Golang如何在goroutine之间共享变量？

I'm learning Go and trying to understand its concurrency features.

I have the following program.

package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup

    for i := 0; i < 5; i++ {
        wg.Add(1)

        x := i

        go func() {
            defer wg.Done()
            fmt.Println(x)
        }()

    }

    wg.Wait()
    fmt.Println("Done")
}

When executed I got:

4
0
1
3
2

It's just what I want. However, if I make slight modification to it:

package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup

    for i := 0; i < 5; i++ {
        wg.Add(1)

        go func() {
            defer wg.Done()
            fmt.Println(i)
        }()

    }

    wg.Wait()
    fmt.Println("Done")
}

What I got will be:

5
5
5
5
5

I don't quite understand the difference. Can anyone help to explain what happened here and how Go runtime execute this code?

You have new variable on each run of x := i,
This code shows difference well, by printing the address of x inside goroutine:
The Go Playground:

package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        wg.Add(1)
        x := i
        go func() {
            defer wg.Done()
            fmt.Println(&x)
        }()
    }
    wg.Wait()
    fmt.Println("Done")
}

output:

0xc0420301e0
0xc042030200
0xc0420301e8
0xc0420301f0
0xc0420301f8
Done

And build your second example with go build -race and run it:
You will see: WARNING: DATA RACE

And this will be fine The Go Playground:

//go build -race
package main

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        wg.Add(1)
        go func(i int) {
            defer wg.Done()
            fmt.Println(i)
        }(i)
    }
    wg.Wait()
    fmt.Println("Done")
}

output:

0
4
1
2
3
Done

The general rule is, don't share data between goroutines. In the first example, you essentially give each goroutine their own copy of x, and they print it out in whatever order they get to the print statement. In the second example, they all reference the same loop variable, and it is incremented to 5 by the time any of them print it. I don't believe the output there is guaranteed, it just happens that the loop creating goroutines finished faster than the goroutines themselves got to the printing part.