How to use sync.WaitGroup in Golang
sync.WaitGroup is used to wait for multiple goroutines to complete, increase the count by Add, Done reduces the count, and Wait blocks until the count is zero. 2. In the example, multiple worker goroutines are started, each call Done ends, and the main function call Wait is waiting for all to complete.
When working with concurrency in Go, you'll often need to wait for multiple goroutines to finish before proceeding. sync.WaitGroup is a synchronization primitive that helps you manage this efficiently. It's ideal when you know in advance how many goroutines you're launching and want to wait for all of them to complete.
Understanding WaitGroup Basics
A WaitGroup maintains an internal counter that starts at zero. You can increase it using Add(n) , decrease it with Done() , and block until it reaches zero using Wait() . The typical pattern is:
- Call Add(n) before starting n goroutines
- Each goroutine calls Done() when finished
- Use Wait() to block until all goroutines have called Done()
Basic Usage Example
Here's a simple example showing how to use WaitGroup:
package main
import (
"fmt"
"sync"
"time"
)
func worker(id int, wg *sync.WaitGroup) {
defer wg.Done() // Ensures Done is called
fmt.Printf("Worker %d starting\n", id)
time.Sleep(time.Second)
fmt.Printf("Worker %d done\n", id)
}
func main() {
var wg sync.WaitGroup
for i := 1; i In this example, we start three goroutines and wait for all of them to finish. Using defer wg.Done() ensures the counter is decremented even if the function panics.
Common Pitfalls and Best Practices
While WaitGroup is straightforward, there are a few things to watch out for:
- Always pass the WaitGroup by pointer to goroutines — never copy it
- Ensure every Add(n) has a corresponding n calls to Done(), or you'll deadlock
- Avoid calling Add after Wait has been invoked in another goroutine
- Use defer wg.Done() to prevent forgetting to call Done
If you're launching goroutines in a loop, make sure loop variables are properly captured by passing them as arguments.
When to Use WaitGroup
WaitGroup works well when you have a fixed number of tasks and just need to wait for completion. It's not suitable for cases where you need to collect results or handle cancellation. For more complex scenarios, consider using channels or context.WithCancel alongside WaitGroup.
Basically, if you're doing parallel work and just need to wait for everything to finish, sync.WaitGroup is a clean, efficient choice.
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