Modifying function that malloc's a 2d array to a 3d array in C

Question

I'm very new to C, this is the first program I'm writing in it. My professor gave us a function for allocating memory for a 2d array, called malloc2d. I am supposed to modify it to allocate memory for a 3d array, but being so new to C I am not sure how to go about it. I've tried looking at other malloc functions for 3d arrays but none of them look similar to the one I was given. Similarly, we have a free2d function that also needs to be modified for a 3d array. Here are the functions to be modified:

void** malloc2D(size_t rows, size_t cols, size_t sizeOfType){
    void* block = malloc(sizeOfType * rows * cols);
    void** matrix = malloc(sizeof(void*) * rows);
    for (int row = 0; row < rows; ++row) {
        matrix[row] = block + cols * row * sizeOfType;
    }//for
    return matrix;
}//malloc2D

void free2D(void*** matrix){
    free((*matrix)[0]);
    free((*matrix));
    matrix = NULL;
}//free2D

Any help or a start would be greatly appreciated.

One of the first things you should learn is that you should not blindly trust the return value of `malloc()` becaus it might return `NULL`, and even though it almost never does, you should not assume that it wont. Also, if the intention of your `free()` function is to prevent having a dangling pointer then `*matrix = NULL` is the correct way. Note that `*matrix` and `matrix[0]` are interchangeable. — Iharob Al Asimi, Feb 18 '18 at 20:42
Also [`void*` pointer arithmetic is illegal](https://stackoverflow.com/a/3524270/1480131). Tell your professor that's it's wrong. It should be `char *block`. — Pablo, Feb 18 '18 at 20:45
Also I don't like this way of requesting memory, because you cannot reallocate or free memory at different parts of the cells, you lose flexibility. — Pablo, Feb 18 '18 at 20:47

score 1 · Accepted Answer · answered Feb 19 '18 at 00:54

I find it difficult to believe this is a first exercise; it is moderately tricky, at least.

Fix the 2D code

The first step should be to clean up the malloc2D() function so it doesn't casually use a GCC extension — indexing a void * — because Standard C does not allow that (because sizeof(void) is undefined in Standard C; GNU C defines it as 1). Also, the bug in free2D() needs to be fixed; the last line of the function should read *matrix = NULL; (the * was omitted). That code should be tested too, because the correct way to access the matrix is not obvious.

Here's some modified code (variables renamed for consistency with the 3D version) that tests the revised 2D code:

/* SO 4885-6272 */

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

/* Should be declared in a header for use in other files */
extern void **malloc2D(size_t rows, size_t cols, size_t sizeOfType);
extern void free2D(void ***matrix);

void **malloc2D(size_t rows, size_t cols, size_t sizeOfType)
{
    void *level2 = malloc(sizeOfType * rows * cols);
    void **level1 = malloc(sizeof(void *) * rows);
    if (level2 == NULL || level1 == NULL)
    {
        free(level2);
        free(level1);
        return NULL;
    }
    for (size_t row = 0; row < rows; ++row)
    {
        level1[row] = (char *)level2 + cols * row * sizeOfType;
    }
    return level1;
}

void free2D(void ***matrix)
{
    free((*matrix)[0]);
    free((*matrix));
    *matrix = NULL;
}

static void test2D(size_t m2_rows, size_t m2_cols)
{
    printf("rows = %zu; cols = %zu\n", m2_rows, m2_cols);
    void **m2 = malloc2D(m2_rows, m2_cols, sizeof(double));
    if (m2 == NULL)
    {
        fprintf(stderr, "Memory allocation failed for 2D array of size %zux%zu doubles\n",
                m2_rows, m2_cols);
        return;
    }

    printf("m2 = 0x%.12" PRIXPTR "; m2[0] = 0x%.12" PRIXPTR "\n",
           (uintptr_t)m2, (uintptr_t)m2[0]);

    for (size_t i = 0; i < m2_rows; i++)
    {
        for (size_t j = 0; j < m2_cols; j++)
            ((double *)m2[i])[j] = (i + 1) * 10 + (j + 1);
    }

