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
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.