[Golang (Go)] Use sync.Once to execute only the first call

Posted on 2020-04-25 Edited on 2025-06-10 In Programming Language , Golang (Go) , Synchronization Views: Word count in article: 1.2k Reading time ≈ 1 mins.

sync.Once

Once is an object that will perform exactly one action.

.Do calls the function f if and only if .Do is being called for the first time for this instance of Once. In other words, given var once Once if once.Do(f) is called multiple times, only the first call will invoke f,
even if f has a different value in each invocation.

A new instance of Once is required for each function to execute.

Examples

// main.go

package main

import (
	"fmt"
	"sync"
)

func main() {
	var once sync.Once
	onceBody := func() {
		fmt.Println("Only once")
	}
	done := make(chan bool)
	for i := 0; i < 10; i++ {
		go func() {
			once.Do(onceBody) // Execute only the first call
			done <- true
		}()
	}
	for i := 0; i < 10; i++ {
		<-done
	}
}