    for (size_t i = 0; i < m2_rows; i++)
    {
        for (size_t j = 0; j < m2_cols; j++)
            printf("%4.0f", ((double *)m2[i])[j]);
        putchar('\n');
    }

    free2D(&m2);
    printf("m2 = 0x%.16" PRIXPTR "\n", (uintptr_t)m2);
}

int main(void)
{
    test2D(4, 5);
    test2D(10, 3);
    test2D(3, 10);
    //test2D(300000000, 1000000000);  /* 2132 PiB - should fail to allocate on sane systems! */
    return 0;
}

When run on a MacBook Pro running macOS High Sierra 10.13.3, compiling with GCC 7.3.0, I get the output:

rows = 4; cols = 5
m2 = 0x7F83C04027F0; m2[0] = 0x7F83C0402750
  11  12  13  14  15
  21  22  23  24  25
  31  32  33  34  35
  41  42  43  44  45
m2 = 0x0000000000000000
rows = 10; cols = 3
m2 = 0x7F83C0402750; m2[0] = 0x7F83C04028C0
  11  12  13
  21  22  23
  31  32  33
  41  42  43
  51  52  53
  61  62  63
  71  72  73
  81  82  83
  91  92  93
 101 102 103
m2 = 0x0000000000000000
rows = 3; cols = 10
m2 = 0x7F83C04027A0; m2[0] = 0x7F83C04028C0
  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
m2 = 0x0000000000000000

With the monster allocation included, the trace ended:

alloc3d19(8985,0x7fffa5d79340) malloc: *** mach_vm_map(size=2400000000000000000) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
Memory allocation failed for 2D array of size 300000000x1000000000 doubles

Adapt to 3D code

I chose to call the leading dimension of the 3D array a 'plane'; each plane contains a 2D array with r rows by c columns.

For me, I drew myself a diagram to convince myself I was getting the assignments correct — after I'd messed up a couple of times. In each cell in the first two tables, the first number is the index number of the cell in the containing array (level1 in the first table) and the second is the index number of the cell in the next level (level2 in the first table). The numbers in the level3 table are simply the indexes into the array of doublea.

level1 (planes: 4)
╔═══════╗
║ 0: 00 ║
║ 1: 05 ║
║ 2: 10 ║
║ 3: 15 ║
╚═══════╝

level2 (planes: 4; rows: 5)
╔════════╦════════╦════════╦════════╦════════╗
║ 00: 00 ║ 01: 06 ║ 02: 12 ║ 03: 18 ║ 04: 24 ║
║ 05: 30 ║ 06: 36 ║ 07: 42 ║ 08: 48 ║ 09: 54 ║
║    …   ║    …   ║    …   ║    …   ║    …   ║
╚════════╩════════╩════════╩════════╩════════╝

level3 (planes: 4; rows: 5; cols: 6)
╔════╦═════╦═════╦═════╦═════╦═════╗
║  0 ║   1 ║   2 ║   3 ║   4 ║   5 ║
║  6 ║   7 ║   8 ║   9 ║  10 ║  11 ║
║ 12 ║  13 ║  14 ║  15 ║  16 ║  17 ║  Plane 0
║ 18 ║  19 ║  20 ║  21 ║  22 ║  23 ║
║ 24 ║  25 ║  26 ║  27 ║  28 ║  29 ║
╠════╬═════╬═════╬═════╬═════╬═════╣
║ 30 ║  31 ║  32 ║  33 ║  34 ║  35 ║
║ 36 ║  37 ║  38 ║  39 ║  40 ║  41 ║  Plane 1
║ …  ║  …  ║  …  ║  …  ║  …  ║  …  ║
╚════╩═════╩═════╩═════╩═════╩═════╝

With that diagram in place — or a paper and pen version with arrows scrawled over it, the values in the cell of plane p in level1 is p * rows; the values of in the cell of plane p, row r in level2 is p * rows + r) * cols; the values in the cell of plane p, row r, cell c in level3 is (p * rows + r) * cols + c. But the values are not integers; they're pointers. Consequently, the values have to be scaled by an appropriate size and added to the base address for the level1, level2 or level3 space.

