用于C中具有负索引的数组的Calloc

Question

我有一个带有负索引的数组。它是一个具有实际维度 [dim_y + 40][dim_x + 40] 的数组，但用户使用该数组就像它具有维度 [dim_y][dim_x] 一样。 首先我有全局并且已经定义了维度dim_x，dim_y，所以我有这个

int map_boundaries[dim_y + 40][dim_x + 40];
int (*map)[dim_x+40] = (int(*)[dim_x+40])&map_boundaries[20][20];

效果很好。 现在我需要维度 dim_y 和 dim_x 是可变的，我的意思是我希望数组“map”不是固定大小而是动态的，我需要从用户和数组“map”中读取 dim_y、dim_x是全球性的，所以我有

int **map_boundaries;

我在main()中使用calloc

map_boundaries = (int **)calloc(dim_y + 40,sizeof(int*)); 
for(i = 0; i < dim_y + 40; i++){
    map_boundaries[i] = (int *)calloc(dim_x + 40,sizeof(int));
}

，但我不知道如何声明第二行

为了更好地理解边界，我做了第二条评论中发布的内容：http://everything2.com/title/Negative+array+indexing

Answer 1

我会分配一个数组：

int* storage = calloc((dim_x + 40) * (dim_y + 40), sizeof(int));

然后我会以这样的不透明结构将其返回给用户：

struct map {
  int* storage;
  size_t dim_x;
  size_t dim_y;
};

然后我会为用户定义一个索引函数：

int* get_cell(struct map* m, x, y) {
  return &m->storage[(x + 20) + (m->dim_x + 40) * (y + 20)];
}

如果用户想在一行中读/写多个元素，也许会得到一整行：

int* get_row(struct map* m, y) {
  return get_cell(m, 0, y);
}

size_t get_row_size(struct map* m) {
  return m->dim_x;
}

由于您希望在运行时确定维度，因此无论如何都需要进行运行时索引计算。

Answer 2

这是我的代码版本。 int **map 值是用户和库用于访问数据的值。 Matrix 结构封装了有关数组的所有信息，包括用于初始化map 的成员。代码设置大小，分配和初始化结构，然后初始化数组，然后像用户和库一样使用它。最后，分配的内存被释放。

在实践中，您可能希望将其拆分为多个单独的函数。

来源：2da.c：

#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include "stderr.h"

typedef struct Matrix
{
    int  *map_base;
    int **map_rows;
    int   x_dim;
    int   y_dim;
    int   extra;
    int **map;
} Matrix;

int main(void)
{
    Matrix m;
    int x_dim = 8;
    int y_dim = 6;
    int extra = 5;

    /* Allocation */
    m.extra = extra;
    m.x_dim = x_dim;
    m.y_dim = y_dim;
    int x_cells = x_dim + 2 * extra;
    int y_cells = y_dim + 2 * extra;
    m.map_base = calloc(x_cells * y_cells, sizeof(**m.map));
    if (m.map_base == 0)
        err_syserr("Failed to allocate %zu bytes memory\n",
                   x_cells * y_cells * sizeof(**m.map));
    m.map_rows = calloc(x_cells, sizeof(*m.map));
    if (m.map_rows == 0)
        err_syserr("Failed to allocate %zu bytes memory\n",
                   x_cells * sizeof(*m.map));
    //printf("Map base: 0x%.8" PRIXPTR "\n", (uintptr_t)m.map_base);
    //printf("Map rows: 0x%.8" PRIXPTR "\n", (uintptr_t)m.map_rows);
    for (int i = 0; i < x_cells; i++)
    {
        m.map_rows[i] = &m.map_base[i * y_cells + extra];
        //printf("Row[%2d]   0x%.8" PRIXPTR "\n", i, (uintptr_t)m.map_rows[i]);
    }
    m.map = &m.map_rows[extra];

    int **map = m.map;
    //printf("Map:      0x%.8" PRIXPTR "\n", (uintptr_t)map);

