30 Input/output library [input.output]

30.10 File systems [filesystems]

30.10.2 Conformance [fs.conformance] File system race behavior [fs.race.behavior]

Behavior is undefined if calls to functions provided by this subclause 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, Requires: is not specified for the function.
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