gcc: Library functions implementation

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1 4.14 Library Functions
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1 The behavior of most of these points are dependent on the implementation
1 of the C library, and are not defined by GCC itself.
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1    * 'The null pointer constant to which the macro 'NULL' expands (C90
1      7.1.6, C99 7.17, C11 7.19).'
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1      In '<stddef.h>', 'NULL' expands to '((void *)0)'.  GCC does not
1      provide the other headers which define 'NULL' and some library
1      implementations may use other definitions in those headers.
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