what is null in c

June 28, 2026

In C, NULL is a special macro that represents a null pointer constant —a pointer that points to no valid object or function at all.

Quick Scoop: What is `NULL` in C?

NULL is a macro defined in standard headers like stdio.h, stddef.h, stdlib.h, etc.

It expands to an implementation-defined null pointer constant, typically 0 or ((void*)0).

It is used only with pointers to mean “this pointer does not point anywhere valid.”

Example:

c

#include <stdio.h>
#include <stddef.h>

int main(void) {
    int *p = NULL;      // p is a null pointer
    if (p == NULL) {
        printf("p points to nothing\n");
    }
    return 0;
}

Mini Section: Why `NULL` Exists

You use NULL to make pointer code safer and clearer.

Common uses:

Initialize pointers when you don’t yet have a valid address.
```
c

int *ptr = NULL;
```

Check before dereferencing to avoid crashes like segmentation faults.
```
c

if (ptr != NULL) {
    printf("%d\n", *ptr);
}
```

Signal “no object” in APIs (e.g., return NULL if a search fails, or accept NULL when the caller has “nothing” to pass).
```
c

char *find_user(const char *name); // returns NULL if not found
```

Mark ends in data structures , such as the last node of a linked list using next = NULL.

Mini Section: `NULL` vs `0` vs “null”

In C, NULL is really just a nicer way to write the null pointer constant.

Writing
```
c

int *p = NULL;
int *q = 0;
```

is equivalent: both p and q are null pointers.

The C standard allows the null pointer constant to be 0 or ((void*)0), and NULL is defined accordingly in headers.

Many languages say “null” generally means “no value,” but in C it specifically means a null pointer.

Mini Section: What `NULL` Is Not

It’s easy to mix up a few concepts when learning C pointers:

NULL is not the same as an empty string ("").
NULL is not the same as integer 0 in a variable like int x = 0;; that is just a number, not a pointer.

A null pointer does not point to a valid memory location you can use , so dereferencing it (*ptr when ptr == NULL) is undefined behavior and can crash the program.

A small illustration:

c

int x = 0;        // integer with value 0, perfectly usable
int *p = NULL;    // pointer to nothing (null pointer)

// *p here would be invalid, but x is fine to read and write

Mini Section: Real-World Style Example

You’ll often see NULL in functions that work with dynamic memory:

c

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

int main(void) {
    int *numbers = malloc(100 * sizeof(int));

    if (numbers == NULL) {            // check for allocation failure
        printf("Memory allocation failed.\n");
        return 1;
    }

    // ... use numbers ...

    free(numbers);                    // release memory
    numbers = NULL;                   // avoid dangling pointer
    return 0;
}

Here, NULL is used:

To detect malloc failure (numbers == NULL).
To “reset” the pointer after free so it clearly no longer refers to valid memory.

TL;DR

In C, NULL is a macro that represents a null pointer constant, usually 0 or ((void*)0), and it’s used with pointers to mean “this points to nothing.”

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what is null in c

Quick Scoop: What is NULL in C?

Mini Section: Why NULL Exists