    /* Initialization */
    int x_min = -extra;
    int y_min = -extra;
    int x_max = x_dim + extra;
    int y_max = y_dim + extra;
    printf("Initialization:\n");
    for (int i = x_min; i < x_max; i++)
    {
        for (int j = y_min; j < y_max; j++)
        {
            map[i][j] = i * 100 + j;
            //printf("[%2d,%2d] = %4d\n", i, j, map[i][j]);
        }
    }

    printf("User view:\n");
    for (int i = 0; i < x_dim; i++)
    {
        for (int j = 0; j < y_dim; j++)
            printf("[%2d,%2d] = %4d\n", i, j, map[i][j]);
    }

    printf("Library view:\n");
    for (int i = x_min; i < x_max; i++)
    {
        for (int j = y_min; j < y_max; j++)
            printf("[%2d,%2d] = %4d\n", i, j, map[i][j]);
    }

    /* Deallocation */
    free(m.map_base);
    free(m.map_rows);

    return 0;
}

示例输出

当在带有 GCC 4.9.1 的 Mac OS X 10.9.5 上在 valgrind 3.10.0 下运行时，我得到以下输出。 "stderr.h" 标头在$HOME/inc 中并声明err_syserr()，而libjl.a 在$HOME/lib/64 中并提供err_syserr() 的实现，该函数报告给定错误消息和由errno 标识的错误并退出。这次我懒得在这个程序中写代码了。 （在 SO 中搜索 '[c] user:15168 err_syserr' 以找到 Protect a shared memory segment with semaphore does not work，它具有基本等效的代码。我没有在此答案的代码中调用 err_setarg0()。）

