Channel closing behavior in Go

Introduction

Channels let goroutines talk to each other. Closing a channel tells others no more values will come.

When you finish sending data and want to tell receivers to stop waiting.

When you want to signal that a task is done.

When you want to avoid deadlocks by closing channels after use.

Syntax

Go

close(channelName)

You only close a channel from the sender side.

Closing a channel twice causes a panic (runtime error).

Examples

This creates a channel and then closes it immediately.

Go

ch := make(chan int)
close(ch)

Looping over a channel stops automatically when the channel is closed.

Go

for v := range ch {
    fmt.Println(v)
}

Reading from a closed channel returns zero value and ok=false.

Go

v, ok := <-ch
if !ok {
    fmt.Println("Channel closed, no more data")
}

Sample Program

This program sends numbers 1 to 3 into a channel, then closes it. The main goroutine reads and prints all values until the channel closes. Then it tries to read once more to show the zero value and ok=false after closing.

Go

package main

import (
    "fmt"
)

func main() {
    ch := make(chan int)

    go func() {
        for i := 1; i <= 3; i++ {
            ch <- i
        }
        close(ch)
    }()

    for v := range ch {
        fmt.Println(v)
    }

    v, ok := <-ch
    fmt.Printf("After close: value=%d, ok=%v\n", v, ok)
}

Important Notes

Do not send on a closed channel; it causes a panic.

Receivers can detect channel closure by checking the second value from receive.

Closing a channel is optional but useful to signal no more data.

Summary

Use close(channel) to tell receivers no more values will come.

Reading from a closed channel returns zero value and ok=false.

Looping with for v := range channel stops when channel closes.