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

### 4.16 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 `NULL` expands (C90 7.1.6, C99 7.17, C11 7.19, C23 7.21).

  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.