Improvements to dynamically allocate memory for a double pointer to struct inside a pointer to struct

Question

I wrote the code below to dynamically allocate memory for the nested struct: Product **product; The purpose of my code is for me to learn the right or better way to dynamically allocate memory for using double pointer to a struct inside another pointer to struct. The code runs fine.

Question: Any corrections or improvements for the code? Thanks in advance for sharing your experiences.

typedef struct {
    int price;
} Product;

typedef struct {
  Product **product;
  int id;
} Inventory;

int main() {
  int i, j, k, count=0;
  int n1=2, n2=3, n3=2;

  Inventory *inventory = malloc(n1 * sizeof *inventory);
  for (i=0; i<n1; i++) {
    inventory[i].product = malloc(n2 * sizeof *inventory[i].product);
    for (j=0; j<n2; j++) {
      inventory[i].product[j] = malloc(n3 * sizeof *inventory[i].product[j]);
    }
  }

  for (i=0; i<n1; i++) {
    for (j=0; j<n2; j++) {
      for (k=0; k<n3; k++) {
        inventory[i].product[j][k].price = count++;
        printf("%d " , inventory[i].product[j][k].price);
      }
    }
  }
  return 0;
}

Output: 0 1 2 3 4 5 6 7 8 9 10 11

Answer 1

I have tried to use larger n1 , n2 and n3 , and your code works fine, too.

But here are two points needed to be noticed:

1. You need to add the free() after you use the malloc() to allocate memory.
2. If you want to use a c++ compiler (for example, g++) to compile this code, you need to cast the pointers' type returned by the malloc() function.

The following is the code I tested. It will cost some time to run it:

#include "stdlib.h"
#include "stdio.h"

typedef struct {
    int price;
} Product;

typedef struct {
  Product **product;
  int id;
} Inventory;

int main() {
  int i, j, k, count=0;
  int n1=525, n2=33, n3=141;

  Inventory *inventory = (Inventory*)malloc(n1 * sizeof *inventory);
  for (i=0; i<n1; i++) {
    inventory[i].product = (Product**)malloc(n2 * sizeof *inventory[i].product);
    for (j=0; j<n2; j++) {
      inventory[i].product[j] = (Product*)malloc(n3 * sizeof *inventory[i].product[j]);
      for (k=0; k<n3; k++) {
        inventory[i].product[j][k].price = k*i*j;
      }
    }
  }

  for (i=0; i<n1; i++) {
    for (j=0; j<n2; j++) {
      for (k=0; k<n3; k++) {
        printf("%d\n", inventory[i].product[j][k].price);
      }
    }
  }

  for (i=0; i<n1; i++) {
    for (j=0; j<n2; j++) {
      free(inventory[i].product[j]);
    }
    free(inventory[i].product);
  }
  free(inventory);

  return 0;
}

Hope it helps.

Answer 2

The problem is product is a pointer to a pointer, so if you intended to only an array of product under Inventory , you must use -> notation instead of . notation for price . (See example 1 below) If however, you intended to actually nest an array of struct Product under each pointer-to-Product you allocate (ie subproducts ), see example 2 :

Note: after each allocation, you must check the return from malloc to insure the allocation succeeded. (omitted in example 1 below for brevity, shown in full in example 2 )

example 1

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

typedef struct {
    int price;
} Product;

typedef struct {
    Product **product;
    int id;
} Inventory;

int main () {

    int i, j;
    int n1 = 2, n2 = 3;

    /* allocated / initiaize Inventory and Product */
    Inventory *inventory = malloc (n1 * sizeof *inventory);
    for (i = 0; i < n1; i++) 
    {
        inventory[i].id = i;
        inventory[i].product = malloc (n2 * sizeof *inventory[i].product);

        for (j = 0; j < n2; j++) 
        {
            inventory[i].product[j] = malloc (sizeof *inventory[i].product[j]);
            inventory[i].product[j]->price = (j + 1) * 2;
        }
    }

    /* print the inventory / procduct price */
    for (i = 0; i < n1; i++) 
    {
        printf ("\n product id : %d\n", inventory[i].id);
        for (j = 0; j < n2; j++) 
        {
            printf ("   price : %d\n", inventory[i].product[j]->price);
        }
    }

