NULL

 

C

Compiler support

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (C11)

Technical Specifications

Symbol index

 

Type support

size_t

ptrdiff_t

nullptr_t (C23)

NULL

max_align_t (C11)

offsetof

Numeric limits

Fixed width integer types (C99)

 

Defined in header <locale.h>
Defined in header <stddef.h>
Defined in header <stdio.h>
Defined in header <stdlib.h>
Defined in header <string.h>
Defined in header <time.h>
Defined in header <wchar.h>
#define NULL /*implementation-defined*/

The macro NULL is an implementation-defined null pointer constant, which may be

• an integer constant expression with the value ​0​
• an integer constant expression with the value ​0​ cast to the type void*

predefined constant nullptr

(since C23)

A null pointer constant may be converted to any pointer type; such conversion results in the null pointer value of that type.

Notes

POSIX requires NULL to be defined as an integer constant expression with the value ​0​ cast to void*.

Possible implementation

// C++ compatible: #define NULL 0 // C++ incompatible: #define NULL (10*2 - 20) #define NULL ((void*)0) // since C23 (compatible with C++11 and later) #define NULL nullptr

Example

Run this code

#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    // any kind of pointer can be set to NULL
    int* p = NULL;
    struct S *s = NULL;
    void(*f)(int, double) = NULL;
    printf("%p %p %p\n", (void*)p, (void*)s, (void*)(long)f);
 
    // many pointer-returning functions use null pointers to indicate error
    char *ptr = malloc(0xFULL);
    if (ptr == NULL)
        printf("Out of memory");
    else
        printf("ptr = %#" PRIxPTR"\n", (uintptr_t)ptr);
    free(ptr);
}

Possible output:

(nil) (nil) (nil)
ptr = 0xc001cafe

See also

nullptr_t (C23)	the type of the predefined null pointer constant nullptr (typedef)
C++ documentation for NULL