Three obvious problems, actually: 1) Thread A closes fd X for File Y, Thread B opens fd X for File Y. Thread A sees fd X referring to File Y, closes it again o

Three obvious problems, actually: 1) Thread A closes fd X for File Y, Thread B opens fd X for File Y. Thread A sees fd X referring to File Y, closes it again out from under Thread B. 2) Since the behavior is unspecified, it's possible you could have fd X briefly continue to point to File Y. This could lead to a case where your fstat() results cease to be valid by the time you run close() again. 3) fd doesn't have to be a normal on-disk file, and POSIX is somewhat vague about what the stat struct's values are for the various possible types of "file".