note of C FAQ
颠覆了我多年的观念
5.9: 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 9.2 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.
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 ”
if you must.
References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102.