Description

The NULL macro is defined by many headers to expand to an implementation-defined null pointer constant. The advantage over using literal 0 is the warnings produced by current compilers on most forms of improper use. It might simply be defined as:

#define NULL 0

Definition

Defined in <stddef.h>, <locale.h>, <stdio.h>, <stdlib.h>, <string.h>, <time.h>.

In standard C, this can be implemented as:

#if !defined(NULL)
    #define NULL ((void*)0)
#endif

Code snippet, portable C90; public domain; past reviewers: none; current reviews: none

Interpretation wrangling

Some people claim that by a strict reading of the Standard, after parenthesising the expression (void *)0, it's no longer a null pointer constant, even though the parentheses are simultaneously required by the Standard to protect the macro expansion. No one seems to dispute that it's the Standard's intention that ((void *)0) is a valid definition of NULL, nor that any other reading is sensible.

References

• The C Standard: 7.17#3, 17.19.1#3, 7.20#3, 7.21.1#1, 7.23.1#2, 7.24.1#3 (N1124 numbering)
• FAQ section 5 Null Pointers