Add "Waiting For Goroutines To Finish" example.

Also link to it from the Channel Synchronization example

examples.txt

@@ -23,6 +23,7 @@ Goroutines
 Channels
 Channel Buffering
 Channel Synchronization
+Waiting For Goroutines To Finish
 Channel Directions
 Select
 Timeouts

examples/channel-synchronization/channel-synchronization.go

@@ -1,6 +1,8 @@
 // We can use channels to synchronize execution
 // across goroutines. Here's an example of using a
 // blocking receive to wait for a goroutine to finish.
+// When waiting for multiple goroutines to finish,
+// you may prefer to [use a WaitGroup](waiting-for-goroutines-to-finish).
 
 package main
 

examples/channel-synchronization/channel-synchronization.hash

@@ -1,2 +1,2 @@
-fe3e2ea1a67d0f95ce4cb18f3e8aa16d416de0ce
+df90432a53832c045472981a9da23d1139fb2b5c
-0DfW-1RMqi
+f6qTtt1agmN

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.go

@@ -0,0 +1,47 @@
+// To wait for multiple goroutines to finish, we can
+// use a sync.WaitGroup.
+
+package main
+
+import (
+    "fmt"
+    "math/rand"
+    "sync"
+    "time"
+)
+
+// This is the function we'll run in every goroutine.
+// wg is the WaitGroup it uses to notify that it's done.
+// Note that a WaitGroup must be passed to functions by
+// pointer.
+func worker(id int, wg *sync.WaitGroup) {
+    fmt.Printf("Worker %d starting\n", id)
+
+    // Sleep for a random duration between 500-700 ms
+    // to simulate work. See the [random numbers](random-numbers)
+    // example for more details on *rand*.
+    msToSleep := time.Duration(500 + rand.Intn(200))
+    time.Sleep(msToSleep * time.Millisecond)
+    fmt.Printf("Worker %d done\n", id)
+
+    // Notify the WaitGroup that we're done.
+    wg.Done()
+}
+
+func main() {
+
+    // This WaitGroup is used to wait for all the
+    // goroutines launched here to finish.
+    var wg sync.WaitGroup
+
+    // Launch several goroutines and increment the WorkGroup
+    // counter for each.
+    for i := 1; i <= 5; i++ {
+        wg.Add(1)
+        go worker(i, &wg)
+    }
+
+    // Block until the WorkGroup counter goes back to 0;
+    // all the workers notified they're done.
+    wg.Wait()
+}

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.hash

@@ -0,0 +1,2 @@
+f068072d11ed9469174c18f5b7a6a7d9d8d3dafb
+koKzXfbq8kg

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.sh

@@ -0,0 +1,14 @@
+$ go run waiting-for-goroutines-to-finish.go
+Worker 5 starting
+Worker 3 starting
+Worker 4 starting
+Worker 1 starting
+Worker 2 starting
+Worker 4 done
+Worker 1 done
+Worker 2 done
+Worker 5 done
+Worker 3 done
+
+# The order of workers starting up and finishing
+# is likely to be different for each invocation.

public/channel-synchronization

@@ -27,7 +27,9 @@
           <td class="docs">
             <p>We can use channels to synchronize execution
 across goroutines. Here&rsquo;s an example of using a
-blocking receive to wait for a goroutine to finish.</p>
+blocking receive to wait for a goroutine to finish.
+When waiting for multiple goroutines to finish,
+you may prefer to <a href="waiting-for-goroutines-to-finish">use a WaitGroup</a>.</p>
 
           </td>
           <td class="code empty leading">
@@ -41,7 +43,7 @@ blocking receive to wait for a goroutine to finish.</p>
             
           </td>
           <td class="code leading">
-	        <a href="http://play.golang.org/p/0DfW-1RMqi"><img title="Run code" src="play.png" class="run" /></a>
+	        <a href="http://play.golang.org/p/f6qTtt1agmN"><img title="Run code" src="play.png" class="run" /></a>
             <div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
 </pre></div>
 
@@ -169,7 +171,7 @@ started.</p>
       
       
       <p class="next">
-        Next example: <a href="channel-directions">Channel Directions</a>.
+        Next example: <a href="waiting-for-goroutines-to-finish">Waiting For Goroutines To Finish</a>.
       </p>
       
       <p class="footer">

public/index.html

@@ -85,6 +85,8 @@
       
         <li><a href="channel-synchronization">Channel Synchronization</a></li>
       
+        <li><a href="waiting-for-goroutines-to-finish">Waiting For Goroutines To Finish</a></li>
+      
         <li><a href="channel-directions">Channel Directions</a></li>
       
