选择语句

Question

A short program about select statement for channels.

package main

import "fmt"

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
        case s := <-quit:
            fmt.Println("quit =",s)
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println(<-c)
        }
        quit <- 9
    }()
    fibonacci(c, quit)
}

The result of the code above:

0
1
1
2
3
5
8
13
21
34
quit = 9

It worked fine. But after I changed (in func fibonacci)

case s := <-quit:
    fmt.Println("quit =",s)

to

case <-quit:
    fmt.Println(<-quit)

an fatal error occurred:

0
1
1
2
3
5
8
13
21
34
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.fibonacci(0x18348040, 0x18348080)
    /tmp/compile42.go:12 +0xf9
main.main()
    /tmp/compile42.go:27 +0x11c

Where does the error come from?

Answer 1

In second case you are getting the value form the channel two times. Every time you do something like <-channel you pop one value out of channel.

Hence program is waiting indefinitely on line

fmt.Println(<-quit)

but fortunately go is intelligent enough to detect this situation and panic with error "all goroutines are asleep - deadlock!"