semaphore.go

package main

import (
	"fmt"
	"sync"
	"time"
)

var wg sync.WaitGroup

func printer(sem chan struct{}) {
	defer wg.Done()

	// An artificial sleep for simulating a long running operation.
	time.Sleep(2 * time.Second)
	fmt.Println("hello")
	<-sem
}

func main() {
	// A buffered channel only has enough room for 2 items in this case.
	sem := make(chan struct{}, 2)

	wg.Add(10)
	for i := 0; i < 10; i++ {
		// "Enqueue" our work and fire off a goroutine.
		// This will block when our channel becomes full.
		sem <- struct{}{}
		// This won't fire until our channel releases in `printer()`.
		go printer(sem)
	}
	wg.Wait()
}