c - Two Dimensional Array Implementation Using Double Pointer

Question

Please consider the following code:

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

#define NUM_ARRAYS     4
#define NUM_ELEMENTS   4
#define INVALID_VAL   -1

int main()
{
   int index            = INVALID_VAL;
   int array_index      = INVALID_VAL;
   int **ptr            = NULL;

   ptr = malloc(sizeof(int*)*NUM_ARRAYS);

   if (!ptr)
   {
      printf ("
Memory Allocation Failure !

");
      exit (EXIT_FAILURE);
   }

   for (index=0; index<NUM_ARRAYS; index++)
   {
      *(ptr+index) = malloc(sizeof(int)*NUM_ELEMENTS); 

      if (!*(ptr+index))
      {
         printf ("
Memory Allocation Failure !
");
         exit (EXIT_FAILURE);
      }
   }

   /* Fill Elements Into This 2-D Array */
   for (index=0; index<NUM_ARRAYS; index++)
   {
      for (array_index = 0; array_index<NUM_ELEMENTS; array_index++)
      {
         *(*(ptr+index)+array_index) = (array_index+1)*(index+1);
      }
   }

   /* Print Array Elements */
   for (index = 0; index<NUM_ARRAYS; index++)
   {
      printf ("
Array %d Elements:
", index);
      for (array_index = 0; array_index<NUM_ELEMENTS; array_index++)
      {
         printf (" %d ", *(*(ptr+index)+array_index));
      }
      printf ("

");
   }

   return 0;
}

There is no problem with my code. It works fine.

Output:

Array 0 Elements:
 1  2  3  4 


Array 1 Elements:
 2  4  6  8 


Array 2 Elements:
 3  6  9  12 


Array 3 Elements:
 4  8  12  16 

I have a question about pointer arithmetic:

*(ptr+0) = Pointer to COMPLETE BLOCK (First Array)
*(ptr+1) = Pointer to COMPLETE BLOCK (Second Array).

But what is: (*ptr+1)?

GDB Output:

(gdb) p *(*ptr+1)
$1 = 2
(gdb) p *(*ptr+2)
$2 = 3
(gdb) p *(*ptr+3)
$3 = 4
(gdb) p *(*ptr+4)
$4 = 0

I am getting confused on this. Please provide me some explanation to resolve this doubt.

Answer 1

                                 (*ptr)      (*ptr+1)     (*ptr+2)
                                   |            |            |
             __________      ______v____________v____________v____________
  ptr------>|   *ptr   |--->|  *(*ptr)   |  *(*ptr+1)  |*(*ptr+2) |       |
            |__________|    |____________|_____________|__________|_______|
 (ptr+1)--->| *(ptr+1) |     ____________ _____________ __________________
            |__________|--->|*(*(ptr+1)) |*(*(ptr+1)+1)|          |       |
            |          |    |____________|_____________|__________|_______|
            |__________|          ^             ^
                                  |             |
                              *(ptr+1)     *(ptr+1)+1

2D array with double pointers that means that you have a main array and the elements of the main array are pointers (or addresses) to a sub arrays. As indicated in above figure

so if you have defined a double pointer as a pointer of this 2D array let's say int **ptr

so ptr is ponting to the main array which will contains pointers to sub arrays. ptr is ponting to the main array that's means ptr is pointing to the first element of the main array so ptr + 1 is pointing to the second element of the main array.

*ptr this means the content of the first element which the ptr is pointing on. And it is a pointer to a subarray. so *ptr is a pointer to the first subarray (the subarray is an array of int). so *ptr is pointing to the first element in the first subarray. so *ptr + 1 is a pointer to the second element in the first subarray