This example taken from tour.golang.org/#63
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
go say("world")
say("hello")
}
The output
hello
world
hello
world
hello
world
hello
world
hello
Why world is printed only 4 times instead of 5 ?
Edit: The answer can be quoted from golang specification:
Program execution begins by initializing the main package and then
invoking the function main. When the function main returns, the
program exits. It does not wait for other (non-main) goroutines to
complete.
When your main function ends your program ends, i.e. all goroutines are terminated.
Your main terminates before go say("world") is done. If you sleep some time at the end of main you should see the last world.
Here is how you solve that synchronization problem properly - with sync.WaitGroup
Playground link
package main
import (
"fmt"
"sync"
"time"
)
func say(s string, wg *sync.WaitGroup) {
defer wg.Done()
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
wg := new(sync.WaitGroup)
wg.Add(2)
go say("world", wg)
go say("hello", wg)
wg.Wait()
fmt.Println("All done")
}
Because the calling gorouting terminates before the second one you spawned does. This causes the second to shut down. To illustrate, modify your code slightly:
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Print(i)
fmt.Println(":"+s)
}
}
func main() {
go say("world")
say("hello")
}
Try putting in a "wait" or a sleep to the end of the main function.
Related
I don't understand why Goroutine in Windows didn't finish properly like Goroutine in Linux?
I already run the code in Powershell, VSCode, Goland, and even CMD, but the code never finish properly like Linux output.
below is the code:
import (
"fmt"
"time"
)
func count() {
for i := 0; i < 5; i++ {
fmt.Println(i)
time.Sleep(time.Millisecond * 1)
}
}
func main() {
go count()
time.Sleep(time.Millisecond * 2)
fmt.Println("Hello World")
time.Sleep(time.Millisecond * 5)
}
Windows Output:
0
1
Hello World
Linux Output (Which is expected output):
0
1
2
Hello World
3
4
Kindly help me understand or how to fix this.
p/s: I just started learning Go.
You seem to be trying to use time.Sleep to synchronize your go routines - in other words, so that main doesn't end before count.
This is the wrong way to synchronize goroutines. Keep in mind that most general purpose operating systems such as Linux and Windows do not guarantee timing; you need a real-time OS for that. So while you might usually get lucky and have the goroutines execute in the expected order, there is no guarantee that sleeps will make things happen in the expected order even on linux. The timing between these goroutines is simply not deterministic.
One of the correct ways to synchronize goroutines is with a sync.WaitGroup.
The following code works with or without the sleeps.
package main
import (
"fmt"
"sync"
)
func count(wg *sync.WaitGroup) {
defer wg.Done()
for i := 0; i < 5; i++ {
fmt.Println(i)
}
}
func main() {
var wg sync.WaitGroup
wg.Add(1)
go count(&wg)
fmt.Println("Hello World")
wg.Wait()
}
syc.WaitGroup is a convenient way to make sure the all goroutines are complete, in this case before the main function exits thus ending the program.
You could also do it with a channel:
package main
import (
"fmt"
)
func count(done chan bool) {
for i := 0; i < 5; i++ {
fmt.Println(i)
}
done <- true
}
func main() {
var done = make(chan bool)
go count(done)
fmt.Println("Hello World")
<-done
}
It is a well-known issue: starting from Go 1.16 time.Sleep in MS Windows uses low resolution timer, as bad as 1/64 second. Sleeping for 1 ms in Windows is anything between 1 and 16 ms.
Try printing microsecond timestamps:
package main
import (
"fmt"
"time"
)
func count() {
fmt.Println(time.Now().Nanosecond() / 1000)
for i := 0; i < 5; i++ {
fmt.Println(i, time.Now().Nanosecond()/1000)
time.Sleep(time.Millisecond * 1)
}
}
func main() {
// if runtime.GOOS == "windows" {
// initTimer()
// }
ts := time.Now().UnixNano()
fmt.Println("Main started: ", ts, (ts%1_000_000_000)/1000)
go count()
time.Sleep(time.Millisecond * 2)
fmt.Println("Hello World", time.Now().Nanosecond()/1000)
time.Sleep(time.Millisecond * 5)
fmt.Println("Main done", time.Now().Nanosecond()/1000)
}
On my Windows 10:
Main started: 1663345297398120000 398120
398703
0 398703
Hello World 405757
1 405757
2 421481
Main done 421481
The numbers are microseconds. See, how big are the intervals.
