2. Pointer Basics

2.1 Declaring Pointers

Syntax:

data_type *pointer_name;

Examples:

int *ptr;        // Pointer to an integer
float *fptr;     // Pointer to a float
char *cptr;      // Pointer to a character
double *dptr;    // Pointer to a double

Important Notes:

• The * (asterisk) indicates that the variable is a pointer
• The asterisk can be placed next to the type or the variable name
• All three declarations below are equivalent:

  int *ptr;
  int* ptr;
  int * ptr;
  

• Convention: Most C programmers use int *ptr style

Multiple Pointer Declaration:

int *p1, *p2, *p3;    // Three pointers to int
int *p1, p2, *p3;     // p1 and p3 are pointers, p2 is int
int* p1, p2, p3;      // Only p1 is pointer! p2 and p3 are int

2.2 Pointer Operators

There are two main operators for working with pointers:

2.3 Initializing Pointers

Method 1: Initialize with address of existing variable

int num = 42;
int *ptr = #  // ptr now points to num

Method 2: Initialize to NULL

int *ptr = NULL;  // Pointer points to nothing (safe initialization)

Method 3: Uninitialized (DANGEROUS)

int *ptr;  // Contains garbage value - DO NOT USE until initialized!

Visual Representation:

Memory Layout:

Variable: num = 42
Address:  0x1000
         ┌──────┐
0x1000:  │  42  │  num
         └──────┘

Variable: ptr
Address:  0x2000
         ┌────────┐
0x2000:  │ 0x1000 │  ptr (points to num)
         └────────┘

2.4 Using Pointers - The & and * Operators

#include <stdio.h>

int main() {
    int num = 100;
    int *ptr;
    
    ptr = &num;  // Store address of num in ptr
    
    printf("Value of num: %d\n", num);           // Direct access
    printf("Address of num: %p\n", (void*)&num); // Address of num
    printf("Value of ptr: %p\n", (void*)ptr);    // Address stored in ptr
    printf("Value pointed to by ptr: %d\n", *ptr); // Dereference ptr
    
    // Modify through pointer
    *ptr = 200;
    
    printf("\nAfter *ptr = 200:\n");
    printf("Value of num: %d\n", num);           // num changed!
    printf("Value pointed to by ptr: %d\n", *ptr);
    
    return 0;
}

/* Output:
Value of num: 100
Address of num: 0x7ffd5c8e4a3c
Value of ptr: 0x7ffd5c8e4a3c
Value pointed to by ptr: 100

After *ptr = 200:
Value of num: 200
Value pointed to by ptr: 200
*/

Understanding the Operations:

int num = 42;
int *ptr = #

// These are equivalent:
num = 100;     // Direct modification
*ptr = 100;    // Indirect modification through pointer

// Both operations change the same memory location