31 Input/output library [input.output]

31.12 File systems [filesystems]

31.12.2 Conformance [fs.conformance] General [fs.conformance.general]

Conformance is specified in terms of behavior.
Ideal behavior is not always implementable, so the conformance subclauses take that into account. POSIX conformance [fs.conform.9945]

Some behavior is specified by reference to POSIX.
How such behavior is actually implemented is unspecified.
[Note 1: 
This constitutes an “as if” rule allowing implementations to call native operating system or other APIs.
— end note]
Implementations should 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 should 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 [fs.err.report].
[Note 2: 
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 file system.
[Example 1: 
The FAT file system used by some memory cards, camera memory, and floppy disks does not support hard links, symlinks, and many other features of more capable file systems, so implementations are not required to support those features on the FAT file system but instead are required to report an error as described above.
— end example] Operating system dependent behavior conformance [fs.conform.os]

Behavior that is specified as being operating system dependent is dependent upon the behavior and characteristics of an operating system.
The operating system an implementation is dependent upon is implementation-defined.
It is permissible for an implementation to be dependent upon an operating system emulator rather than the actual underlying operating system. File system race behavior [fs.race.behavior]

A file system race is the condition that occurs when multiple threads, processes, or computers interleave access and modification of the same object within a file system.
Behavior is undefined if calls to functions provided by subclause [filesystems] introduce a file system race.
If the possibility of a file system race would make it unreliable for a program to test for a precondition before calling a function described herein, Preconditions: is not specified for the function.
[Note 1: 
As a design practice, preconditions are not specified when it is unreasonable for a program to detect them prior to calling the function.
— end note]