为什么golang中的select仅适用于goroutine中的通道？

Question

Consider the following go playground

    package main

    import "fmt"

    func main() {
        messages := make(chan string)

        messages <- "my msg"

        select {
        case msg := <-messages:
            fmt.Println("received message", msg)
        }

    }

The code above will reach error

fatal error: all goroutines are asleep - deadlock!

However

if I change it to

    package main

    import "fmt"

    func main() {
        messages := make(chan string)

        go func() {
            messages <- "my msg"
        }()
        select {
        case msg := <-messages:
            fmt.Println("received message", msg)
        }

    }

It will work.

Is there a particular reason for this behavior?

Shouldn't the code execute in a sequential manner in the first case so that by the time the select statement is reached, the msg will be passed and it will catch the case msg := <-messages ?

Answer 1

Shouldn't the code execute in a sequential manner in the first case so that by the time the select statement is reached, the msg will be passed and it will catch the case msg := <-messages ?

The select statement is never reached, thats the problem in your first code.

The statement

messages <- "my msg"

Wants to push a string into the channel, but since you created a unbuffered channel

messages := make(chan string)

the goroutine keeps waiting for someone to actually read from the channel, so it can push the string to the channel. You can only push something to a unbuffered channel if there is somewhere a goroutine reading from it!

Try the first example with a buffered channel:

messages := make(chan string, 1)

It should work as you expected.