---
name: "NULL"
section: "3const"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/NULL"
---

# NULL(3const)

null pointer constant

## NAME

NULL - null pointer constant

## LIBRARY

Standard C library (*libc*)

## SYNOPSIS

```
#include <stddef.h>
```

**#define NULL  ((void \*) 0)**

## DESCRIPTION

**NULL** represents a null pointer constant, that is, a pointer that does not point to anything.

## STANDARDS

C11, POSIX.1-2024.

## HISTORY

C89, POSIX.1-1988.

## NOTES

The following headers also provide **NULL**: *\<locale.h\>*, *\<stdio.h\>*, *\<stdlib.h\>*, *\<string.h\>*, *\<time.h\>*, *\<unistd.h\>*, and *\<wchar.h\>*.

## CAVEATS

It is undefined behavior to dereference a null pointer, and that usually causes a segmentation fault in practice.

It is also undefined behavior to perform pointer arithmetic on it.

*NULL - NULL* is undefined behavior, according to ISO C, but is defined to be 0 in C++.

To avoid confusing human readers of the code, do not compare pointer variables to **0**, and do not assign **0** to them. Instead, always use **NULL**.

**NULL** shouldn't be confused with **NUL**, which is an [**ascii**(7)](/ascii/7.md) character, represented in C as **'\\0'**.

## BUGS

When it is necessary to set a pointer variable to a null pointer, it is not enough to use [**memset**(3)](/memset/3.md) to zero the pointer (this is usually done when zeroing a struct that contains pointers), since ISO C and POSIX don't guarantee that a bit pattern of all 0s represent a null pointer. See the EXAMPLES section in [**getaddrinfo**(3)](/getaddrinfo/3.md) for an example program that does this correctly.

## SEE ALSO

[**void**(3type)](/void/3type.md)