To improve timer resolution you can call timeBeginPeriod function.
//go:build windows
// +build windows
package main
import "syscall"
func initTimer() {
winmmDLL := syscall.NewLazyDLL("winmm.dll")
procTimeBeginPeriod := winmmDLL.NewProc("timeBeginPeriod")
procTimeBeginPeriod.Call(uintptr(1))
}
Calling initTimer helps a lot:
Main started: 1663345544132793500 132793
132793
0 133301
Hello World 134854
1 134854
2 136403
3 137964
4 139627
Main done 140696
Still the resolution is not 1 ms, but better than 2 ms.
The full code is here: https://go.dev/play/p/LGPv74cgN_h
Discussion thread in Golang issues: https://github.com/golang/go/issues/44343
This question already has answers here:
No output from goroutine
(3 answers)
Closed 4 years ago.
Can someone explain how this works:
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
say("world")
}
But this doesnt work once i add the word go to the routine in main
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
go say("world")
}
I think its because its finishing before executing the goroutine.
The "world" goroutine does not run or complete because main returns and the program exits.
When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.
https://golang.org/ref/spec#Program_execution
While SayHello() executes as expected, the goroutine prints nothing.
package main
import "fmt"
func SayHello() {
for i := 0; i < 10 ; i++ {
fmt.Print(i, " ")
}
}
func main() {
SayHello()
go SayHello()
}
When your main() function ends, your program ends as well. It does not wait for other goroutines to finish.
Quoting from the Go Language Specification: Program Execution:
Program execution begins by initializing the main package and then invoking the function main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.
See this answer for more details.
You have to tell your main() function to wait for the SayHello() function started as a goroutine to complete. You can synchronize them with channels for example:
func SayHello(done chan int) {
for i := 0; i < 10; i++ {
fmt.Print(i, " ")
}
if done != nil {
done <- 0 // Signal that we're done
}
}
func main() {
SayHello(nil) // Passing nil: we don't want notification here
done := make(chan int)
go SayHello(done)
<-done // Wait until done signal arrives
}
Another alternative is to signal the completion by closing the channel:
func SayHello(done chan struct{}) {
for i := 0; i < 10; i++ {
fmt.Print(i, " ")
}
if done != nil {
close(done) // Signal that we're done
}
}
func main() {
SayHello(nil) // Passing nil: we don't want notification here
done := make(chan struct{})
go SayHello(done)
<-done // A receive from a closed channel returns the zero value immediately
}
Notes:
According to your edits/comments: if you want the 2 running SayHello() functions to print "mixed" numbers randomly: you have no guarantee to observe such behaviour. Again, see the aforementioned answer for more details. The Go Memory Model only guarantees that certain events happen before other events, you have no guarantee how 2 concurrent goroutines are executed.
You might experiment with it, but know that the result will not be deterministic. First you have to enable multiple active goroutines to be executed with:
runtime.GOMAXPROCS(2)
And second you have to first start SayHello() as a goroutine because your current code first executes SayHello() in the main goroutine and only once it finished starts the other one:
runtime.GOMAXPROCS(2)
done := make(chan struct{})
go SayHello(done) // FIRST START goroutine
SayHello(nil) // And then call SayHello() in the main goroutine
<-done // Wait for completion
Alternatively (to icza's answer) you can use WaitGroup from sync package and anonymous function to avoid altering original SayHello.
package main
import (
"fmt"
"sync"
)
func SayHello() {
for i := 0; i < 10; i++ {
fmt.Print(i, " ")
}
}
func main() {
SayHello()
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
SayHello()
}()
wg.Wait()
}
In order to print numbers simultaneously run each print statement in separate routine like the following
package main
import (
"fmt"
"math/rand"
"sync"
"time"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 10; i++ {
wg.Add(1)
go func(fnScopeI int) {
defer wg.Done()
// next two strings are here just to show routines work simultaneously
amt := time.Duration(rand.Intn(250))
time.Sleep(time.Millisecond * amt)
fmt.Print(fnScopeI, " ")
}(i)
}
wg.Wait()
}
A Go program exits when the main function returns.
One option is to use something like sync.WaitGroup to wait on the other goroutines that main has spawned before returning from main.
Another option is to call runtime.Goexit() in main. From the godoc:
Goexit terminates the goroutine that calls it. No other goroutine is affected. Goexit runs all deferred calls before terminating the goroutine. Because Goexit is not a panic, any recover calls in those deferred functions will return nil.