         <li><a href="select">Select</a></li>

public/waiting-for-goroutines-to-finish

@@ -0,0 +1,223 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Go by Example: Waiting For Goroutines To Finish</title>
+    <link rel=stylesheet href="site.css">
+  </head>
+  <script type="text/javascript">
+    if (window.location.host == "gobyexample.com") {
+      var _gaq = _gaq || [];
+      _gaq.push(['_setAccount', 'UA-34996217-1']);
+      _gaq.push(['_trackPageview']);
+      (function() {
+        var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
+        ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
+        var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
+      })();
+    }
+  </script>
+  <body>
+    <div class="example" id="waiting-for-goroutines-to-finish">
+      <h2><a href="./">Go by Example</a>: Waiting For Goroutines To Finish</h2>
+      
+      <table>
+        
+        <tr>
+          <td class="docs">
+            <p>To wait for multiple goroutines to finish, we can
+use a sync.WaitGroup.</p>
+
+          </td>
+          <td class="code empty leading">
+	        
+            
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            
+          </td>
+          <td class="code leading">
+	        <a href="http://play.golang.org/p/koKzXfbq8kg"><img title="Run code" src="play.png" class="run" /></a>
+            <div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
+    <span class="s">&quot;fmt&quot;</span>
+    <span class="s">&quot;math/rand&quot;</span>
+    <span class="s">&quot;sync&quot;</span>
+    <span class="s">&quot;time&quot;</span>
+<span class="p">)</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>This is the function we&rsquo;ll run in every goroutine.
+wg is the WaitGroup it uses to notify that it&rsquo;s done.
+Note that a WaitGroup must be passed to functions by
+pointer.</p>
+
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre><span class="kd">func</span> <span class="nx">worker</span><span class="p">(</span><span class="nx">id</span> <span class="kt">int</span><span class="p">,</span> <span class="nx">wg</span> <span class="o">*</span><span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span><span class="p">)</span> <span class="p">{</span>
+    <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;Worker %d starting\n&quot;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>Sleep for a random duration between 500-700 ms
+to simulate work. See the <a href="random-numbers">random numbers</a>
+example for more details on <em>rand</em>.</p>
+
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre>    <span class="nx">msToSleep</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Duration</span><span class="p">(</span><span class="mi">500</span> <span class="o">+</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">200</span><span class="p">))</span>
+    <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">msToSleep</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span><span class="p">)</span>
+    <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;Worker %d done\n&quot;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>Notify the WaitGroup that we&rsquo;re done.</p>
+
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre>    <span class="nx">wg</span><span class="p">.</span><span class="nx">Done</span><span class="p">()</span>
+<span class="p">}</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>This WaitGroup is used to wait for all the
+goroutines launched here to finish.</p>
+
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre>    <span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>Launch several goroutines and increment the WorkGroup
+counter for each.</p>
+
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre>    <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">;</span> <span class="nx">i</span> <span class="o">&lt;=</span> <span class="mi">5</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
+        <span class="nx">wg</span><span class="p">.</span><span class="nx">Add</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
+        <span class="k">go</span> <span class="nx">worker</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="o">&amp;</span><span class="nx">wg</span><span class="p">)</span>
+    <span class="p">}</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>Block until the WorkGroup counter goes back to 0;
+all the workers notified they&rsquo;re done.</p>
+
+          </td>
+          <td class="code">
+	        
+            <div class="highlight"><pre>    <span class="nx">wg</span><span class="p">.</span><span class="nx">Wait</span><span class="p">()</span>
+<span class="p">}</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+      </table>
+      
+      <table>
+        
+        <tr>
+          <td class="docs">
+            
+          </td>
+          <td class="code leading">
+	        
+            <div class="highlight"><pre><span class="gp">$</span> go run waiting-for-goroutines-to-finish.go
+<span class="go">Worker 5 starting</span>
+<span class="go">Worker 3 starting</span>
+<span class="go">Worker 4 starting</span>
+<span class="go">Worker 1 starting</span>
+<span class="go">Worker 2 starting</span>
+<span class="go">Worker 4 done</span>
+<span class="go">Worker 1 done</span>
+<span class="go">Worker 2 done</span>
+<span class="go">Worker 5 done</span>
+<span class="go">Worker 3 done</span>
+</pre></div>
+
+          </td>
+        </tr>
+        
+        <tr>
+          <td class="docs">
+            <p>The order of workers starting up and finishing
+is likely to be different for each invocation.</p>
+
+          </td>
+          <td class="code empty">
+	        
+            
+          </td>
+        </tr>
+        
+      </table>
+      
+      
+      <p class="next">
+        Next example: <a href="channel-directions">Channel Directions</a>.
+      </p>
+      
+      <p class="footer">
+        by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/waiting-for-goroutines-to-finish">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
+      </p>
+    </div>
+  </body>
+</html>