I'm learning Go and trying to understand its concurrency features.
I have the following program.
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
x := i
go func() {
defer wg.Done()
fmt.Println(x)
}()
}
wg.Wait()
fmt.Println("Done")
}
When executed I got:
4
0
1
3
2
It's just what I want. However, if I make slight modification to it:
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func() {
defer wg.Done()
fmt.Println(i)
}()
}
wg.Wait()
fmt.Println("Done")
}
What I got will be:
5
5
5
5
5
I don't quite understand the difference. Can anyone help to explain what happened here and how Go runtime execute this code?
You have new variable on each run of x := i
,
This code shows difference well, by printing the address of x
inside goroutine:
The Go Playground:
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
x := i
go func() {
defer wg.Done()
fmt.Println(&x)
}()
}
wg.Wait()
fmt.Println("Done")
}
output:
0xc0420301e0
0xc042030200
0xc0420301e8
0xc0420301f0
0xc0420301f8
Done
And build your second example with go build -race
and run it:
You will see: WARNING: DATA RACE
And this will be fine The Go Playground:
//go build -race
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func(i int) {
defer wg.Done()
fmt.Println(i)
}(i)
}
wg.Wait()
fmt.Println("Done")
}
output:
0
4
1
2
3
Done