Go goroutine with channel strange result

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)
}

> 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

My first golang reply, may be completely off

The loop could probably look like this:

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

Basically, you define an anonymous function and call it with a parameter. You could do it differently but this solution looks very functional and sleek