$ gcc -O3 -g -I/Users/jleffler/inc -std=c11 -Wall -Wextra -Wmissing-prototypes \
>     -Wstrict-prototypes -Wold-style-definition -Werror 2da.c -o 2da \
>     -L/Users/jleffler/lib/64 -ljl
$ valgrind ./2da
==26293== Memcheck, a memory error detector
==26293== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==26293== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==26293== Command: 2da
==26293== 
Initialization:
User view:
[ 0, 0] =    0
[ 0, 1] =    1
[ 0, 2] =    2
[ 0, 3] =    3
[ 0, 4] =    4
[ 0, 5] =    5
[ 1, 0] =  100
[ 1, 1] =  101
[ 1, 2] =  102
[ 1, 3] =  103
[ 1, 4] =  104
[ 1, 5] =  105
[ 2, 0] =  200
[ 2, 1] =  201
[ 2, 2] =  202
[ 2, 3] =  203
[ 2, 4] =  204
[ 2, 5] =  205
[ 3, 0] =  300
[ 3, 1] =  301
[ 3, 2] =  302
[ 3, 3] =  303
[ 3, 4] =  304
[ 3, 5] =  305
[ 4, 0] =  400
[ 4, 1] =  401
[ 4, 2] =  402
[ 4, 3] =  403
[ 4, 4] =  404
[ 4, 5] =  405
[ 5, 0] =  500
[ 5, 1] =  501
[ 5, 2] =  502
[ 5, 3] =  503
[ 5, 4] =  504
[ 5, 5] =  505
[ 6, 0] =  600
[ 6, 1] =  601
[ 6, 2] =  602
[ 6, 3] =  603
[ 6, 4] =  604
[ 6, 5] =  605
[ 7, 0] =  700
[ 7, 1] =  701
[ 7, 2] =  702
[ 7, 3] =  703
[ 7, 4] =  704
[ 7, 5] =  705
Library view:
[-5,-5] = -505
[-5,-4] = -504
[-5,-3] = -503
[-5,-2] = -502
[-5,-1] = -501
[-5, 0] = -500
[-5, 1] = -499
[-5, 2] = -498
[-5, 3] = -497
[-5, 4] = -496
[-5, 5] = -495
[-5, 6] = -494
[-5, 7] = -493
[-5, 8] = -492
[-5, 9] = -491
[-5,10] = -490
[-4,-5] = -405
[-4,-4] = -404
[-4,-3] = -403
[-4,-2] = -402
[-4,-1] = -401
[-4, 0] = -400
[-4, 1] = -399
[-4, 2] = -398
[-4, 3] = -397
[-4, 4] = -396
[-4, 5] = -395
[-4, 6] = -394
[-4, 7] = -393
[-4, 8] = -392
[-4, 9] = -391
[-4,10] = -390
[-3,-5] = -305
[-3,-4] = -304
[-3,-3] = -303
[-3,-2] = -302
[-3,-1] = -301
[-3, 0] = -300
[-3, 1] = -299
[-3, 2] = -298
[-3, 3] = -297
[-3, 4] = -296
[-3, 5] = -295
[-3, 6] = -294
[-3, 7] = -293
[-3, 8] = -292
[-3, 9] = -291
[-3,10] = -290
[-2,-5] = -205
[-2,-4] = -204
[-2,-3] = -203
[-2,-2] = -202
[-2,-1] = -201
[-2, 0] = -200
[-2, 1] = -199
[-2, 2] = -198
[-2, 3] = -197
[-2, 4] = -196
[-2, 5] = -195
[-2, 6] = -194
[-2, 7] = -193
[-2, 8] = -192
[-2, 9] = -191
[-2,10] = -190
[-1,-5] = -105
[-1,-4] = -104
[-1,-3] = -103
[-1,-2] = -102
[-1,-1] = -101
[-1, 0] = -100
[-1, 1] =  -99
[-1, 2] =  -98
[-1, 3] =  -97
[-1, 4] =  -96
[-1, 5] =  -95
[-1, 6] =  -94
[-1, 7] =  -93
[-1, 8] =  -92
[-1, 9] =  -91
[-1,10] =  -90
[ 0,-5] =   -5
[ 0,-4] =   -4
[ 0,-3] =   -3
[ 0,-2] =   -2
[ 0,-1] =   -1
[ 0, 0] =    0
[ 0, 1] =    1
[ 0, 2] =    2
[ 0, 3] =    3
[ 0, 4] =    4
[ 0, 5] =    5
[ 0, 6] =    6
[ 0, 7] =    7
[ 0, 8] =    8
[ 0, 9] =    9
[ 0,10] =   10
[ 1,-5] =   95
[ 1,-4] =   96
[ 1,-3] =   97
[ 1,-2] =   98
[ 1,-1] =   99
[ 1, 0] =  100
[ 1, 1] =  101
[ 1, 2] =  102
[ 1, 3] =  103
[ 1, 4] =  104
[ 1, 5] =  105
[ 1, 6] =  106
[ 1, 7] =  107
[ 1, 8] =  108
[ 1, 9] =  109
[ 1,10] =  110
[ 2,-5] =  195
[ 2,-4] =  196
[ 2,-3] =  197
[ 2,-2] =  198
[ 2,-1] =  199
[ 2, 0] =  200
[ 2, 1] =  201
[ 2, 2] =  202
[ 2, 3] =  203
[ 2, 4] =  204
[ 2, 5] =  205
[ 2, 6] =  206
[ 2, 7] =  207
[ 2, 8] =  208
[ 2, 9] =  209
[ 2,10] =  210
