为什么Goroutines之间的共享变量不会更新

Question

Why does the code snippet below print out 0? However, i is correctly updated to 1 without the for loop.

package main

import (
    "fmt"
    "time"
)

func main() {

    var i uint64
    go func() {
        for {
            i++
        }
    }()

    time.Sleep(time.Second)
    fmt.Println(i)
}

Solution 1

package main

import (
        "fmt"
        "runtime"
        "time"
)

func main() {

        runtime.GOMAXPROCS(1)

        var i uint64
        ch := make(chan bool)
        go func() {
                for {
                        select {
                        case <-ch:
                                return
                        default:
                                i++
                        }
                }
        }()

        time.Sleep(time.Second)
        ch <- true
        fmt.Println(i)
}

Solution 2

package main

import (
        "fmt"
        "runtime"
        "sync"
        "time"
)

func main() {

        runtime.GOMAXPROCS(1)

        var i uint64
        var mx sync.Mutex
        go func() {
                for {
                        mx.Lock()
                        i++
                        mx.Unlock()
                }
        }()

        time.Sleep(time.Second)
        mx.Lock()
        fmt.Println(i)
        mx.Unlock()
}

Solution 3

package main

import (
        "fmt"
        "runtime"
        "sync/atomic"
        "time"
)

func main() {

        runtime.GOMAXPROCS(1)

        var i uint64
        go func() {
                for {
                        atomic.AddUint64(&i, 1)
                        runtime.Gosched()
                }
        }()

        time.Sleep(time.Second)
        fmt.Println(atomic.LoadUint64(&i))
}

Answer 1

You have a race condition. The results are undefined.

package main

import (
    "fmt"
    "time"
)

func main() {

    var i uint64
    go func() {
        for {
            i++
        }
    }()

    time.Sleep(time.Second)
    fmt.Println(i)
}

Output:

$ go run -race racer.go
==================
WARNING: DATA RACE
Read at 0x00c0000a2010 by main goroutine:
  main.main()
      /home/peter/src/racer.go:18 +0x95

Previous write at 0x00c0000a2010 by goroutine 6:
  main.main.func1()
      /home/peter/src/racer.go:13 +0x4e

Goroutine 6 (running) created at:
  main.main()
      /home/peter/src/racer.go:11 +0x7a
==================
66259553
Found 1 data race(s)
exit status 66
$ 

---

Reference: Introducing the Go Race Detector