Q: If NULL and 0 are equivalent as null pointer constants, which should I use?
A: Many programmers believe that NULL should be used in all pointer contexts, as a reminder that the value is to be thought of as a pointer. Others feel that the confusion surrounding NULL and 0 is only compounded by hiding 0 behind a macro, and prefer to use unadorned 0 instead. There is no one right answer. (See also questions 9.4 and 17.10.) C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. Any usage of NULL (as opposed to 0) should be considered a gentle reminder that a pointer is involved; programmers should not depend on it (either for their own understanding or the compiler's) for distinguishing pointer 0's from integer 0's.
It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required, even though it might work, because doing so sends the wrong stylistic message. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all in non-pointer contexts.) In particular, do not use NULL when the ASCII null character (NUL) is desired. Provide your own definition
#define NUL '\0'if you must.
K&R1 Sec. 5.4 pp. 97-8
K&R2 Sec. 5.4 p. 102