Add use-sync-waitgroup-to-synchronize-goroutines.md.

Author: NanXiao

SUMMARY.md

@@ -40,4 +40,5 @@
 * [nil channel VS closed channel](posts/nil-channel-vs-closed-channel.md)
 * [Select operation](posts/select-operation.md)
 * [Need not close every channel](posts/need-not-close-every-channel.md)
-* [Processing JSON object](posts/processing-json-object.md)
+* [Processing JSON object](posts/processing-json-object.md)
+* [Use sync.WaitGroup to synchronize goroutines](posts/use-sync-waitgroup-to-synchronize-goroutines.md)

posts/use-sync-waitgroup-to-synchronize-goroutines.md

@@ -0,0 +1,41 @@
+# Use sync.WaitGroup to synchronize goroutines
+----
+[sync.WaitGroup](https://golang.org/pkg/sync/#WaitGroup) provides a goroutine synchronization mechanism, and used for waiting for a collection of goroutines to finish. In the internal of `sync.WaitGroup` struct, there is a `counter` which records how many goroutines need to be waited are living now.  
+
+`sync.WaitGroup` provides `3` methods: `Add`, `Done` and `Wait`. `Add` method is used to identify how many goroutines need to be waited, and it will add `counter` value. When a goroutine exits, it must call `Done`, and it will decrease `counter` value by `1`. The `main` goroutine blocks on `Wait`, once the `counter` becomes `0`, the `Wait` will return, and main goroutine can continue to run.
+
+Let’s see an example:
+
+	package main
+	
+	import (
+	    "sync"
+	    "time"
+	    "fmt"
+	)
+	
+	func sleepFun(sec time.Duration, wg *sync.WaitGroup) {
+	    defer wg.Done()
+	    time.Sleep(sec * time.Second)
+	    fmt.Println("goroutine exit")
+	}
+	
+	func main() {
+	    var wg sync.WaitGroup
+	
+	    wg.Add(2)
+	    go sleepFun(1, &wg)
+	    go sleepFun(3, &wg)
+	    wg.Wait()
+	    fmt.Println("Main goroutine exit")
+	
+	}
+Because the `main` goroutine need to wait `2` goroutines, so the argument for `wg.Add` is `2`. The execution result is like this:
+
+	goroutine exit
+	goroutine exit
+	Main goroutine exit
+Please notice, the `Add` must go ahead of `Done`.  
+
+Reference:  
+[Use sync.WaitGroup in Golang](http://nanxiao.me/en/use-sync-waitgroup-in-golang/).