That leads to code like this:

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

/* Should be declared in a header for use in other files */
extern void ***malloc3D(size_t planes, size_t rows, size_t cols, size_t sizeOfType);
extern void free3D(void ****matrix);

void ***malloc3D(size_t planes, size_t rows, size_t cols, size_t sizeOfType)
{
    void   *level3 = malloc(sizeOfType * planes * rows * cols);
    void  **level2 = malloc(sizeof(void *) * planes * rows);
    void ***level1 = malloc(sizeof(void **) * planes);
    //printf("planes = %zu; rows = %zu; cols = %zu; ", planes, rows, cols);
    //printf("level1 = 0x%.12" PRIXPTR "; level2 = 0x%.12" PRIXPTR "; level3 = 0x%.12" PRIXPTR "\n",
    //        (uintptr_t)level1, (uintptr_t)level2, (uintptr_t)level3);
    fflush(stdout);
    if (level3 == NULL || level2 == NULL || level1 == NULL)
    {
        free(level3);
        free(level2);
        free(level1);
        return NULL;
    }
    for (size_t plane = 0; plane < planes; plane++)
    {
        level1[plane] = (void **)((char *)level2 + plane * rows * sizeof(void **));
        //printf("level1[%zu]   = 0x%.12" PRIXPTR "\n", plane, (uintptr_t)level1[plane]);
        for (size_t row = 0; row < rows; ++row)
        {
            level2[plane * rows + row] = (char *)level3 + (plane * rows + row) * cols * sizeOfType;
            //printf("  level2[%zu] = 0x%.12" PRIXPTR "\n",
            //       plane * rows + row, (uintptr_t)level2[plane * rows + row]);
        }
    }
    return level1;
}

void free3D(void ****matrix)
{
    free((*matrix)[0][0]);
    free((*matrix)[0]);
    free((*matrix));
    *matrix = NULL;
}

static void test3D(size_t m3_plns, size_t m3_rows, size_t m3_cols)
{
    printf("planes = %zu; rows = %zu; cols = %zu\n", m3_plns, m3_rows, m3_cols);
    void ***m3 = malloc3D(m3_plns, m3_rows, m3_cols, sizeof(double));
    if (m3 == NULL)
    {
        fprintf(stderr, "Memory allocation failed for 3D array of size %zux%zux%zu doubles\n",
                m3_plns, m3_rows, m3_cols);
        return;
    }

    printf("m3 = 0x%.12" PRIXPTR "; m3[0] = 0x%.12" PRIXPTR "; m3[0][0] = 0x%.12" PRIXPTR "\n",
           (uintptr_t)m3, (uintptr_t)m3[0], (uintptr_t)m3[0][0]);

    for (size_t i = 0; i < m3_plns; i++)
    {
        for (size_t j = 0; j < m3_rows; j++)
        {
            for (size_t k = 0; k < m3_cols; k++)
                ((double *)m3[i][j])[k] = (i + 1) * 100 + (j + 1) * 10 + (k + 1);
        }
    }

    for (size_t i = 0; i < m3_plns; i++)
    {
        printf("Plane %zu:\n", i + 1);
        for (size_t j = 0; j < m3_rows; j++)
        {
            for (size_t k = 0; k < m3_cols; k++)
                printf("%4.0f", ((double *)m3[i][j])[k]);
            putchar('\n');
        }
        putchar('\n');
    }

    free3D(&m3);
    printf("m3 = 0x%.16" PRIXPTR "\n", (uintptr_t)m3);
}

int main(void)
{
    test3D(4, 5, 6);
    test3D(3, 4, 10);
    test3D(4, 3, 7);
    test3D(4, 9, 7);
    test3D(30000, 100000, 100000000);  /* 2132 PiB - should fail to allocate on sane systems! */
    return 0;
}

