Go by Example: Closing Channels

Closing a channel indicates that no more values will be sent on it. This can be useful to communicate completion to the channel’s receivers.

 
package main
 
import "fmt"

In this example we’ll use a jobs channel to communicate work to be done from the main() goroutine to a worker goroutine. When we have no more jobs for the worker we’ll close the jobs channel.

 
func main() {
    jobs := make(chan int, 5)
    done := make(chan bool)

Here’s the worker goroutine. It repeatedly receives from jobs with j, more := <-jobs. In this special 2-value form of receive, the more value will be false if jobs has been closed and all values in the channel have already been received. We use this to notify on done when we’ve worked all our jobs.

 
    go func() {
        for {
            j, more := <-jobs
            if more {
                fmt.Println("received job", j)
            } else {
                fmt.Println("received all jobs")
                done <- true
                return
            }
        }
    }()

This sends 3 jobs to the worker over the jobs channel, then closes it.

 
    for j := 1; j <= 3; j++ {
        jobs <- j
        fmt.Println("sent job", j)
    }
    close(jobs)
    fmt.Println("sent all jobs")

We await the worker using the synchronization approach we saw earlier.

 
    <-done

Reading from a closed channel succeeds immediately, returning the zero value of the underlying type. The optional second return value is true if the value received was delivered by a successful send operation to the channel, or false if it was a zero value generated because the channel is closed and empty.

 
    _, ok := <-jobs
    fmt.Println("received more jobs:", ok)
}
 
$ go run closing-channels.go 
sent job 1
received job 1
sent job 2
received job 2
sent job 3
received job 3
sent all jobs
received all jobs
received more jobs: false

The idea of closed channels leads naturally to our next example: range over channels.

Next example: Range over Channels.

by Mark McGranaghan and Eli Bendersky | source | license