sync.WaitGroup

In this lesson, explore sync.waitgroup through one small Go program, and connect its output to the language rules behind it.

sync.WaitGroup counts unfinished operations. Call Add before starting a task, Done when that task finishes, and Wait to block until the count reaches zero.

Do not copy a WaitGroup after first use. Ensure new tasks are registered before the corresponding Wait begins when it could reach zero.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"sync"
)

func main() {
	var wg sync.WaitGroup
	results := make([]int, 3)
	for i := range results {
		wg.Add(1)
		go func(n int) { defer wg.Done(); results[n] = n * 2 }(i)
	}
	wg.Wait()
	fmt.Println(results)
}

Output

[0 2 4]

Add tracks three workers. Each writes a different slice element then signals Done. Wait returns after all writes, giving [0 2 4].

• Calling Add inside a goroutine may race with Wait when the count is zero.
• A WaitGroup only waits; it does not make unrelated writes mutually exclusive.

Use WaitGroup to wait for four independent jobs and print an ordered summary.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced sync.WaitGroup in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.