为什么传入通道的值没有被 fmt.Println 打印出来？

Question

func main() {
    links := []string{
        "http://google.com",
        "http://amazon.com",
        "http://golang.org",
        "http://yahoo.com",
        "http://ebay.com",
    }

    c := make(chan string)

    for _, link := range links {
        testRequest(link, c)
    }
    msg := <-c
    fmt.Println(msg)
}
func testRequest(s string, c chan string) {
    _, err := http.Get(s)
    if err != nil {
        fmt.Println(s, "Is down presently")
        c <- "Something might be down"
        return
    }
    fmt.Println(s, "Is working perfectly")
    c <- "Working great!"
}

Channels 应该是引用类型，出于某种原因，该函数没有向通道传递任何内容，因为每次我运行代码都会按预期打印出第一行，但程序不会't stop 运行ning 并且也不执行 fmt.Println(channel) 所以我的假设是没有值被传递到通道，因为它应该是，有什么原因吗？

Answer 1

您的程序在 for 第一个循环后卡住了，发送方尝试发送到通道，但接收方只能在循环完成后才能接收

//This loop will blocks until finished
for _, link := range links {
    testRequest(link, c) // Try to send, receiver can't receive
}

//Until above loop finishs, this never excutes
msg := <-c
mt.Println(msg)

运行发送方循环go所以接收方循环可以执行，但是你需要在所有工作完成之前防止主程序退出。

func main() {
    links := []string{
        "http://google.com",
        "http://amazon.com",
        "http://yahoo.com",
        "http://ebay.com",
    }

    c := make(chan string)

    // This code will not blocks
    go func() {
        for _, link := range links {
            testRequest(link, c)
        }
    }()

    // Above code doesn't block, this code can excutes
    for {
        msg := <-c
        fmt.Println(msg)
    }
}

func testRequest(s string, c chan string) {
    _, err := http.Get(s)
    if err != nil {
        fmt.Println(s, "Is down presently")
        c <- "Something might be down"
        return
    }
    fmt.Println(s, "Is working perfectly")
    c <- "Working great!"
}

Answer 2

对于无缓冲通道，testRequests() 中的写入将在您从 main() 中的通道读取之前阻塞。你陷入僵局了。通常你应该得到一个错误，通常去找出所有 goroutines 何时被阻止。不知道你为什么不。

您可能想运行在不同的 goroutine 中测试 Requests()：

package main

import (
    "fmt"
    "net/http"
)

func main() {
    links := []string{
            "http://google.com",
            "http://amazon.com",
            "http://golang.org",
            "http://yahoo.com",
            "http://ebay.com",
    }

    c := make(chan string)

    go func() {
            for _, link := range links {
                    testRequest(link, c)
            }
            close(c)
    }()
    for msg := range c {
            fmt.Println(msg)
    }
    fmt.Println("Done handling request")
}

func testRequest(s string, c chan string) {
    _, err := http.Get(s)
    if err != nil {
            fmt.Println(s, "Is down presently")
            c <- "Something might be down"
            return
    }
    fmt.Println(s, "Is working perfectly")
    c <- "Working great!"
}

https://play.golang.org/p/g9X1h_NNJAB

为什么传入通道的值没有被 fmt.Println 打印出来？

Why is the value passed into the channel not being printed out by fmt.Println?

go

pointers

channel