Question 5.4

What is NULL and how is it #defined?

As a matter of style, many programmers prefer not to have unadorned 0's scattered through their programs. Therefore, the preprocessor macro NULL is #defined (by <stdio.h> or <stddef.h>) with the value 0, possibly cast to (void *) (see also question 5.6). A programmer who wishes to make explicit the distinction between 0 the integer and 0 the null pointer constant can then use NULL whenever a null pointer is required.

Using NULL is a stylistic convention only; the preprocessor turns NULL back into 0 which is then recognized by the compiler, in pointer contexts, as before. In particular, a cast may still be necessary before NULL (as before 0) in a function call argument. The table under question 5.2 above applies for NULL as well as 0 (an unadorned NULL is equivalent to an unadorned 0).

NULL should only be used for pointers; see question 5.9.

References: K&R1 Sec. 5.4 pp. 97-8
K&R2 Sec. 5.4 p. 102
ANSI Sec. 4.1.5, Sec. 3.2.2.3
ISO Sec. 7.1.6, Sec. 6.2.2.3
Rationale Sec. 4.1.5
H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292

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