Channel causes panic when called on same scope-CodePudding

Can someone explain to me why the first code runs but the second one fails?

First One :

func main() {

  channel := make(chan int)
  go demo(channel)
  num := <-channel
  println(num)

}

func demo(channel chan int) {
  channel <- 2
}

Second One:

func main() {
    demo()
}

func demo() {
    channel := make(chan int)
    channel <- 2
    num := <-channel
    println(num)
}

In the second one, I am taking the input from the channel in the demo function but it still creates a deadlock can someone explain this in brief. What I can see is when I am calling the channel in the same scope it's causing an issue.

CodePudding user response：

Your channel is unbuffered so writing to it will block since no other goroutines can read from it.

Make this change and it will run:

channel := make(chan int, 1)

CodePudding user response：

In the first case, write to channel is in a separate goroutine. When the main goroutine blocks reading from the channel, there is still one goroutine that can write to the channel, thus is it not a deadlock. Eventually that goroutine writes to the channel and the program completes.

In the second case, write to channel blocks, and there are no other goroutines running, so it is a deadlock.