sync.WaitGroup

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.

Exercise Print with WaitGroup

Inside main, add a sync.WaitGroup declaration and print wg done with fmt.Println.

Show example 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()
}

Test yourself

What does wg.Done() do?
- Decrements the counter
- Increments the counter
- Starts waiting
- Kills the goroutine
Show answer

Decrements the counter — decrements the counter. Add increments, Done decrements.

WaitGroup usage

var wg sync.WaitGroup
wg.Add(1)
go func() {
    defer wg.____()
    work()
}()
wg.Wait()

Show answer

var wg sync.WaitGroup
wg.Add(1)
go func() {
    defer wg.Done()
    work()
}()
wg.Wait()

Match WaitGroup concepts

Add

Increment the counter
Done

Decrement the counter
Wait

Wait
sync.WaitGroup

Tip

Next lesson: sync.Once

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