英文:
Golang Concurrency Issue
问题
我正在学习Golang并发，并编写了一个程序来按顺序显示URL。我期望代码返回以下结果：
http://bing.com***
http://google.com***
但实际上它总是返回http:/google.com***。好像变量被覆盖了。由于我使用了goroutine，我希望它同时返回两个值。
func check(u string) string {
tmpres := u+"*****"
return tmpres
}
func IsReachable(url string) string {
ch := make(chan string, 1)
go func() {
ch <- check(url)
}()
select {
case reachable := <-ch:
// 使用错误和回复
return reachable
case <-time.After(3 * time.Second):
// 调用超时
return "none"
}
}
func main() {
var urls = []string{
"http://bing.com/",
"http://google.com/",
}
for _, url := range urls {
go func() {
fmt.Println(IsReachable(url))
}()
}
time.Sleep(1 * time.Second)
}
请注意，由于goroutine的特性，循环中的匿名函数会共享相同的url变量。为了解决这个问题，你可以将url作为参数传递给匿名函数：
for _, url := range urls {
go func(u string) {
fmt.Println(IsReachable(u))
}(url)
}
这样每个goroutine都会使用自己的url值，而不会被覆盖。
英文:
I am learning Golang concurrency and have written a program to display URL's in order. I expect the code to return
http://bing.com***
http://google.com***
But it always returns http:/google.com*** . As if the variable is being overwritten.Since i am using goroutines i would expect it to return both values at the sametime.
func check(u string) string {
tmpres := u+"*****"
return tmpres
}
func IsReachable(url string) string {
ch := make(chan string, 1)
go func() {
ch <- check(url)
}()
select {
case reachable := <-ch:
// use err and reply
return reachable
case <-time.After(3* time.Second):
// call timed out
return "none"
}
}
func main() {
var urls = []string{
"http://bing.com/",
"http://google.com/",
}
for _, url := range urls {
go func() {
fmt.Println(IsReachable(url))
}()
}
time.Sleep(1 * time.Second)
}
答案1
得分: 5
两个问题。首先，你创建了一个竞态条件。通过闭包引用循环变量，你在循环线程和 goroutine 线程之间共享了该变量，这导致了你描述的问题：当为第一个 URL 启动的 goroutine 尝试运行时，变量的值已经改变。你需要将其复制到一个局部变量中，或者将其作为参数传递，例如：
for _, url := range urls {
go func(url string) {
fmt.Println(IsReachable(url))
}(url)
}
其次，你说你想要按顺序显示它们，这与并发/并行一般不兼容，因为你无法控制并行操作的顺序。如果你想要按顺序执行它们，应该在单个线程中按顺序执行。否则，你将不得不收集结果，等待它们全部返回，然后按照所需的顺序对结果进行排序，然后再打印它们。
英文:
Two problems. First, you've created a race condition. By closing over the loop variable, you're sharing it between the thread running the loop and the thread running the goroutine, which is causing your described problem: by the time the goroutine that was started for the first URL tries to run, the value of the variable has changed. You need to either copy it to a local variable, or pass it as an argument, e.g.:
for _, url := range urls {
go func(url string) {
fmt.Println(IsReachable(url))
}(url)
}
Second, you said you wanted to display them "in order", which is not a goal generally compatible with concurrency/parallism, because you cannot control the order of parallel operations. If you want them in order, you should do them in order in a single thread. Otherwise, you'll have to collect the results, wait for all them to come back, then sort the results back into the desired order before printing them.
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