{"id":360,"date":"2024-09-21T12:26:26","date_gmt":"2024-09-21T05:26:26","guid":{"rendered":"https:\/\/gotutorial.org\/?page_id=360"},"modified":"2024-09-22T11:47:09","modified_gmt":"2024-09-22T04:47:09","slug":"go-channels","status":"publish","type":"page","link":"https:\/\/www.gotutorial.org\/go-tutorial\/go-channels\/","title":{"rendered":"Go Channels"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to use Go channels to allow goroutines to communicate with each other.<\/p>\n\n\n\n

Introduction to Go channels<\/h2>\n\n\n\n

In Go, channels are used to pass data between goroutines<\/a>. In other words, goroutines share data by communicating with each other via channels.<\/p>\n\n\n\n

Creating channels<\/h3>\n\n\n\n

To create a channel, you use the var<\/code> keyword, followed by the channel name, the chan<\/code> keyword, and the data type of the channel:<\/p>\n\n\n

var<\/span> channelName chan<\/span> type<\/span><\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
For example:<\/p>\n\n\n
var<\/span> ch chan<\/span> string<\/span><\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we define a channel ch<\/code> with the type string<\/code>. The ch<\/code> channel allows goroutines to send and receive strings only.<\/p>\n\n\n\n
Alternatively, you can use the built-in make()<\/code> function to create a channel:<\/p>\n\n\n
ch := make<\/span>(chan<\/span> string<\/span>)<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Sending data to channels<\/h3>\n\n\n\n
To send data to a channel, you use the <-<\/code> operator:<\/p>\n\n\n
channel <- data<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
For example, the following shows how to send the message \"Hi\"<\/code> to the channel ch<\/code>:<\/p>\n\n\n
ch <- \"Hi\"<\/span><\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
If you attempt to send a value that is not a string to the ch<\/code> channel, you’ll encounter a compilation error. <\/p>\n\n\n\n
Receiving data from channels<\/h3>\n\n\n\n
To receive data from a channel, you use the <-<\/code> operator:<\/p>\n\n\n
<- channel<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example shows how to receive a string message from a channel:<\/p>\n\n\n
mesage := <- channel<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Receiving data from a channel will block <\/em>the goroutine until the message is received.<\/p>\n\n\n\n
Additionally, you can use a for range loop<\/a> to read data from a channel:<\/p>\n\n\n
for<\/span> data := range<\/span> channel {\n    \/\/ process the data<\/span>\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Putting it all together<\/h3>\n\n\n\n
Here’s the complete program that demonstrates how to use a channel to exchange data between goroutines:<\/p>\n\n\n
package<\/span> main\n\nimport<\/span> \"fmt\"<\/span>\n\nfunc<\/span> main<\/span>()<\/span><\/span> {\n    ch := make<\/span>(chan<\/span> string<\/span>)\n\n    go<\/span> func<\/span>()<\/span><\/span> {\n        ch <- \"Hi\"<\/span>\n    }()\n\n    message := <-ch\n    fmt.Println(message)\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, create a channel ch<\/code> with the type of string<\/code>:<\/p>\n\n\n
ch := make<\/span>(chan<\/span> string<\/span>)<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, spawn a new goroutine that sends the message \"Hi\"<\/code> to the ch<\/code> channel:<\/p>\n\n\n
go<\/span> func<\/span>()<\/span><\/span> {\n    ch <- \"Hi\"<\/span>\n}()<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, receive the message<\/code> from the channel ch<\/code> in the main goroutine: <\/p>\n\n\n
message := <-ch<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that the main goroutine will not complete until it receives the message<\/code> from the ch<\/code> channel.<\/p>\n\n\n\n
Finally, display the message on the screen:<\/p>\n\n\n
fmt.Println(message)<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
In the following modified version of the program, the goroutine sends data to the channel after 3 seconds. Therefore, the main<\/code> goroutine will wait for the same amount of time before receiving the data:<\/p>\n\n\n
package<\/span> main\n\nimport<\/span> (\n    \"fmt\"<\/span>\n    \"time\"<\/span>\n)\n\nfunc<\/span> main<\/span>()<\/span><\/span> {\n    fmt.Println(\"Starting the program...\"<\/span>)\n    ch := make<\/span>(chan<\/span> string<\/span>)\n\n    go<\/span> func<\/span>()<\/span><\/span> {\n        time.Sleep(3<\/span> * time.Second)\n        ch <- \"Hi\"<\/span>\n    }()\n\n    message := <-ch\n    fmt.Println(message)\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Starting the program...\nHi<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