Calling Goexit from the main goroutine terminates that goroutine without func main returning. Since func main has not returned, the program continues execution of other goroutines. If all other goroutines exit, the program crashes.
This allows main goroutine to stop executing while the background routines continue to execute. For example:
package main
import (
"fmt"
"runtime"
"time"
)
func f() {
for i := 0; ; i++ {
fmt.Println(i)
time.Sleep(10 * time.Millisecond)
}
}
func main() {
go f()
runtime.Goexit()
}
This can be cleaner than blocking forever in the main function, especially for programs that are infinite. One downside is that if all of the goroutines of a process return or exit (including the main goroutine), Go will detect this as an error and panic:
fatal error: no goroutines (main called runtime.Goexit) - deadlock!
To avoid this, at least one goroutine must call os.Exit before it returns. Calling os.Exit(0) immediately terminates the program and indicates that it did so without error. For example:
package main
import (
"fmt"
"os"
"runtime"
"time"
)
func f() {
for i := 0; i < 10; i++ {
fmt.Println(i)
time.Sleep(10 * time.Millisecond)
}
os.Exit(0)
}
func main() {
go f()
runtime.Goexit()
}
Below is a simple go example. I have omitted error handling etc intentionally to make the example short. I have a simple for loop calling the writeOutput function 5 times using the go keyword to make the function run concurrently.
What I expect to happen is 5 files are created in /tmp/ with the contents of test.
What happens is that no files are created.
However if I remove the go keyword the code executes as expected. Im overlooking something super obvious. My background is dynamically typed languages like PHP/Ruby so just getting to grips with go and can't understand why 5 files are created when the go keyword exists.
package main
import (
"os"
"math/rand"
"strconv"
)
func main() {
for i := 0; i < 5; i++ {
go writeOutput()
}
}
func writeOutput() {
filename := strconv.Itoa(rand.Intn(10000))
file, _ := os.Create("/tmp/" + filename)
defer file.Close()
file.WriteString("test")
}
I managed to solve this with a wait group as suggested in the comments.
package main
import (
"math/rand"
"os"
"strconv"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func() {
defer wg.Done()
writeOutput()
}()
}
wg.Wait()
}
func writeOutput() {
filename := strconv.Itoa(rand.Intn(10000))
file, _ := os.Create("/tmp/" + filename)
defer file.Close()
file.WriteString("test")
}
I started to learn go language days ago. When I tried to start writing some fun codes, I am stuck by a strange behavior.
package main
import "fmt"
func recv(value int) {
if value < 0 {
return
}
fmt.Println(value)
go recv(value-1)
}
func main() {
recv(10)
}
when I run the above code, only 10 is printed. When I remove the go before the call to recv, 10 to 0 are printed out. I believe I am misusing go routine here, but I can not understand why it failed start a go routine this way.
When the main function returns, Go will not wait for any still existing goroutines to finish but instead just exit.
recv will return to main after the first "iteration" and because main has nothing more to do, the program will terminate.
One solution to this problem is to have a channel that signals that all work is done, like the following:
package main
import "fmt"
func recv(value int, ch chan bool) {
if value < 0 {
ch <- true
return
}
fmt.Println(value)
go recv(value - 1, ch)
}
func main() {
ch := make(chan bool)
recv(10, ch)
<-ch
}
Here, recv will send a single boolean before returning, and main will wait for that message on the channel.
For the logic of the program, it does not matter what type or specific value you use. bool and true are just a straightforward example. If you want to be more efficient, using a chan struct{} instead of a chan bool will save you an additional byte, since empty structs do not use any memory.
A sync.Waitgroup is another solution and specifically intended for the purpose of waiting for an arbitrary amount of goroutines to run their course.
package main
import (
"fmt"
"sync"
)
func recv(value int, wg *sync.WaitGroup) {
if value < 0 {
return
}
fmt.Println(value)
wg.Add(1) // Add 1 goroutine to the waitgroup.
go func() {
recv(value-1, wg)
wg.Done() // This goroutine is finished.
}()
}
func main() {
var wg sync.WaitGroup
recv(10, &wg)
// Block until the waitgroup signals
// all goroutines to be finished.
wg.Wait()
}
I did so and also worked. How come?
package main
import "fmt"
func recv(value int) {
if value < 0 {
return
}
fmt.Println(value)
recv(value - 1)
}
func main() {
recv(10)
}