NULL
From cppreference.com
C
Type support
(C23) | ||||
NULL | ||||
(C11) | ||||
| Numeric limits | ||||
| Fixed width integer types (C99) |
| 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 0 cast to the type void*
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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.
Contents |
[edit] Notes
POSIX requires NULL to be defined as an integer constant expression with the value 0 cast to void*.
[edit] 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 |
[edit] 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
[edit] See also
| (C23) |
the type of the predefined null pointer constant nullptr (typedef) [edit] |
| C++ documentation for NULL
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