C NULL

🔹 C NULL

C NULL is a special constant in C used to represent a null pointer, meaning the pointer does not point to any valid memory location.

It is defined in multiple header files such as:

✅ What is NULL?

NULL stands for “no value” or “empty pointer”.

Example:

This means ptr is declared but doesn’t point to any memory address.

🧠 Why Use NULL?

NULL helps avoid:

Dangling pointers
Invalid memory access
Unpredictable program crashes

Using NULL makes it easy to test whether a pointer has a valid address.

📌 Checking NULL Pointer

🧪 Example With Dynamic Memory

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

int main() {
int *p = malloc(sizeof(int));

if (p != NULL) {
*p = 50;
printf("Value: %d\n", *p);
}

free(p);
p = NULL; // Best practice

if (p == NULL) {
printf("Pointer is now NULL.\n");
}

return 0;
}

#include <stdio.h>

#include <stdlib.h>

int main() {

int *p = malloc(sizeof(int));

if (p != NULL) {

*p = 50;

printf("Value: %d\n", *p);

}

free(p);

p = NULL; // Best practice

if (p == NULL) {

printf("Pointer is now NULL.\n");

}

return 0;

}

💡 After freeing memory, setting pointer to NULL prevents accidental reuse.

🔥 NULL vs Uninitialized Pointer

Pointer State	Example	Safe?
NULL pointer	`int *p = NULL;`	✔ Safe to check
Uninitialized pointer	`int *p;`	❌ Dangerous

Uninitialized pointers hold garbage values, which may point to invalid or system memory.

🚨 Don’t Dereference a NULL Pointer

This causes a segmentation fault.

✔ NULL vs 0

In C, NULL is often defined as:

But semantically:

0 is an integer
NULL is a pointer constant

So use NULL for pointers:

🏁 Summary

Concept	Meaning
NULL	Special value indicating no valid address
Used with	Pointers
Common use	Checking valid memory before access
Prevents	Crashes and undefined behavior