Each name declared as an object with external linkage in a header is reserved to the implementation to designate that library object with external linkage,183 both in namespace std and in the global namespace.
Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage. 184
Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, 185 or as a name of namespace scope in the global namespace.
The function signatures declared in <cuchar>, <cwchar>, and <cwctype> are always reserved, notwithstanding the restrictions imposed in subclause 4.5.1 of Amendment 1 to the C Standard for these headers.