(str != NULL);
 822
 823    if (unlikely(*str == NULL)) {
 824       *str = linear_vasprintf(parent, fmt, args);
 825       *start = strlen(*str);
 826       return true;
 827    }
 828
 829    new_length = printf_length(fmt, args);
 830
 831    ptr = linear_realloc(parent, *str, *start + new_length + 1);
 832    if (unlikely(ptr == NULL))
 833       return false;
 834
 835    vsnprintf(ptr + *start, new_length + 1, fmt, args);
 836    *str = ptr;
 837    *start += new_length;
 838    return true;
 839 }
 840
 841 /* helper routine for strcat/strncat - n is the exact amount to copy */
 842 static bool
 843 linear_cat(void *parent, char **dest, const char *str, unsigned n)
 844 {
 845    char *both;
 846    unsigned existing_length;
 847    assert(dest != NULL && *dest != NULL);
 848
 849    existing_length = strlen(*dest);
 850    both = linear_realloc(parent, *dest, existing_length + n + 1);
 851    if (unlikely(both == NULL))
 852       return false;
 853
 854    memcpy(both + existing_length, str, n);
 855    both[existing_length + n] = '\0';
 856
 857    *dest = both;
 858    return true;
 859 }
 860
 861 bool
 862 linear_strcat(void *parent, char **dest, const char *str)
 863 {
 864    return linear_cat(parent, dest, str, strlen(str));
 865 }