    /* free all memory */
    for (i = 0; i < n1; i++) 
    {
        for (j = 0; j < n2; j++) 
            free (inventory[i].product[j]);
        free (inventory[i].product);
    }
    free (inventory);

    return 0;
}

Output

$ ./bin/allocate_ptr_in_struct

 product id : 0
   price : 2
   price : 4
   price : 6

 product id : 1
   price : 2
   price : 4
   price : 6

Memory Use Error Check

$ valgrind ./bin/allocate_ptr_in_struct
==10167== Memcheck, a memory error detector
==10167== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==10167== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==10167== Command: ./bin/allocate_ptr_in_struct
==10167==

 product id : 0
   price : 2
   price : 4
   price : 6

 product id : 1
   price : 2
   price : 4
   price : 6
==10167==
==10167== HEAP SUMMARY:
==10167==     in use at exit: 0 bytes in 0 blocks
==10167==   total heap usage: 9 allocs, 9 frees, 104 bytes allocated
==10167==
==10167== All heap blocks were freed -- no leaks are possible
==10167==
==10167== For counts of detected and suppressed errors, rerun with: -v
==10167== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

---

Allocation of Nested Subproduct

example 2

Per the comment that you intended to allocate a nested struct under **product the change is straightforward:

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

typedef struct {
    int price;
} Product;

typedef struct {
    Product **product;
    int id;
} Inventory;

int main () {

    int i, j, k;
    int n1 = 2, n2 = 3, n3 = 2;

    /* allocated / initiaize Inventory and Product */
    Inventory *inventory = malloc (n1 * sizeof *inventory);
    if (!inventory) {
        fprintf (stderr, "error: virtual memory exhausted.\n");
        exit (EXIT_FAILURE);
    }
    for (i = 0; i < n1; i++) 
    {
        inventory[i].id = i;
        if (!(inventory[i].product = malloc (n2 * sizeof *inventory[i].product))) {
            fprintf (stderr, "error: virtual memory exhausted.\n");
            exit (EXIT_FAILURE);
        }

        for (j = 0; j < n2; j++) 
        {
            if (!(inventory[i].product[j] = malloc (n3 * sizeof *inventory[i].product[j]))) {
                fprintf (stderr, "error: virtual memory exhausted.\n");
                exit (EXIT_FAILURE);
            }
            for (k = 0; k < n3; k++)
                inventory[i].product[j][k].price = (j + 1) * 2 * (k + 1);
        }
    }

    /* print the inventory / procduct price */
    for (i = 0; i < n1; i++) 
    {
        printf ("\n  Inventory id : %d\n", inventory[i].id);
        for (j = 0; j < n2; j++) 
        {
            printf ("\n    Product[%d]\n", j);
            for (k = 0; k < n3; k++)
                printf ("      subproduct[%d][%d] price : %d\n", 
                        j, k, inventory[i].product[j][k].price);
        }
    }

    /* free all memory */
    for (i = 0; i < n1; i++) 
    {
        for (j = 0; j < n2; j++) 
            free (inventory[i].product[j]);
        free (inventory[i].product);
    }
    free (inventory);

    return 0;
}

Output

$ ./bin/allocate_ptr_in_struct

  Inventory id : 0

    Product[0]
      subproduct[0][0] price : 2
      subproduct[0][1] price : 4

    Product[1]
      subproduct[1][0] price : 4
      subproduct[1][1] price : 8

    Product[2]
      subproduct[2][0] price : 6
      subproduct[2][1] price : 12

  Inventory id : 1

    Product[0]
      subproduct[0][0] price : 2
      subproduct[0][1] price : 4

    Product[1]
      subproduct[1][0] price : 4
      subproduct[1][1] price : 8

    Product[2]
      subproduct[2][0] price : 6
      subproduct[2][1] price : 12

Memory Use Error Check

$ valgrind ./bin/allocate_ptr_in_struct
==23024== Memcheck, a memory error detector
==23024== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==23024== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==23024== Command: ./bin/allocate_ptr_in_struct
==23024==

  Inventory id : 0

    Product[0]
      subproduct[0][0] price : 2
      subproduct[0][1] price : 4

    Product[1]
      subproduct[1][0] price : 4
      subproduct[1][1] price : 8

    Product[2]
      subproduct[2][0] price : 6
      subproduct[2][1] price : 12

  Inventory id : 1

    Product[0]
      subproduct[0][0] price : 2
      subproduct[0][1] price : 4

    Product[1]
      subproduct[1][0] price : 4
      subproduct[1][1] price : 8

    Product[2]
      subproduct[2][0] price : 6
      subproduct[2][1] price : 12
==23024==
==23024== HEAP SUMMARY:
==23024==     in use at exit: 0 bytes in 0 blocks
==23024==   total heap usage: 9 allocs, 9 frees, 128 bytes allocated
==23024==
==23024== All heap blocks were freed -- no leaks are possible
==23024==
==23024== For counts of detected and suppressed errors, rerun with: -v
==23024== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)