sync.WaitGroup
- Go course
- Intermediate
- Lesson 21/80
- 4 min
Learn to wait for goroutines with a WaitGroup.
sync.WaitGroup
sync.WaitGroup waits for a set of goroutines to finish. Add(n) increments the counter, each goroutine calls Done() to decrement, the main program blocks on Wait() until zero.
Pattern: wg.Add(1) first, then go func() { defer wg.Done(); ... }(). Call Add outside the goroutine; otherwise a race occurs.
WaitGroup pattern
The correct WaitGroup pattern is:
- Call
wg.Add(1)before starting the goroutine - Inside the goroutine
defer wg.Done()as the first thing - In main
wg.Wait()blocks until all done - In a loop Add(1) each iteration
If you Add inside the goroutine Wait may pass before zero and goroutines leak.
Practice Print with WaitGroup
Inside main, add a sync.WaitGroup declaration and print wg done with fmt.Println.
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
wg.Add(1)
go func() { defer wg.Done(); fmt.Println("wg tamam") }()
wg.Wait()
}Expected output
wg tamam
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does wg.Done() do?
- Decrements the counter Correct answer
- Increments the counter
- Starts waiting
- Kills the goroutine
Explanation: decrements the counter. Add increments, Done decrements.
2 WaitGroup usage
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.____()
work()
}()
wg.Wait()var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
work()
}()
wg.Wait()Explanation: Wait is in main, Done is in the goroutine.
3 Match WaitGroup concepts
| Term | Meaning |
|---|---|
Add |
Increment the counter |
Done |
Decrement the counter |
Wait |
Wait |
sync.WaitGroup |
Tip |