4.14 Library functions
The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
- The null pointer constant to which the macro
NULLexpands (C90 7.1.6, C99 7.17).
((void *)0). GCC does not provide the other headers which define
NULLand some library implementations may use other definitions in those headers.