Pointers in C

1. Definition

A pointer is a variable that stores the memory address of another variable. Pointers allow for efficient manipulation of data and enable advanced programming techniques like dynamic memory allocation.

2. Why Use Pointers?

• Efficient memory management.
• Enables dynamic memory allocation.
• Facilitates passing arrays, structures, and functions as arguments.
• Useful in data structures like linked lists and trees.

3. Syntax

data_type *pointer_name;

Example:

int *ptr;

Here, ptr is a pointer to an integer.

4. Pointer Operators

1. Address-of Operator (&):

   • Used to get the memory address of a variable.

   • Example:

     int a = 10;
     int *ptr = &a; // ptr holds the address of a
     

2. Dereference Operator (\*):

   • Used to access the value at the address stored in the pointer.

   • Example:

     printf("%d", *ptr); // Outputs: 10
     

5. Pointer Initialization

A pointer must be initialized with the address of a variable before it is used.

int a = 5;
int *ptr = &a;

6. Pointer Example

Basic Example:

#include <stdio.h>

int main() {
    int a = 10;
    int *ptr = &a;

    printf("Value of a: %d\n", a);
    printf("Address of a: %p\n", &a);
    printf("Value of ptr: %p\n", ptr);
    printf("Value at ptr: %d\n", *ptr);

    return 0;
}

7. Null Pointer

A pointer that does not point to any valid memory address.

int *ptr = NULL;

Use Case:

• To check if a pointer is valid before dereferencing it.

8. Pointer Arithmetic

Pointers support arithmetic operations like addition and subtraction.

Example:

int arr[3] = {10, 20, 30};
int *ptr = arr;

printf("%d\n", *ptr);       // Outputs: 10
printf("%d\n", *(ptr + 1)); // Outputs: 20
printf("%d\n", *(ptr + 2)); // Outputs: 30

9. Pointers and Arrays

• The name of an array acts as a pointer to its first element.

• Example:

  int arr[3] = {10, 20, 30};
  int *ptr = arr;
  
  printf("%d\n", ptr[1]); // Outputs: 20
  

10. Pointer to Pointer

A pointer that stores the address of another pointer.

int a = 10;
int *ptr = &a;
int **pptr = &ptr;

printf("%d\n", **pptr); // Outputs: 10

11. Dynamic Memory Allocation

Pointers are used for dynamic memory allocation using functions like malloc, calloc, and free.

Example:

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

int main() {
    int *ptr = (int *)malloc(sizeof(int));
    *ptr = 25;
    printf("Value: %d\n", *ptr);
    free(ptr); // Freeing allocated memory
    return 0;
}

12. Common Errors with Pointers

• Uninitialized Pointers: Can lead to undefined behavior.
• Dangling Pointers: Occur when memory is deallocated but the pointer still points to it.
• Memory Leaks: Forgetting to free dynamically allocated memory.

13. Advantages of Pointers

• Enables dynamic memory usage.
• Simplifies the manipulation of arrays and structures.
• Makes it possible to create complex data structures like linked lists.