Library functions implementation (Using the GNU Compiler Collection (GCC))

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 NULL expands (C90 7.1.6, C99 7.17, C11 7.19).
In <stddef.h>, NULL expands to ((void *)0). GCC does not provide the other headers which define NULL and some library implementations may use other definitions in those headers.