Blog / Go Language Spec

Close

September 6, 2024

Today we’re looking at the close built-in function. There’s not really any new information here, as we’ve already talked about channels, but it’s a good opportunity to review.

Close

For an argument ch with a core type that is a channel, the built-in function close records that no more values will be sent on the channel. It is an error if ch is a receive-only channel. Sending to or closing a closed channel causes a run-time panic. Closing the nil channel also causes a run-time panic. After calling close, and after any previously sent values have been received, receive operations will return the zero value for the channel’s type without blocking. The multi-valued receive operation returns a received value along with an indication of whether the channel is closed.

I think the most important thing to rember about close, is that it only directly affects send operations.

That is to say, if you have a (buffered) channel with a number of unread elements in it, calling close will not affect those elements—they can still be read.

ch := make(chan int, 10)
ch <- 1
ch <- 2
ch <- 3
close(ch)
for i := range ch {
  fmt.Println("read value:", i)
}

See it in the playground

Quotes from The Go Programming Language Specification Language version go1.23 (June 13, 2024)

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Closing channels

Channel types … A channel may be closed with the built-in function close. The multi-valued assignment form of the receive operator reports whether a received value was sent before the channel was closed. Closing a channel is a simple matter of using the built-in close function: // ch must be of type `chan T` or `chan<- T`. A receive-only channel (`<-chan T`) cannot be closed. close(ch) Calling close on a channel takes immediate effect, and prevents further values from being written to the channel (any items already in the channel may still be read).

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