NULL
From cppreference.com
| Defined in header <locale.h>
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| Defined in header <stddef.h>
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| Defined in header <stdio.h>
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| Defined in header <stdlib.h>
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| Defined in header <string.h>
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| Defined in header <time.h>
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| Defined in header <wchar.h>
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#define NULL /*implementation-defined*/
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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
0cast to the typevoid*
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(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
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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
(C23) |
the type of the predefined null pointer constant nullptr (typedef) |
C++ documentation for NULL
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