Go goroutine
last modified April 11, 2024
In this article we show how to work with goroutines in Golang. A goroutine is a lightweight execution thread.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
Goroutine definition
Goroutine is a lightweight execution thread. It is a function that runs concurrently alongside other running code. Note that concurrent execution may or may not parallel. In Go, every program has at least one goroutine: the main goroutine.
A goroutine is started with the go keywords.
Executing functions in sequence
The following example runs a function one by one.
package main
import (
"fmt"
)
func main() {
hello("Martin")
hello("Lucia")
hello("Michal")
hello("Jozef")
hello("Peter")
}
func hello(name string) {
fmt.Printf("Hello %s!\n", name)
}
The program runs the hello function in a sequence.
$ go run main.go Hello Martin! Hello Lucia! Hello Michal! Hello Jozef! Hello Peter!
The output is always the same.
Executing functions concurrently
Now we run the hello function concurrently.
package main
import (
"fmt"
)
func main() {
go hello("Martin")
go hello("Lucia")
go hello("Michal")
go hello("Jozef")
go hello("Peter")
fmt.Scanln()
}
func hello(name string) {
fmt.Printf("Hello %s!\n", name)
}
With the go keyword, we run the hello function concurrently. The
fmt.Scanln function waits for the input from user. If we comment
out this function, the program finishes before we can see output from the
goroutines.
$ go run main.go Hello Lucia! Hello Michal! Hello Martin! Hello Jozef! Hello Peter! $ go run main.go Hello Martin! Hello Peter! Hello Lucia! Hello Michal! Hello Jozef!
We run the program twice. Note that the output is different.
Go sync.WaitGroup
The sync.WaitGroup is a synchronization tool which waits for a
collection of goroutines to finish.
package main
import (
"fmt"
"sync"
"time"
)
func main() {
var wg sync.WaitGroup
wg.Add(2)
go func() {
count("oranges")
wg.Done()
}()
go func() {
count("apples")
wg.Done()
}()
wg.Wait()
}
func count(thing string) {
for i := 0; i < 4; i++ {
fmt.Printf("counting %s\n", thing)
time.Sleep(time.Millisecond * 500)
}
}
In the program, we synchronize the execution of two goroutines with
sync.WaitGroup.
var wg sync.WaitGroup wg.Add(2)
With Add, we tell how may goroutines we wait for.
go func() {
count("oranges")
wg.Done()
}()
We create an anonymous goroutine. We tell the Go runtime that the goroutine has
finished with Done.
wg.Wait()
The Wait function blocks until all goroutines have finished.
time.Sleep(time.Millisecond * 500)
In demonstrational programs, time.Sleep is often used to slow down
the execution of goroutines.
$ go run main.go counting apples counting oranges counting apples counting oranges counting oranges counting apples counting apples counting oranges
Go async requests
The next example uses goroutines to make asynchronous requests.
package main
import (
"fmt"
"io/ioutil"
"log"
"net/http"
"regexp"
"sync"
)
func main() {
urls := []string{
"http://webcode.me",
"https://example.com",
"http://httpbin.org",
"https://www.perl.org",
"https://www.php.net",
"https://www.python.org",
"https://code.visualstudio.com",
"https://clojure.org",
}
var wg sync.WaitGroup
for _, u := range urls {
wg.Add(1)
go func(url string) {
defer wg.Done()
content := doReq(url)
title := getTitle(content)
fmt.Println(title)
}(u)
}
wg.Wait()
}
func doReq(url string) (content string) {
resp, err := http.Get(url)
if err != nil {
log.Println(err)
return
}
defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
log.Println(err)
return
}
return string(body)
}
func getTitle(content string) (title string) {
re := regexp.MustCompile("<title>(.*)</title>")
parts := re.FindStringSubmatch(content)
if len(parts) > 0 {
return parts[1]
} else {
return "no title"
}
}
We make multiple asynchronous HTTP requests. We get the contents of the
title tag of each of the web pages. Each request is wrapped inside
one goroutine.
go func(url string) {
defer wg.Done()
content := doReq(url)
title := getTitle(content)
fmt.Println(title)
}(u)
Withing the goroutine, we generate a GET request, receive the response, get the title from the response and print it to the terminal.
$ go run main.go The Perl Programming Language - www.perl.org Welcome to Python.org Visual Studio Code - Code Editing. Redefined PHP: Hypertext Preprocessor Example Domain httpbin.org Clojure My html page
Goroutine channnels
Goroutines communicate via channnels. They allow to send and receive values with
the channel operator, <-.
c := make(chan string)
A new channel is created with the make function.
c <- v // send v := <-c // receive
The channel operator sends and receives values between goroutines.
package main
import (
"fmt"
"time"
)
func main() {
c := make(chan string)
go hello("Martin", c)
for msg := range c {
fmt.Println(msg)
}
}
func hello(name string, c chan string) {
for i := 0; i < 5; i++ {
msg := fmt.Sprintf("Hello %s!", name)
c <- msg
time.Sleep(time.Millisecond * 500)
}
close(c)
}
In the program, two goroutines communicate: main and hello.
c := make(chan string)
A channel is created with make.
go hello("Martin", c)
A hello goroutine is created with go. We pass the
channel as a parameter.
for msg := range c {
fmt.Println(msg)
}
With the range keyword, we go through the messages and print them
to the console.
func hello(name string, c chan string) {
for i := 0; i < 5; i++ {
msg := fmt.Sprintf("Hello %s!", name)
c <- msg
time.Sleep(time.Millisecond * 500)
}
close(c)
}
In the hello function, we create five messages and send them via
the channel to the main goroutine. When the goroutine is finished,
we close the channel with close.
$ go run main.go Hello Martin! Hello Martin! Hello Martin! Hello Martin! Hello Martin!
Calculating fibonacci values with goroutines
In the next example, we calculate fibonacci numbers with goroutines.
package main
import (
"fmt"
)
func fib(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c)
}
func main() {
c := make(chan int, 10)
go fib(cap(c), c)
for i := range c {
fmt.Println(i)
}
}
A series of fibonacci values is generated inside the fib goroutine.
The values are one by one send to the caller goroutine via a channel.
$ go run main.go 0 1 1 2 3 5 8 13 21 34
Source
The Go Programming Language Specification
This was an introductory tutorial to Golang's goroutines. We have presented a few simple examples demonstrating the usage of goroutines.
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