用频道奇怪的结果去goroutine

Question

When i run the goroutines, i generally get 40 as value, i know its about the concurrency but why is the last number coming? I suppose the output must be:

Page number:  34  
Page number:  12  
Page number:  8  
Page number:  2  
Page number:  29

example source code:

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
)

func getWebPageContent(url string, c chan int, val int) interface{} {

    if r, err := http.Get(url); err == nil {
        defer r.Body.Close()
        if body, err := ioutil.ReadAll(r.Body); err == nil {
            c <- val
            return string(body)
        }
    } else {
        fmt.Println(err)
    }
    return "XoX"

}

const MAX_TH = 40

func main() {

    // pln := fmt.Println
    messages := make(chan int)
    for j := 0; j < MAX_TH; j++ {
        go func() { getWebPageContent("http://www.example.com", messages, j) }()
    }

    routine_count := 0
    var page_number int
    for {
        page_number = <-messages
        routine_count++
        fmt.Println("Page number: ", page_number)
        if routine_count == MAX_TH {
            break
        }
    }
    close(messages)
}

Answer 1

The Go Programming Language

Frequently Asked Questions (FAQ)

What happens with closures running as goroutines?

Some confusion may arise when using closures with concurrency. Consider the following program:

func main() {
    done := make(chan bool)

    values := []string{"a", "b", "c"}
    for _, v := range values {
        go func() {
            fmt.Println(v)
            done <- true
        }()
    }

    // wait for all goroutines to complete before exiting
    for _ = range values {
        <-done
    }
}

One might mistakenly expect to see a, b, c as the output. What you'll probably see instead is c, c, c. This is because each iteration of the loop uses the same instance of the variable v, so each closure shares that single variable. When the closure runs, it prints the value of v at the time fmt.Println is executed, but v may have been modified since the goroutine was launched. To help detect this and other problems before they happen, run go vet.

To bind the current value of v to each closure as it is launched, one must modify the inner loop to create a new variable each iteration. One way is to pass the variable as an argument to the closure:

for _, v := range values {
    go func(u string) {
        fmt.Println(u)
        done <- true
    }(v)
}

In this example, the value of v is passed as an argument to the anonymous function. That value is then accessible inside the function as the variable u.

Even easier is just to create a new variable, using a declaration style that may seem odd but works fine in Go:

for _, v := range values {
    v := v // create a new 'v'.
    go func() {
        fmt.Println(v)
        done <- true
    }()
}

Therefore, in your case, create a new variable by adding the statement j := j,

for j := 0; j < MAX_TH; j++ {
    j := j
    go func() { getWebPageContent("http://www.example.com", messages, j) }()
}

For example,

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
)

func getWebPageContent(url string, c chan int, val int) interface{} {
    if r, err := http.Get(url); err == nil {
        defer r.Body.Close()
        if body, err := ioutil.ReadAll(r.Body); err == nil {
            c <- val
            return string(body)
        }
    } else {
        fmt.Println(err)
    }
    return "XoX"
}

const MAX_TH = 40

func main() {

    // pln := fmt.Println
    messages := make(chan int)
    for j := 0; j < MAX_TH; j++ {
        j := j
        go func() { getWebPageContent("http://www.example.com", messages, j) }()
    }

    routine_count := 0
    var page_number int
    for {
        page_number = <-messages
        routine_count++
        fmt.Println("Page number: ", page_number)
        if routine_count == MAX_TH {
            break
        }
    }
    close(messages)
}

Output:

Page number:  23
Page number:  6
Page number:  1
Page number:  3
Page number:  28
Page number:  32
Page number:  18
Page number:  22
Page number:  0
Page number:  36
Page number:  7
Page number:  21
Page number:  12
Page number:  2
Page number:  5
Page number:  4
Page number:  33
Page number:  13
Page number:  20
Page number:  27
Page number:  29
Page number:  8
Page number:  31
Page number:  10
Page number:  17
Page number:  25
Page number:  19
Page number:  35
Page number:  14
Page number:  38
Page number:  15
Page number:  30
Page number:  37
Page number:  39
Page number:  26
Page number:  9
Page number:  16
Page number:  11
Page number:  24
Page number:  34