The message Hi<\/code> appears after 3 seconds after the program starts.<\/p>\n\n\n\n
Deadlocks<\/h2>\n\n\n\n
The following program causes a deadlock <\/em>that you may encounter when start working with channels:<\/p>\n\n\n
package<\/span> main\n\nimport<\/span> \"fmt\"<\/span>\n\nfunc<\/span> main<\/span>()<\/span><\/span> {\n    ch := make<\/span>(chan<\/span> string<\/span>)\n\n    go<\/span> func<\/span>()<\/span><\/span> {\n        ch <- \"Hi\"<\/span>\n    }()\n\n    message := <-ch\n    fmt.Println(message)\n\n    \/\/ Error: deadlock<\/span>\n    message = <-ch\n    fmt.Println(message)\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Hi\nfatal error: all goroutines are asleep - deadlock!<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example:<\/p>\n\n\n\n
    \n
  • First, a goroutine sends the message \"Hi\"<\/code> to the channel.<\/li>\n\n\n\n
  • Second, the main<\/code> goroutine receives and prints the message.<\/li>\n\n\n\n
  • Third, the main goroutine attempts to receive more data from the channel. However, no more data is sent to the channel, and the second receiving operation blocks indefinitely<\/em>, causing a deadlock.<\/li>\n<\/ul>\n\n\n\n
    To prevent deadlocks, you should ensure every send has a matching receipt. In other words, there should be a corresponding receipt for every message you send, and vice versa.<\/p>\n\n\n\n
    A practical example of Go channels<\/h2>\n\n\n\n
    The following example shows how to check the status of a list of URLs<\/code> concurrently:<\/p>\n\n\n
    package<\/span> main\n\nimport<\/span> (\n    \"fmt\"<\/span>\n    \"net\/http\"<\/span>\n)\n\nfunc<\/span> check<\/span>(url string<\/span>, ch chan<\/span> string<\/span>)<\/span><\/span> {\n    _, err := http.Get(url)\n    if<\/span> err != nil<\/span> {\n        ch <- fmt.Sprintf(\"The %s is down.\"<\/span>, url)\n    } else<\/span> {\n        ch <- fmt.Sprintf(\"The %s is up.\"<\/span>, url)\n    }\n}\n\nfunc<\/span> main<\/span>()<\/span><\/span> {\n    urls := []string<\/span>{\n        \"https:\/\/google.com\"<\/span>,\n        \"https:\/\/go.dev\"<\/span>,\n        \"https:\/\/abcdef.test\/\"<\/span>,\n    }\n    ch := make<\/span>(chan<\/span> string<\/span>)\n    for<\/span> _, url := range<\/span> urls {\n        go<\/span> check(url, ch)\n    }\n\n    for<\/span> i := 0<\/span>; i < len<\/span>(urls); i++ {\n        fmt.Println(<-ch)\n    }\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    How it works.<\/p>\n\n\n\n
    First, define a function that accepts a URL to check and a channel to send data to once the checking completes:<\/p>\n\n\n
    func<\/span> check<\/span>(url string<\/span>, ch chan<\/span> string<\/span>)<\/span><\/span> {\n    _, err := http.Get(url)\n    if<\/span> err != nil<\/span> {\n        ch <- fmt.Sprintf(\"The %s is down.\"<\/span>, url)\n    } else<\/span> {\n        ch <- fmt.Sprintf(\"The %s is up.\"<\/span>, url)\n    }\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    Next, declare and initialize a list of URLs<\/code> to check:<\/p>\n\n\n
    urls := []string<\/span>{\n    \"https:\/\/google.com\"<\/span>,\n    \"https:\/\/go.dev\"<\/span>,\n    \"https:\/\/abcdef.test\/\"<\/span>,\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    Then, create a channel to exchange the messages:<\/p>\n\n\n
    ch := make<\/span>(chan<\/span> string<\/span>)<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    After that, spawn goroutines to check the URLs<\/code> concurrently:<\/p>\n\n\n
    for<\/span> _, url := range<\/span> urls {\n    go<\/span> check(url, ch)\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    Finally, receive the URL status from the ch channel in the main goroutine:<\/p>\n\n\n
    for<\/span> i := 0<\/span>; i < len<\/span>(urls); i++ {\n    fmt.Println(<-ch)\n}<\/code><\/span>Code language:<\/span> Go<\/span> (<\/span>go<\/span>)<\/span><\/small><\/pre>\n\n\n
    Summary<\/h2>\n\n\n\n
      \n
    • Use channels to send and receive data between goroutines.<\/li>\n\n\n\n
    • Use the channel <- data<\/code> syntax to send data to a channel.<\/li>\n\n\n\n
    • Use the data := <- channel<\/code> to receive data from a channel.<\/li>\n\n\n\n
    • Ensure the number of sends and receives are matched to avoid deadlocks.<\/li>\n<\/ul>\n
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