What does an empty select do?

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I found the following code in net/http/httptest and wonder what the empty select statement does in Go.

go s.Config.Serve(s.Listener)
if *serve != "" {
  fmt.Fprintln(os.Stderr, "httptest: serving on", s.URL)
  select {}
}

An empty select{} statement blocks forever. It is similar to an empty for{} statement.

On most (all?) supported Go architectures, the empty select will yield CPU. An empty for-loop won't, i.e. it will "spin" on 100% CPU.

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On Mac OS X, in Go, for { } will cause the CPU% to max, and the process's STATE will be running

select { }, on the other hand, will not cause the CPU% to max, and the process's STATE will be sleeping

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The empty select statement just blocks the current goroutine.

As for why you'd do this, here is one reason. This snippet is equivalent

if *serve != "" {
  fmt.Fprintln(os.Stderr, "httptest: serving on", s.URL)
  s.Config.Serve(s.Listener)
} else {
  go s.Config.Serve(s.Listener)
}

It's better in that there isn't a wasted goroutine. It's worse in that now there is code repetition. The author optimized for less code repetition over a wasted resource. Note however the permanently block goroutine is trivial to detect and may have zero cost over the duplicating version.

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IS_EMPTY and IS_NOT_EMPTY functions, The primary table key of a standard table can be empty. An empty Explicitly using an inline declaration INTO TABLE @DATA(itab) in the statement SELECT. LINQ Query Issues when trying to select from empty collection. Ask Question Asked 6 years, 7 months ago. Active 6 years, 7 months ago. on an empty var.

Comments
  • That was my guess. However I still find it semantically awkward, because select suggests a condition and not a loop.
  • AFAIK an empty for would be intensive on the cpu (at least that's how empty infinite loops work in other languages). While in OP's example it's likely that select{} is used to prevent the main goroutine from exiting.
  • In CSP-speak, the empty select is like STOP, a process that never proceeds. This is akin to a self-deadlock and I imagine it doesn't get used all that often. However, the question proposed an interesting and genuine use-case for this pattern. An empty for{} loop is a self-livelock, different because it consumes cpu resources.
  • It appears that it's idiomatic Go to use select{} to block the main goroutine which yields the CPU. Since the main goroutine hasn't returned, this leaves the process around and one can analyze the behavior of the other goroutines.