[ 3,-5] =  295
[ 3,-4] =  296
[ 3,-3] =  297
[ 3,-2] =  298
[ 3,-1] =  299
[ 3, 0] =  300
[ 3, 1] =  301
[ 3, 2] =  302
[ 3, 3] =  303
[ 3, 4] =  304
[ 3, 5] =  305
[ 3, 6] =  306
[ 3, 7] =  307
[ 3, 8] =  308
[ 3, 9] =  309
[ 3,10] =  310
[ 4,-5] =  395
[ 4,-4] =  396
[ 4,-3] =  397
[ 4,-2] =  398
[ 4,-1] =  399
[ 4, 0] =  400
[ 4, 1] =  401
[ 4, 2] =  402
[ 4, 3] =  403
[ 4, 4] =  404
[ 4, 5] =  405
[ 4, 6] =  406
[ 4, 7] =  407
[ 4, 8] =  408
[ 4, 9] =  409
[ 4,10] =  410
[ 5,-5] =  495
[ 5,-4] =  496
[ 5,-3] =  497
[ 5,-2] =  498
[ 5,-1] =  499
[ 5, 0] =  500
[ 5, 1] =  501
[ 5, 2] =  502
[ 5, 3] =  503
[ 5, 4] =  504
[ 5, 5] =  505
[ 5, 6] =  506
[ 5, 7] =  507
[ 5, 8] =  508
[ 5, 9] =  509
[ 5,10] =  510
[ 6,-5] =  595
[ 6,-4] =  596
[ 6,-3] =  597
[ 6,-2] =  598
[ 6,-1] =  599
[ 6, 0] =  600
[ 6, 1] =  601
[ 6, 2] =  602
[ 6, 3] =  603
[ 6, 4] =  604
[ 6, 5] =  605
[ 6, 6] =  606
[ 6, 7] =  607
[ 6, 8] =  608
[ 6, 9] =  609
[ 6,10] =  610
[ 7,-5] =  695
[ 7,-4] =  696
[ 7,-3] =  697
[ 7,-2] =  698
[ 7,-1] =  699
[ 7, 0] =  700
[ 7, 1] =  701
[ 7, 2] =  702
[ 7, 3] =  703
[ 7, 4] =  704
[ 7, 5] =  705
[ 7, 6] =  706
[ 7, 7] =  707
[ 7, 8] =  708
[ 7, 9] =  709
[ 7,10] =  710
[ 8,-5] =  795
[ 8,-4] =  796
[ 8,-3] =  797
[ 8,-2] =  798
[ 8,-1] =  799
[ 8, 0] =  800
[ 8, 1] =  801
[ 8, 2] =  802
[ 8, 3] =  803
[ 8, 4] =  804
[ 8, 5] =  805
[ 8, 6] =  806
[ 8, 7] =  807
[ 8, 8] =  808
[ 8, 9] =  809
[ 8,10] =  810
[ 9,-5] =  895
[ 9,-4] =  896
[ 9,-3] =  897
[ 9,-2] =  898
[ 9,-1] =  899
[ 9, 0] =  900
[ 9, 1] =  901
[ 9, 2] =  902
[ 9, 3] =  903
[ 9, 4] =  904
[ 9, 5] =  905
[ 9, 6] =  906
[ 9, 7] =  907
[ 9, 8] =  908
[ 9, 9] =  909
[ 9,10] =  910
[10,-5] =  995
[10,-4] =  996
[10,-3] =  997
[10,-2] =  998
[10,-1] =  999
[10, 0] = 1000
[10, 1] = 1001
[10, 2] = 1002
[10, 3] = 1003
[10, 4] = 1004
[10, 5] = 1005
[10, 6] = 1006
[10, 7] = 1007
[10, 8] = 1008
[10, 9] = 1009
[10,10] = 1010
[11,-5] = 1095
[11,-4] = 1096
[11,-3] = 1097
[11,-2] = 1098
[11,-1] = 1099
[11, 0] = 1100
[11, 1] = 1101
[11, 2] = 1102
[11, 3] = 1103
[11, 4] = 1104
[11, 5] = 1105
[11, 6] = 1106
[11, 7] = 1107
[11, 8] = 1108
[11, 9] = 1109
[11,10] = 1110
[12,-5] = 1195
[12,-4] = 1196
[12,-3] = 1197
[12,-2] = 1198
[12,-1] = 1199
[12, 0] = 1200
[12, 1] = 1201
[12, 2] = 1202
[12, 3] = 1203
[12, 4] = 1204
[12, 5] = 1205
[12, 6] = 1206
[12, 7] = 1207
[12, 8] = 1208
[12, 9] = 1209
[12,10] = 1210
==26293== 
==26293== HEAP SUMMARY:
==26293==     in use at exit: 29,341 bytes in 374 blocks
==26293==   total heap usage: 452 allocs, 78 frees, 36,581 bytes allocated
==26293== 
==26293== LEAK SUMMARY:
==26293==    definitely lost: 0 bytes in 0 blocks
==26293==    indirectly lost: 0 bytes in 0 blocks
==26293==      possibly lost: 0 bytes in 0 blocks
==26293==    still reachable: 4,096 bytes in 1 blocks
==26293==         suppressed: 25,245 bytes in 373 blocks
==26293== Rerun with --leak-check=full to see details of leaked memory
==26293== 
==26293== For counts of detected and suppressed errors, rerun with: -v
==26293== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
$

