20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints]

20.5.4.3 Reserved names [reserved.names]

20.5.4.3.3 External linkage [extern.names]

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,178 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.179
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,180 or as a name of namespace scope in the global namespace.
The list of such reserved names includes errno, declared or defined in <cerrno>.
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.