31 Input/output library [input.output]

31.12 File systems [filesystems]

31.12.2 Conformance [fs.conformance] 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]