Mutexes

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

A sync.Mutex protects a critical section: only one goroutine may hold the lock at a time. Lock before accessing shared mutable state, then unlock when finished.

A mutex protects an invariant, not merely a single statement. Every writer and reader of protected state must follow the same locking discipline.

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 mu sync.Mutex
	var wg sync.WaitGroup
	count := 0
	for i := 0; i < 4; i++ {
		wg.Add(1)
		go func() { defer wg.Done(); mu.Lock(); count++; mu.Unlock() }()
	}
	wg.Wait()
	fmt.Println("Count:", count)
}

Output

Count: 4

The four workers increment shared count inside the lock. The main goroutine waits for all workers, then safely prints 4.

• Never copy a mutex after use.
• A mutex does not protect code that accesses the same variable without locking.

Make each worker increment count twice while holding the lock; predict the new count.

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

You have practiced Mutexes 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.