Race Detection

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

The Go race detector instruments memory accesses to find data races that happen during a test or run. Use go test -race ./... or go run -race main.go.

It reports races observed during execution; a clean run does not prove all possible paths are race-free.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

Command-line check: go run -race main.go. Read the race detector guide.

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 wg sync.WaitGroup
	numbers := make([]int, 3)
	for i := range numbers {
		wg.Add(1)
		go func(index int) { defer wg.Done(); numbers[index] = index + 1 }(i)
	}
	wg.Wait()
	fmt.Println(numbers)
}

Output

[1 2 3]

Each worker writes a different slice element and Wait establishes completion before the printed read. It is suitable to run with -race as a passing example.

• -race adds overhead and does not replace code review.
• Two goroutines writing different indexes of a pre-sized slice is different from concurrently appending to its header.

Run this with go run -race main.go; then experiment with an intentionally shared counter in a disposable scratch program.

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

You have practiced Race Detection 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.