Golang channels - buffered semaphor

We try to run 20 concurrent goroutines (all of them will start), in example below We’re setting channel buffer value to 5, go blocks on write where there is no more room in our channel buffer (we currently have 5) when first 5 goroutines run buffer will be full and next 15 will wait on write. After first goroutine complete and read from channel next hunged goroutine write data to goroutine.

package main

import (

var sem = make(chan int, 5)

func main() {
	for i := 0; i < 20; i++ {
		go handle()

	time.Sleep(time.Second * 10)

func handle() {
	sem <- 1
	fmt.Println("Step", time.Now())

Run it on play.golang.org

Code Summary

In context of writing to channels there are some important things:

  • Writing to channels blocks
  • If buffer is set (second param in make function) writing is blocked after buffer is full until that all goroutines can write to this channel.
  • In our example all goroutines have started (5 are working, 15 are blocked on write)

Play around change buffer value to other, look how it will work with your changes.

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