Closing Channels

In this lesson, explore closing channels through one small Go program, and connect its output to the language rules behind it.

Closing a channel announces that no more values will be sent. A receiver can continue draining buffered values, then receives the zero value and ok == false.

Typically the sender that owns the channel closes it. Sending on a closed channel panics; closing twice also panics. Do not use channel closure as a universal task-cancellation mechanism.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	numbers := make(chan int, 2)
	numbers <- 3
	numbers <- 5
	close(numbers)
	for n := range numbers {
		fmt.Println(n)
	}
	_, ok := <-numbers
	fmt.Println("More data:", ok)
}

Output

3
5
More data: false

The range loop drains 3 and 5 and terminates when the closed channel is empty. The final receive reports ok=false.

• Only close when no sender can send again.
• Receiving from a closed channel is allowed; sending to one is not.

Change the channel to hold three numbers; use a range loop to calculate their sum after closing it.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Closing Channels in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.