并发编程
goroutine  channel
goroutine  函数的并发执行方式
channel 用来在goroutine之间进行参数传递
main函数本身也运行在一个goroutine中
go functionxxx  创建一个新的goroutine，并在这个新的goroutine中执行这个函数
make函数创建了一个传递string类型参数的channel

当一个goroutine尝试在一个channel上做send或者receive操作时
这个goroutine会阻塞在调用处，直到另一个goroutine从这个channel里接收或者写入值
每一个fetch函数在执行时都会往channel里发送一个值（ch <- expression），主函数负责接收这些值（<-ch）

在main函数里接收所有fetch函数传回的字符串 





fetchall.go

// Fetchall fetches URLs in parallel and reports their times and sizes.
package main

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

func main() {
    start := time.Now()
    ch := make(chan string)
    for _, url := range os.Args[1:] {
        go fetch(url, ch) // start a goroutine
    }
    for range os.Args[1:] {
        fmt.Println(<-ch) // receive from channel ch
    }
    fmt.Printf("%.2fs elapsed\n", time.Since(start).Seconds())
}

func fetch(url string, ch chan<- string) {
    start := time.Now()
    resp, err := http.Get(url)
    if err != nil {
        ch <- fmt.Sprint(err) // send to channel ch
        return
    }
    nbytes, err := io.Copy(ioutil.Discard, resp.Body)
    resp.Body.Close() // don't leak resources
    if err != nil {
        ch <- fmt.Sprintf("while reading %s: %v", url, err)
        return
    }
    secs := time.Since(start).Seconds()
    ch <- fmt.Sprintf("%.2fs  %7d  %s", secs, nbytes, url)
}



go build fetchall.go

./fetchall http://codefun007.xyz http://codefun007.xyz/about.htm http://codefun007.xyz/links.htm

0.12s      933  http://codefun007.xyz/about.htm
0.14s     1562  http://codefun007.xyz/links.htm
0.14s     4981  http://codefun007.xyz
0.14s elapsed