NULL

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

(since C23)

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

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

#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

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