Why does my `for-select` statement keeps receiving from my channel even after I close it?

Question

I have the following code:

package main

import (
    "fmt"
    "time"
)

func main() {
    ch := make(chan int)
    ch2 := make(chan int)
    go func(c chan int, c2 chan int) {
        for {
            select {
            case v := <-c:
                fmt.Println(v)
            case v := <-c2:
                fmt.Println(v)
            default:
            }
        }
    }(ch, ch2)
    ch <- 1
    close(ch)
    close(ch2)
    time.Sleep(10 * time.Second)
}

When I run this, it prints 1 to the stdout, and then keeps printing 0 . Why is this?

I know I can check whether the channel is closed in my goroutine, but I just want to know the reason for this.

Also, suppose I want to exit from a goroutine after all(multiple) channels are closed, is that possible? I was assuming that once all channels are closed, I could potentially exit from the goroutine in the default case after all channels are closed

Answer 1

That's the expected behavior per Spec: Receive operator:

A receive operation on a closed channel can always proceed immediately, yielding the element type's zero value after any previously sent values have been received.

See How does a non initialized channel behave?

If you want to terminate a loop once all values have been received from a channel (that were sent on it before it was closed), use the for... range construct, for example:

c := make(chan int) // Initialize some channel

for v := range c {
    fmt.Println("Received:", v)
}

If you have multiple channels and you want to receive from all, you may use multiple goroutines, each having a for range for a designated channel.

Another solution is:

A function can read from multiple inputs and proceed until all are closed by multiplexing the input channels onto a single channel that's closed when all the inputs are closed. This is called fan-in.

Read more about it in The Go Blog Go Concurrency Patterns: Pipelines and cancellation: Fan-out- fain-in .

Answer 2

The Go Programming Language Specificatio n

Close

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.

Receive operator

A receive operation on a closed channel can always proceed immediately, yielding the element type's zero value after any previously sent values have been received.