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Pointers in C

A pointer in C is a variable that stores the memory address of another variable. Instead of storing a value directly, a pointer holds the location where the value is stored.
For example, imagine a house where various items (values) are stored. The house address represents the memory location, and the items inside represent the stored data. Just like you use an address to locate a house, a pointer in C is used to access a value stored at a specific memory location.

Syntax:

int *ptr;
Example 1: 
#include <stdio.h>

int main() {
    int num = 10;  // A normal integer variable
    int *ptr;      // Pointer declaration

    ptr = &num;    // Assign address of num to ptr

    printf("Value of num: %d\n", num);
    printf("Address of num: %p\n", &num);
    printf("Pointer stores address: %p\n", ptr);
    printf("Value pointed by ptr: %d\n", *ptr); // Dereferencing the pointer

    return 0;
}

Output:

Value of num: 10
Address of num: 0x7ffd7f20e5d4
Pointer stores address: 0x7ffd7f20e5d4
Value pointed by ptr: 10

We use %p in printf to print memory addresses because %p is specifically designed to format and display pointer (address) values. Addresses are large values (often 8 bytes on 64-bit systems), and using %p ensures they are displayed correctly.

Explanation:

The format 0x7ffd7f20e5d4 is a memory address in hexadecimal notation. Let's break it down:
1. 0x Prefix:
✰ This indicates that the number is in hexadecimal (base 16) format.
✰ Hexadecimal uses digits 0-9 and letters A-F to represent values.

2. 7ffd7f20e5d4 (Hexadecimal Number):
✰ This is the actual memory address where the variable (or pointer) is stored.
✰ It is a virtual memory address assigned by the operating system.
✰ On a 64-bit system, addresses are typically 12-16 hex digits long.
✰ On a 32-bit system, addresses are usually 8 hex digits long.

Example Interpretation:
If you convert 0x7ffd7f20e5d4 to decimal, you get a large number, which is the actual memory location in RAM.

Memory Address Breakdown:
✰ 7ffd → Could indicate a stack memory segment (depends on OS).
✰ 7f20e5d4 → The specific location within that segment.
Key Takeaways:
✰ This address is dynamically assigned and varies each time the program runs.
✰ Addresses in heap and stack regions may look different (stack addresses are often close together).
✰ If Address Space Layout Randomization (ASLR) is enabled, the address may change every execution for security reasons.

Example 2: Swapping Two numbers using Pointers.
#include <stdio.h>

int main() {
    int a = 10, b = 20;
    int *num1 = &a, *num2 = &b;  // Pointers storing the addresses of a and b

    printf("Before Swap: a = %d, b = %d\n", a, b);

    // Swapping using pointers
    int temp = *num1;  
    *num1 = *num2;
    *num2 = temp;

    printf("After Swap: a = %d, b = %d\n", a, b);

    return 0;
}

Output:

Before swapping: num1 = 10, num2 = 20
After swapping: num1 = 20, num2 = 10
Example 3: Swapping Two numbers without using third variable in Pointers.
#include <stdio.h>

int main() {
    int a = 5, b = 15;
    int *p1 = &a, *p2 = &b;  // Pointers to a and b

    printf("Before Swap: a = %d, b = %d\n", a, b);

    // Swapping using arithmetic operations with pointers
    *p1 = *p1 + *p2; 
    *p2 = *p1 - *p2;  
    *p1 = *p1 - *p2; 

    printf("After Swap: a = %d, b = %d\n", a, b);

    return 0;
}

Output: 

Before Swap: a = 5, b = 15
After Swap: a = 15, b = 5