Pointers in C

Addresses and pointers in C

Let us declare a single variable and array, and see how they are placed in memory

/* Online C Compiler and Editor */
#include <stdio.h>

int main()
{
    printf("Declaring an integer and an integer array\n");
    int num=7853;
    int d[3] = {1,2,7};
    printf("num= %d\n",num);
    for(int i =0;i<3;i++)
        printf("Integer array element d[%d] = %d\n",i,d[i]);
    
    int  * pd = d;
    printf("addresss of d = %ld\n",pd);
    
    int  * pd0 = &(d[0]);
    printf("addresss of d[0] = %ld\n",pd0);

    int  * pe = d+1;
    printf("addresss of e = %ld\n",pe);
    
    int  * pd1 = &(d[1]);
    printf("addresss of d[1] = %ld\n",pd1);
        
    return 0;
}

Output:

Declaring an integer and an integer array
num= 7853
Integer array element d[0] = 1
Integer array element d[1] = 2
Integer array element d[2] = 7
addresss of d = 140727830638268
addresss of d[0] = 140727830638268
addresss of e = 140727830638272
addresss of d[1] = 140727830638272

Notes:

• The address of a variable is the location of it's first byte
• Pointers and arrays are closely related
• The address of a variable can be stored in another variable
• Addresses must be stored in special types of variables
• In C language,these special variables called pointers.
  • the C compiler does not allow to store pointers in normal integers/long variables
  • this is possible to trick the compiler via casting etc, but this is not reccomended.
• By storing addresses in 'pointer' variables,
  • C language knows the value is a pointer and treats it in special ways
  • C language also takes into account, the 'size' of the data type held in a pointer
• Recap: Addresses and pointers:
  • Address is the location.
  • Pointer is a variable that stores the location.
• if d is an array (of any type), say int d[10];
  • then the address of d and address of d[0] are the same
  • the first element of the array
    • can be written as: d[0]
    • can also be written as: *(d) or *d
    • i.e. *(d) refers to first element, not entire array
    • you can think of operator * as "content-of"
  • the array indices in above case starts at 0 and ends at 9 (not 10)
  • the nth element of the array
    • can be written as d[n]
    • can also be written as *(d+n)
  • Adding index n to d produces address of d[n]
  • this means the content of *(d+n) and d[n] all point to the same value
• C is aware if array contains 'integers' or 'char' or 'double'
• C is aware of the byte size of each of these variables
• to get address of [n] the element,
  • C compiler does not simply add n to d (i.e. d+n)
  • instead, it adds (sizeof int)* n to d.