What are goroutines and how do you use sync.WaitGroup to wait for them?

A goroutine is a lightweight, concurrently executing function managed by the Go runtime. The cost to create one is ~2 KB of stack and ~300 ns — roughly 1000× cheaper than an OS thread. The runtime multiplexes goroutines onto OS threads with its own scheduler.

// Launch a goroutine with the 'go' keyword
go fmt.Println("running concurrently")

// Anonymous goroutine
go func(msg string) {
    fmt.Println(msg)
}("hello from goroutine")

// PROBLEM: main() may return before goroutines finish

// SOLUTION: sync.WaitGroup
var wg sync.WaitGroup

for i := 0; i < 5; i++ {
    wg.Add(1)           // register ONE more goroutine — do this BEFORE go
    go func(n int) {
        defer wg.Done() // signal this goroutine is complete
        fmt.Printf("worker %d\n", n)
    }(i)
}

wg.Wait()  // block until all goroutines call Done() — counter reaches 0
fmt.Println("all workers finished")

// Output (order may vary):
// worker 3
// worker 1
// worker 0
// worker 4
// worker 2
// all workers finished

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