Difference between Null Pointer and Dangling Pointer

Last Updated on November 27, 2023 by Ankit Kochar

Understanding pointers in programming languages is fundamental, yet distinguishing between specific types of pointers, such as Null Pointers and Dangling Pointers, can be perplexing for many developers. Pointers play a pivotal role in memory management, and misconceptions about these types of pointers can lead to unexpected program behaviors, crashes, or security vulnerabilities. This article aims to elucidate the disparities between Null Pointers and Dangling Pointers, shedding light on their definitions, characteristics, and implications in programming.

Null Pointers

These are the type of pointers that do not point to a valid memory address. Instead, they point to a special value which is known as a null pointer value. In programming languages like C and C++, these values are represented by 0 or a constant NULL. They are usually used to indicate that a pointer is not currently pointing to a valid memory address or to initialize pointers to a known value.

The process when the program tries to the memory location of the pointer and uses the value at that particular memory location is known as dereferencing. So when a null pointer is dereferenced an access violation error or segmentation fault will occur. This is because the null pointer is not associated with any valid memory address so any attempt to access that address will result in a memory access violation.

Example of Null Pointer

Now we will look at an example of the null pointer with its code implementation.

#include <stdio.h>
 
int main(void) {
int* ptr = NULL;
*ptr = 10;
 
}

Output

Segmentation Error

Explanation of the above code
In the above example we have created a null pointer ptr and then try to assign the value 10 to the memory location pointed by it. But ptr is a null pointer so it does not point to any valid memory address and when you try to dereference it segmentation fault error will occur.

Dangling Pointers

A dangling pointer is a type of pointer that points to a memory location that has been deallocated or otherwise freed. When you have deallocated memory block then the memory is returned to the system and the program will not be able to access that memory any longer. And if the pointer still points to that memory location it becomes a dangling pointer.
There are many problems associated with dangling pointers depending on how they are used. In some cases, it can cause segmentation fault error or access violation error when trying to deference them just like a null pointer. By simplifying the above statement the program may continue to run but it will give unexpected or incorrect results.

Example of Dangling Pointer

In this section, we will see an example of dangling pointer with code and implementation.

#include <stdio.h>

int main(void) {
int* ptr = (int*) malloc(sizeof(int));
*ptr = 10;
free(ptr);
*ptr = 20;
    return 0;
}

Output

sample.c: In function 'main':
sample.c:4:19: warning: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
 int* ptr = (int*) malloc(sizeof(int));
                   ^~~~~~
sample.c:4:19: warning: incompatible implicit declaration of built-in function 'malloc'
sample.c:4:19: note: include '' or provide a declaration of 'malloc'
sample.c:6:1: warning: implicit declaration of function 'free' [-Wimplicit-function-declaration]
 free(ptr);
 ^~~~
sample.c:6:1: warning: incompatible implicit declaration of built-in function 'free'
sample.c:6:1: note: include '' or provide a declaration of 'free'

Explanation of the above code
This code dynamically allocates a block of memory large enough to hold an integer using the malloc function and then assigns the value 10 to the memory location pointed to by ptr. Next, the free function is called to deallocate the memory block pointed to by ptr. Finally, the program attempts to assign the value 20 to the memory location pointed to by ptr.

Since the memory block pointed to by ptr has been deallocated, ptr is now a dangling pointer. Attempting to dereference it (i.e., assigning the value 20 to the memory location pointed to by ptr) will result in undefined behavior. In some cases, this may cause the program to crash or produce incorrect results.

Difference Between Null Pointer and Dangling Pointer

In this section we will see various differences between a null pointer and a dangling pointer.

Conclusion
Distinguishing between Null Pointers and Dangling Pointers is critical for writing robust and secure programs. While Null Pointers represent intentional absence or uninitialized values, Dangling Pointers stem from referencing deallocated memory. Understanding their differences and employing best practices in pointer management is imperative to avoid unexpected runtime errors, crashes, and security vulnerabilities in software development. By grasping these nuances, programmers can fortify their code against common pitfalls associated with these pointer types, ensuring more reliable and resilient applications.

Frequently Asked Questions Related to Difference Between Null Pointer and Dangling Pointer

Here are some of the frequently asked questions about the difference between a null pointer and a dangling pointer.

1. What is a Null Pointer?
A Null Pointer is a pointer that doesn’t point to any memory location. It explicitly holds a null value, typically represented as 0 in most programming languages. Dereferencing a Null Pointer often leads to a runtime error or undefined behavior, as it doesn’t reference valid memory.

2. What is a Dangling Pointer?
A Dangling Pointer is a pointer that points to a memory location that has been freed or deallocated. When the memory it points to is no longer available, using or dereferencing a Dangling Pointer can result in unpredictable behavior, leading to crashes or security vulnerabilities.

3. How are Null Pointers and Dangling Pointers different?
Null Pointers explicitly point to no memory location, serving as a deliberate placeholder to indicate absence or uninitialized values. Dangling Pointers, on the other hand, refer to memory locations that were previously valid but have since been freed, resulting in pointers that inadvertently point to inaccessible or undefined memory.

4. What are the common pitfalls associated with Null and Dangling Pointers?
Mishandling Null Pointers might cause unexpected crashes or undefined behavior when attempting to access nonexistent memory. Dangling Pointers, if accessed after the memory they point to has been deallocated, can lead to security vulnerabilities, data corruption, or program crashes.

5. How can programmers prevent issues related to these pointers?
Utilizing defensive programming practices, such as initializing pointers to null values, validating pointers before dereferencing, and avoiding referencing memory after deallocation, can help mitigate Null and Dangling Pointer issues. Additionally, using modern programming languages or tools equipped with features like automatic memory management can reduce the likelihood of these problems.