Race Conditions

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

A data race occurs when two goroutines access the same memory concurrently, at least one writes, and the accesses are not properly synchronized. Such a program has invalid assumptions even if its output sometimes looks right.

The example avoids a race by sending updates to one owner goroutine through a channel; no worker writes the shared counter directly.

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"

func main() {
	updates := make(chan int, 3)
	for i := 0; i < 3; i++ {
		go func() { updates <- 1 }()
	}
	sum := 0
	for i := 0; i < 3; i++ {
		sum += <-updates
	}
	fmt.Println("Total:", sum)
}

Output

Total: 3

Each goroutine sends 1. Only main changes sum, and each receive ensures one update arrived. The total is 3.

• Running code repeatedly without errors does not prove race freedom.
• Protect read and write operations, not only writes.

Compare this owner-loop approach with protecting sum using a mutex.

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

You have practiced Race Conditions 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.