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,144
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.145
Each name from the C standard library declared with external linkage
is reserved to the implementation for use as a name with
extern"C"
linkage,
both in namespace std and in the global namespace.
Each function signature from the C standard library declared with
external linkage
is reserved to the implementation for use as
a function signature with both
extern"C"
and
extern"C++"
linkage,146
or as a name of namespace scope in the global namespace.
The list of such reserved function
signatures with external linkage includes
setjmp(jmp_buf),
declared or defined in <csetjmp>,
and
va_end(va_list),
declared or defined in
<cstdarg>.
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.