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Table of Contents
Basic structure and usage
Points that are easy to ignore when using
Common usage in actual scenarios
Home Backend Development Golang Go sync.WaitGroup example

Go sync.WaitGroup example

Jul 09, 2025 am 01:48 AM
go

sync.WaitGroup is used to wait for a group of goroutines to complete the task. Its core is to work together through three methods: Add, Done, and Wait. 1. Add(n) sets the number of goroutines to wait; 2. Done() is called at the end of each goroutine, and the count is reduced by one; 3. Wait() blocks the main coroutine until all tasks are completed. When using it, please note: Add should be called outside the goroutine, avoid duplicate Wait, and be sure to ensure that Don is called. It is recommended to use it with defer. It is common in concurrent crawling of web pages, batch data processing and other scenarios, and can effectively control the concurrency process.

Go sync.WaitGroup example

It is actually quite common to use Go's sync.WaitGroup to manage concurrent tasks, especially when syncing multiple goroutines. Simply put, it is a counter that is used to wait for a set of coroutines to complete.

Go sync.WaitGroup example

The following is a simple example to explain how to use it, which is suitable for friends who are new to concurrent programming.


Basic structure and usage

There are three main methods of WaitGroup: Add(n) , Done() and Wait() .

Go sync.WaitGroup example
  • Add(n) : Sets the number of goroutines to wait for.
  • Done() : Called once after each goroutine is executed, which is equivalent to decrementing the counter by one.
  • Wait() : Blocks the current coroutine until all tasks call Done() .

Let's give a basic example:

 package main

import (
    "fmt"
    "sync"
    "time"
)

func main() {
    var wg sync.WaitGroup

    for i := 0; i < 3; i {
        wg.Add(1)
        go func(id int) {
            defer wg.Done()
            fmt.Printf("goroutine %d start\n", id)
            time.Sleep(time.Second)
            fmt.Printf("goroutine %d done\n", id)
        }(id)
    }

    wg.Wait()
    fmt.Println("All goroutines finished.")
}

In this example, three goroutines are started, and the main function will wait until they are executed before exiting.

Go sync.WaitGroup example

Points that are easy to ignore when using

Although WaitGroup is simple, some details are prone to errors if you don’t pay attention to:

  • Add is called outside goroutine
    If you call Add in goroutine, it may cause Wait() to be executed before it is added, thus ending early.

  • Avoid duplicate Wait
    A WaitGroup is not recommended for reuse unless you explicitly reset the status. Otherwise, the second call Wait() may not block.

  • Done must be called
    If you forget to call Done() , the program will get stuck on Wait() . It is a good habit to use defer , make sure to execute it.


Common usage in actual scenarios

Sometimes we use WaitGroup with other structures, such as batch processing of data or parallel requesting external interfaces.

Suppose we want to crawl multiple web page content concurrently, we can write it like this:

 urls := []string{"https://example.com/1", "https://example.com/2", ...}

var wg sync.WaitGroup

for _, url := range urls {
    wg.Add(1)
    go func(u string) {
        defer wg.Done()
        // Simulate request resp, err := http.Get(u)
        if err != nil {
            fmt.Println("Error fetching:", u)
            Return
        }
        fmt.Println("Fetched:", resp.Status)
    }(u)
}

wg.Wait()

This mode is very common in crawler and background task processing.


Basically that's it. By mastering the basic usage and precautions of sync.WaitGroup , you can control the concurrency process well.

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