Annex D (normative) Compatibility features [depr]

D.9 C headers [depr.c.headers]

D.9.6 Other C headers [depr.c.headers.other]

1
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Every C header other than <complex.h> ([depr.complex.h.syn]), <iso646.h> ([depr.iso646.h.syn]), <stdalign.h> ([depr.stdalign.h.syn]), <stdbool.h> ([depr.stdbool.h.syn]), and <tgmath.h> ([depr.tgmath.h.syn]), 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.
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[Example
:
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
]