“仍然可达”块是stdout 的缓冲区；如果我在从main() 返回之前包含fclose(stdout);，它就会消失。 Mac OS X 运行时库也分配了大量内存，但在隐藏信息中会识别出这种使用情况。

---

评论和回复

正如您在 2 行中看到的那样，我进行了整个初始化。但问题不在于初始化；问题是在每一帧中，地图的值都会改变。所以我只需要一个直接指向“边界图”的“地图”。 map[0][0] 是 boundaries_map[20][20] 等等。我试图声明全局**map，然后将我的第二行写入主行，但我收到有关演员表的错误。 (*map)[dim_x+40] 这是**map 不是吗？

我不确定我是否完全理解您的需求，这是此回复需要很长时间的原因之一。

您不能拥有属于 VLA 的全局变量；它们只能在函数中分配，可以作为局部变量在堆栈上分配，也可以通过malloc() 等在堆上分配。我展示的原始代码允许您通过一个全局变量定义一个数组，一个双指针，可以从任何可以读取双指针的代码访问。

下面的代码可以看作是构建 VLA 的练习。它使用malloc() 创建一个VLA，在指向VLA 的指针中记录它的创建位置。这可以传递给函数，就像可以传递基于堆栈的 VLA 一样。代码在 Mac OS X 10.9.5 上的 GCC 4.9.1 下编译干净：

$ gcc -g -O3 -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes -Werror \
>     2dv.c -o 2dv 

根据valgrind，它运行干净且无泄漏。

来源：2dv.c：

#include <stdio.h>
#include <stdlib.h>

static void dump_vla_1(int x_size, int y_size, int vla[x_size][y_size]);
static void dump_vla_2(int x_min, int y_min, int x_size, int y_size,
                       int vla[x_size][y_size]);
static void dump_vla_3(int x_min, int x_max, int y_min, int y_max,
                       int x_size, int y_size, int vla[x_size][y_size]);

int main(void)
{
    int extra = 3;
    int x_base = 5;
    int y_base = 4;
    int x_size = x_base + 2 * extra;
    int y_size = y_base + 2 * extra;

    int (*vla)[x_size][y_size] = calloc(x_size * y_size, sizeof(int));

    for (int i = 0; i < x_size; i++)
        for (int j = 0; j < y_size; j++)
            (*vla)[i][j] = (i + 1) * 100 + (j + 1);

    for (int i = 0; i < x_size; i++)
    {
        for (int j = 0; j < y_size; j++)
            printf(" %4d", (*vla)[i][j]);
        putchar('\n');
    }

    dump_vla_1(x_size, y_size, *vla);
    dump_vla_2(0, 0, x_size, y_size, *vla);

    /* Harsh cast! */
    int (*vla_2)[x_size][y_size] = (int (*)[x_size][y_size])&(*vla)[extra][extra];

    dump_vla_2(-extra, -extra, x_size, y_size, *vla_2);

    dump_vla_3(-extra, x_size - extra, -extra, y_size - extra, x_size, y_size, *vla_2);
    dump_vla_3(0, x_base, 0, y_base, x_size, y_size, *vla_2);

    free(vla);
    return 0;
}

static void dump_vla_1(int x_size, int y_size, int vla[x_size][y_size])
{
    printf("Matrix %dx%d\n", x_size, y_size);
    for (int i = 0; i < x_size; i++)
    {
        for (int j = 0; j < y_size; j++)
            printf(" %4d", vla[i][j]);
        putchar('\n');
    }
}

