What is the goroutine loop-variable capture bug and how do you fix it?

One of the most common Go concurrency interview puzzles. When a goroutine closure captures a loop variable by reference, all goroutines in the loop share the same variable — which has already advanced to its final value by the time any goroutine runs.

// BUG (Go 1.21 and earlier): all goroutines may print 5
for i := 0; i < 5; i++ {
    go func() {
        fmt.Println(i) // captures &i — reads whatever i is RIGHT NOW
    }()
}
// By the time goroutines execute, the loop finished and i == 5
// Typical output: 5 5 5 5 5

// FIX 1: pass as argument — creates an independent copy per iteration
for i := 0; i < 5; i++ {
    go func(n int) { // n is a local copy of i at this point
        fmt.Println(n) // 0 1 2 3 4 (in any order)
    }(i)
}

// FIX 2: shadow the loop variable inside the loop body
for i := 0; i < 5; i++ {
    i := i // new variable, shadows the outer i
    go func() { fmt.Println(i) }()
}

// GO 1.22+: loop variables are per-iteration by default
// for i := range 5 { go func() { fmt.Println(i) }() } // safe in 1.22!

// Same bug with range over slice
names := []string{"Alice", "Bob", "Carol"}
for _, name := range names {
    go func() { fmt.Println(name) }() // BUG: all may print "Carol"
}

// Fix
for _, name := range names {
    go func(n string) { fmt.Println(n) }(name)
}

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.