Entities in the C++ standard library have external linkage ([basic.link]).
Unless otherwise specified, objects and functions have the default
Whether a name from the C standard library declared with
external linkage has
linkage is implementation-defined. It is recommended that an
linkage for this purpose.
Objects and functions
defined in the library and required by a C++ program are included in
the program prior to program startup.
replacement functions ([replacement.functions]),
run-time changes ([handler.functions]).