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### 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 `NULL` expands (C90 7.1.6, C99 7.17).

  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.