Example output (with outsize memory allocation):

planes = 4; rows = 5; cols = 6
m3 = 0x7FFCC94027F0; m3[0] = 0x7FFCC9402750; m3[0][0] = 0x7FFCC9402850
Plane 1:
 111 112 113 114 115 116
 121 122 123 124 125 126
 131 132 133 134 135 136
 141 142 143 144 145 146
 151 152 153 154 155 156

Plane 2:
 211 212 213 214 215 216
 221 222 223 224 225 226
 231 232 233 234 235 236
 241 242 243 244 245 246
 251 252 253 254 255 256

Plane 3:
 311 312 313 314 315 316
 321 322 323 324 325 326
 331 332 333 334 335 336
 341 342 343 344 345 346
 351 352 353 354 355 356

Plane 4:
 411 412 413 414 415 416
 421 422 423 424 425 426
 431 432 433 434 435 436
 441 442 443 444 445 446
 451 452 453 454 455 456

m3 = 0x0000000000000000
planes = 3; rows = 4; cols = 10
m3 = 0x7FFCC94027F0; m3[0] = 0x7FFCC9402750; m3[0][0] = 0x7FFCC9402840
Plane 1:
 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

Plane 2:
 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

Plane 3:
 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

m3 = 0x0000000000000000
planes = 4; rows = 3; cols = 7
m3 = 0x7FFCC94027F0; m3[0] = 0x7FFCC9402750; m3[0][0] = 0x7FFCC9402840
Plane 1:
 111 112 113 114 115 116 117
 121 122 123 124 125 126 127
 131 132 133 134 135 136 137

Plane 2:
 211 212 213 214 215 216 217
 221 222 223 224 225 226 227
 231 232 233 234 235 236 237

Plane 3:
 311 312 313 314 315 316 317
 321 322 323 324 325 326 327
 331 332 333 334 335 336 337

Plane 4:
 411 412 413 414 415 416 417
 421 422 423 424 425 426 427
 431 432 433 434 435 436 437

m3 = 0x0000000000000000
planes = 4; rows = 9; cols = 7
m3 = 0x7FFCC94027F0; m3[0] = 0x7FFCC9402840; m3[0][0] = 0x7FFCC9802000
Plane 1:
 111 112 113 114 115 116 117
 121 122 123 124 125 126 127
 131 132 133 134 135 136 137
 141 142 143 144 145 146 147
 151 152 153 154 155 156 157
 161 162 163 164 165 166 167
 171 172 173 174 175 176 177
 181 182 183 184 185 186 187
 191 192 193 194 195 196 197

Plane 2:
 211 212 213 214 215 216 217
 221 222 223 224 225 226 227
 231 232 233 234 235 236 237
 241 242 243 244 245 246 247
 251 252 253 254 255 256 257
 261 262 263 264 265 266 267
 271 272 273 274 275 276 277
 281 282 283 284 285 286 287
 291 292 293 294 295 296 297

Plane 3:
 311 312 313 314 315 316 317
 321 322 323 324 325 326 327
 331 332 333 334 335 336 337
 341 342 343 344 345 346 347
 351 352 353 354 355 356 357
 361 362 363 364 365 366 367
 371 372 373 374 375 376 377
 381 382 383 384 385 386 387
 391 392 393 394 395 396 397

Plane 4:
 411 412 413 414 415 416 417
 421 422 423 424 425 426 427
 431 432 433 434 435 436 437
 441 442 443 444 445 446 447
 451 452 453 454 455 456 457
 461 462 463 464 465 466 467
 471 472 473 474 475 476 477
 481 482 483 484 485 486 487
 491 492 493 494 495 496 497

m3 = 0x0000000000000000

planes = 30000; rows = 100000; cols = 100000000
alloc3d79(9018,0x7fffa5d79340) malloc: *** mach_vm_map(size=2400000000000000000) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
Memory allocation failed for 3D array of size 30000x100000x100000000 doubles

Modifying function that malloc's a 2d array to a 3d array in C

1 Answers1

Fix the 2D code

Adapt to 3D code

Linked