C - Dereference operator

The dereference operator (*) in C is used to access the value stored at a particular memory address, typically through a pointer. It is essential for working with pointers and dynamic memory.

Learning Objectives

• Use * to read and write through pointers.
• Avoid undefined behavior by checking pointer validity.

Prerequisites

• C - Pointers
• C - Address-of operator

What is the Dereference Operator?

The dereference operator (*) allows you to access or modify the value pointed to by a pointer variable.

Syntax

*pointer_variable

Simple Example

#include <stdio.h>

int main(void) {
    int x = 20;
    int *ptr = &x;
    printf("Value of x: %d\n", x);
    printf("Value via pointer: %d\n", *ptr);
    return 0;
}

Output:

Value of x: 20
Value via pointer: 20

Modifying Values via Pointers

#include <stdio.h>

void setToZero(int *p) {
    *p = 0;
}

int main(void) {
    int num = 5;
    setToZero(&num);
    printf("num after setToZero: %d\n", num);
    return 0;
}

Output:

num after setToZero: 0

Checks for Understanding

1. What happens if you dereference a NULL pointer?
2. How do you change the value that a pointer points to?

Common Pitfalls

• Dereferencing an uninitialized or NULL pointer leads to undefined behavior (crash).
• Always ensure the pointer is valid before dereferencing.

Summary

The dereference operator * is crucial for pointer manipulation and indirect access to variables in C. Practice using * with pointers to understand its behavior.