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{{c/title|NULL}}
{{c/types/navbar}}
{{dcl begin}}
{{dcl header|locale.h}}
{{dcl header|stddef.h}}
{{dcl header|stdio.h}}
{{dcl header|stdlib.h}}
{{dcl header|string.h}}
{{dcl header|time.h}}
{{dcl header|wchar.h}}
{{dcl|
#define NULL /*implementation-defined*/
}}
{{dcl end}}

The macro {{tt|NULL}} is an implementation-defined null pointer constant, which may be
* an integer [[c/language/constant_expression#Integer_constant_expression|constant expression]] with the value {{c|0}}
* an integer constant expression with the value {{c|0}} [[c/language/conversion#Pointer conversions|cast to the type]] {{c|void*}}
{{rrev|since=c23|
* predefined constant {{ltt|c/language/nullptr}}
}}

A null pointer constant may be [[c/language/conversion#Pointer conversions|converted]] to any pointer type; such conversion results in the null pointer value of that type.

===Notes===
[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stddef.h.html POSIX requires] {{tt|NULL}} to be defined as an integer constant expression with the value {{c|0}} cast to {{c|void*}}.

===Possible implementation===
{{eq fun
|1=
// 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===
{{example
|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);
}
|p=true
|output=
(nil) (nil) (nil)
ptr = 0xc001cafe
}}

===See also===
{{dsc begin}}
{{dsc inc|c/types/dsc nullptr_t}}
{{dsc see cpp|cpp/types/NULL}}
{{dsc end}}

{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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