NULL
From cppreference.com
| Defined in header <stddef.h>
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| Defined in header <string.h>
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| Defined in header <wchar.h>
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| Defined in header <time.h>
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| Defined in header <locale.h>
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| Defined in header <stdio.h>
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| Defined in header <stdlib.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*
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) |
Example
Run this code
#include <stdlib.h> #include <stdio.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; // many pointer-returning functions use null pointers to indicate error char *ptr = malloc(10); if (ptr == NULL) printf("Out of memory"); free(ptr); }