17 Language support library [support]

17.14 C headers [support.c.headers]

17.14.7 Other C headers [support.c.headers.other]

Every C header other than <complex.h>, <iso646.h>, <stdalign.h>,
<stdatomic.h>, <stdbool.h>, and <tgmath.h>, each of which has a name of the form <name.h>, behaves as if each name placed in the standard library namespace by the corresponding <cname> header is placed within the global namespace scope, except for the functions described in [sf.cmath], the declaration of std​::​byte ([cstddef.syn]), and the functions and function templates described in [support.types.byteops].
It is unspecified whether these names are first declared or defined within namespace scope ([basic.scope.namespace]) of the namespace std and are then injected into the global namespace scope by explicit using-declarations ([namespace.udecl]).
[Example 1:
The header <cstdlib> assuredly provides its declarations and definitions within the namespace std.
It may also provide these names within the global namespace.
The header <stdlib.h> assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard.
It may also provide these names within the namespace std.
— end example]