static void dump_vla_2(int x_min, int y_min, int x_size, int y_size,
                       int vla[x_size][y_size])
{
    printf("Matrix %dx%d (%d..%d, %d..%d)\n",
           x_size, y_size, x_min, x_min + x_size - 1, y_min, y_min + y_size - 1);
    for (int i = x_min; i < x_min + x_size; i++)
    {
        for (int j = y_min; j < y_min + y_size; j++)
            printf(" %4d", vla[i][j]);
        putchar('\n');
    }
}

static void dump_vla_3(int x_min, int x_max, int y_min, int y_max,
                       int x_size, int y_size, int vla[x_size][y_size])
{
    printf("Matrix %dx%d (%d..%d, %d..%d)\n",
           x_size, y_size, x_min, x_max, y_min, y_max);
    for (int i = x_min; i < x_max; i++)
    {
        for (int j = y_min; j < y_max; j++)
            printf(" %4d", vla[i][j]);
        putchar('\n');
    }
}

注意使用*vla 和*vla_2 将VLA 传递给函数。由于vla 和vla_2 变量是指向VLA 的指针，所以*vla 是函数期望的VLA。

输出：

  101  102  103  104  105  106  107  108  109  110
  201  202  203  204  205  206  207  208  209  210
  301  302  303  304  305  306  307  308  309  310
  401  402  403  404  405  406  407  408  409  410
  501  502  503  504  505  506  507  508  509  510
  601  602  603  604  605  606  607  608  609  610
  701  702  703  704  705  706  707  708  709  710
  801  802  803  804  805  806  807  808  809  810
  901  902  903  904  905  906  907  908  909  910
 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010
 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110
Matrix 11x10
  101  102  103  104  105  106  107  108  109  110
  201  202  203  204  205  206  207  208  209  210
  301  302  303  304  305  306  307  308  309  310
  401  402  403  404  405  406  407  408  409  410
  501  502  503  504  505  506  507  508  509  510
  601  602  603  604  605  606  607  608  609  610
  701  702  703  704  705  706  707  708  709  710
  801  802  803  804  805  806  807  808  809  810
  901  902  903  904  905  906  907  908  909  910
 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010
 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110
Matrix 11x10 (0..10, 0..9)
  101  102  103  104  105  106  107  108  109  110
  201  202  203  204  205  206  207  208  209  210
  301  302  303  304  305  306  307  308  309  310
  401  402  403  404  405  406  407  408  409  410
  501  502  503  504  505  506  507  508  509  510
  601  602  603  604  605  606  607  608  609  610
  701  702  703  704  705  706  707  708  709  710
  801  802  803  804  805  806  807  808  809  810
  901  902  903  904  905  906  907  908  909  910
 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010
 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110
Matrix 11x10 (-3..7, -3..6)
  101  102  103  104  105  106  107  108  109  110
  201  202  203  204  205  206  207  208  209  210
  301  302  303  304  305  306  307  308  309  310
  401  402  403  404  405  406  407  408  409  410
  501  502  503  504  505  506  507  508  509  510
  601  602  603  604  605  606  607  608  609  610
  701  702  703  704  705  706  707  708  709  710
  801  802  803  804  805  806  807  808  809  810
  901  902  903  904  905  906  907  908  909  910
 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010
 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110
Matrix 11x10 (-3..8, -3..7)
  101  102  103  104  105  106  107  108  109  110
  201  202  203  204  205  206  207  208  209  210
  301  302  303  304  305  306  307  308  309  310
  401  402  403  404  405  406  407  408  409  410
  501  502  503  504  505  506  507  508  509  510
  601  602  603  604  605  606  607  608  609  610
  701  702  703  704  705  706  707  708  709  710
  801  802  803  804  805  806  807  808  809  810
  901  902  903  904  905  906  907  908  909  910
 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010
 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110
Matrix 11x10 (0..5, 0..4)
  404  405  406  407
  504  505  506  507
  604  605  606  607
  704  705  706  707
  804  805  806  807