Some behavior is specified by reference to POSIX. How such behavior is actually implemented is unspecified.
[ Note: This constitutes an “as if” rule allowing implementations to call native operating system or other APIs. — end note ]
Implementations are encouraged to provide such behavior as it is defined by POSIX. Implementations shall document any behavior that differs from the behavior defined by POSIX. Implementations that do not support exact POSIX behavior are encouraged to provide behavior as close to POSIX behavior as is reasonable given the limitations of actual operating systems and file systems. If an implementation cannot provide any reasonable behavior, the implementation shall report an error as specified in Error Reporting ([err.report]).
[ Note: This allows users to rely on an exception being thrown or an error code being set when an implementation cannot provide any reasonable behavior. — end note ]
Implementations are not required to provide behavior that is not supported by a particular operating system.