What are the consequences of closing the read / write channel?

stay go In the interview of , Undoubtedly, the most frequently asked knowledge point is channel 了 .

When channel What happens when you read data after closing , Most often asked .

One 、 natural channel

First, let's start with a piece of normal operation code ：

ch := make(chan int, 3)
ch <- 1
ch <- 2
ch <- 3

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

This code should be very familiar , Is there a problem with this writing ？

The result of execution ：

$ go run n.go 
1
2
3
fatal error: all goroutines are asleep - deadlock!

If one channel If it is not closed in a certain cooperation process , our for range The error of deadlock will be reported .

Two 、 Close and then read

1、 Use for range Read closed channel

Now we are for range Close this before channel, See what happens ：

ch := make(chan int, 3)
 ch <- 1
 ch <- 2
 ch <- 3
 close(ch)
 //  If it is not closed, a deadlock will be reported   fatal error: all goroutines are asleep - deadlock!
 for item := range ch {
  fmt.Println(item)
 }

What's wrong with writing code like this ？

$ go run n.go 
1
2
3

There's nothing wrong , Normal reading .

2、 Direct independent value

Now let's change the way of direct independent value ：

ch := make(chan int, 3)
ch <- 1
ch <- 2
ch <- 3
close(ch)
item, state := <-ch      // state  Whether to read the value 
fmt.Println(item, state)

state It will return whether the value has been obtained , The execution result of this code is to get the first value ：

$ go run n.go 
1 true

What if we read it repeatedly ？

ch := make(chan int, 3)
ch <- 1
ch <- 2
ch <- 3
close(ch)
<-ch
<-ch
<-ch
item, state := <-ch      // state  Whether to read the value 
fmt.Println(item, state) // After reading all the values , You can't report an error if you read it again , You will only get zero 

I read before 3 Time , After reading the data inside , Read it again, and it will be zero .

$ go run n.go 
0 false

So the conclusion is ： If channel There are elements that have not been read , Will read it correctly , Even if he has closed .

3、 ... and 、 Write the value inside ？

Finally, let's take a look at , When channel After closing , Write the value in it ？

ch := make(chan int, 3)
ch <- 1
ch <- 2
ch <- 3
close(ch)
ch <- 4

The result of this code execution ：

$ go run n.go 
panic: send on closed channel

Will be submitted to the panic, We can see the specific reasons go Source code , Path is ：src/runtime/chan.go