u/CyberRabbi is absolutely correct. It's true that for _some_ kinds of devices you could expect fast close(2) IF the device documents that. But as you can see

u/CyberRabbi is absolutely correct. It's true that for _some_ kinds of devices you could expect fast close(2) IF the device documents that. But as you can see, implementing this can be hard even for devices where you'd think close(2) has to be fast. Even a tmpfs might have trouble making close(2) fast due to concurrency issues. The correct thing to do when you don't care about the result of close(2) is to call it in a worker thread. Ideally there would be async system calls for everything including closing open resources. Ideally there would be only async system calls except for a "wait for next event on this(these) handle(s)" system call.