NULL − null pointer constant
Standard C library (libc)
#include <stddef.h>
#define NULL ((void *) 0)
NULL represents a null pointer constant, that is, a pointer that does not point to anything.
C99 and later; POSIX.1-2001 and later.
The following headers also provide NULL: <locale.h>, <stdio.h>, <stdlib.h>, <string.h>, <time.h>, <unistd.h>, and <wchar.h>.
It is undefined behavior to dereference a null pointer, and that usually causes a segmentation fault in practice.
It is also undefined behavior to perform pointer arithmetic on it.
NULL − NULL is undefined behavior, according to ISO C, but is defined to be 0 in C++.
To avoid confusing human readers of the code, do not compare pointer variables to 0, and do not assign 0 to them. Instead, always use NULL.
NULL shouldn’t be confused with NUL, which is an ascii(7) character, represented in C as '\0'.
When it is necessary to set a pointer variable to a null pointer, it is not enough to use memset(3) to zero the pointer (this is usually done when zeroing a struct that contains pointers), since ISO C and POSIX don’t guarantee that a bit pattern of all 0s represent a null pointer. See the EXAMPLES section in getaddrinfo(3) for an example program that does this correctly.
void(3type)