为啥以下代码会产生deadlock?

import (
    "fmt"
)

func main() {
    AlterPrint()
}

// POINT: communicate between goroutines by channel

func AlterPrint(){
  letter, number := make(chan bool), make(chan bool)
  letterDone := make(chan bool)
  numberDone := make(chan bool)
  go func() {
    i := 1
    for {
      if i > 28 {
          numberDone <- true
          return
      }
      select{
        case <-number: {
          fmt.Print(i)
          i++
          fmt.Print(i)
          i++
          letter <- true
          break
        }
        default: {
          break
        }
      }

    }
  }()

  go func(){
    i := 'A'
    for {
      if i > 'Z' {
        letterDone <- true
        return
      }
      select{
        case <-letter: {
          fmt.Print(string(i))
          i++
          fmt.Print(string(i))
          i++
          number <- true
          break
        }
        default: {
          break
        }
      }
    }
  }()
  number <- true
  <- letterDone
  <- numberDone
}

I expect the output of "12AB34CD56EF78GH910IJ1112KL1314MN1516OP1718QR1920ST2122UV2324WX2526YZ2728", but the actual output is

goroutine 1 [chan receive]:
main.AlterPrint()
    /tmp/54841538.go:66 +0x183
main.main()
    /tmp/54841538.go:7 +0x14

goroutine 5 [chan send]:
main.AlterPrint.func1(0xc82000c240, 0xc82000c180, 0xc82000c120)
    /tmp/54841538.go:31 +0x25a
created by main.AlterPrint
    /tmp/54841538.go:40 +0xde
exit status 2

package main import ( "fmt" ) func main() { AlterPrint() } // POINT: communicate between goroutines by channel func AlterPrint(){ letter, number := make(chan bool), make(chan bool) letterDone := make(chan bool) numberDone := make(chan bool) go func() { i := 1 for { if i > 28 { numberDone <- true return } select{ case <-number: { fmt.Print(i) i++ fmt.Print(i) i++ if i != 29 { letter <- true } break } default: { break } } } }() go func(){ i := 'A' for { if i > 'Z' { letterDone <- true return } select{ case <-letter: { fmt.Print(string(i)) i++ fmt.Print(string(i)) i++ number <- true break } default: { break } } } }() number <- true <- letterDone <- numberDone }

StoneFlying · #1 · 6年之前

```go
package main

import (
    "fmt"
)

func main() {
    AlterPrint()
}

// POINT: communicate between goroutines by channel

func AlterPrint(){
  letter, number := make(chan bool), make(chan bool)
  letterDone := make(chan bool)
  numberDone := make(chan bool)
  go func() {
    i := 1
    for {
      if i > 28 {
          numberDone <- true
          return
      }
      select{
        case <-number: {
          fmt.Print(i)
          i++
          fmt.Print(i)
		  i++
		  if i != 29 {
			  letter <- true
		  }
          break
        }
        default: {
          break
        }
      }

}
  }()

go func(){
    i := 'A'
    for {
      if i > 'Z' {
        letterDone <- true
        return
      }
      select{
        case <-letter: {
          fmt.Print(string(i))
          i++
          fmt.Print(string(i))
          i++
          number <- true
          break
        }
        default: {
          break
        }
      }
    }
  }()
  number <- true
  <- letterDone
  <- numberDone
}
```

stayfoo · #2 · 6年之前

1、两个goroutine中的代码，在互相等待 chan 的值

number 和 letter 永远不会接收到数据，导致 numberDone 和 letterDone 也永远不会接收到数据。

case <-number: {
                fmt.Print(i)
                i++
                fmt.Print(i)
                i++
                letter <- true
                break
            }

case <-letter: {
                fmt.Print(string(i))
                i++
                fmt.Print(string(i))
                i++
                number <- true
                break
            }

2、发送到 chan 的数据，没有被接收。

main 中最后 number <- true ，发送给 number 了一个true，没有被接收。

使用如下代码就不会崩溃：

number <- true
fmt.Println("number: ", <- number)

linture · #3 · 6年之前

Solved. letterDone 被读出之后，第二个goroutine完全退出，第一个goroutine阻塞在了letter <- true，主线程阻塞在了<- numberDone。@StoneFlying's answer solved this dead lock.

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