C - Pointers

 Jargon

• What's a Jargon?

– Jargon may refer to terminology used in a certain profession, such as computer jargon, or it may refer to any nonsensical language that is not understood by most people.

– Speech or writing having unusual or pretentious
vocabulary, convoluted phrasing, and vague meaning.

• Pointer are perceived difficult

– Because of jargonification

• So, let's dejargonify & understand them

Pointers

• Just like a book analogy, Computers contains different different sections (Code) in the memory
• All sections have different purposes
• Every section has a address and we need to point to them whenever required
• In fact everything (Instructions and Data) in a particular section has a address!!
• So the pointer concept plays a big role here

Use of Pointers:

• To have C as a low level language being a high level language
• Returning more than one value from a function
• To achieve the similar results as of ”pass by variable”
• Parameter passing mechanism in function, by passing the reference 
• To have the dynamic allocation mechanism

The 7 Rules of Pointer:

• Rule 1 - Pointer is an Integer
• Rule 2 - Referencing and De-referencing
• Rule 3 - Pointing means Containing
• Rule 4 - Pointer Type
• Rule 5 - Pointer Arithmetic
• Rule 6 - Pointing to Nothing
• Rule 7 - Static vs Dynamic Allocation

Pointer - Multilevel

• A pointer, pointing to another pointer which can be pointing to others pointers and so on is know as multilevel pointers.
• We can have any level of pointers.
• As the depth of the level increase we have to bit careful while dealing with it.

 Example :
#include <stdio.h>
int main()
{
      int num = 10;
      int *ptr1 = &num;
      int **ptr2 = &ptr1;
      int ***ptr3 = &ptr2;
      printf( “%d” , ptr3);
      printf( “%d” , *ptr3);
      printf( “%d” , **ptr3);
      printf( “%d” , ***ptr3);
    return 0;
}
 Pointers – Constant Pointer

• A read only pointer
• Valid address MUST be assigned while defining constant pointer
• Once address assigned, it can't be changed

• Pointers – 2D Array:

#include <stdio.h>
int main()
{
       int a[2][3] = {1, 2, 3, 4, 5, 6};
       return 0;
}

          "Total Memory: ROWS * COLS * sizeof(datatype) Bytes"

•   Pointers – 2D Array - Referencing:

   

  •  Pointers – 2D Array - Dereferencing:

    

    Example 1: Say a[0][1] is to be accessed, then decomposition happens like,:
    a[0][1] = *(a[0] + 1 * sizeof(type))
    A[0][1] = *(*(a + 0 * sizeof(1D array)) + 1 * sizeof(type))
    A[0][1] = *(*(1000 + 0 * 12) + 1 * 4)
    A[0][1] = *(*(1000 + 0) + 4)
    A[0][1] = *(*(1004))
    A[0][1] = 2
    Example 2: Say a[1][2] is to be accessed, then decomposition happens like,
    a[1][2] = *(a[1] + 2 * sizeof(type))
    A[0][1] = *(*(a + 1 * sizeof(1D array)) + 2 * sizeof(type))
    A[0][1] = *(*(1000 + 1 * 12) + 2 * 4)
    A[0][1] = *(*(1000 + 12) + 8)
    A[0][1] = *(*(1020))
    A[0][1] = 6