blob: 10b42fddc2f2e2ab50cec49176f6f6a1f1002278 [file] [log] [blame]
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package sync
import (
// Once is an object that will perform exactly one action.
type Once struct {
m Mutex
done uint32
// 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.
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
// config.once.Do(func() { config.init(filename) })
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
func (o *Once) Do(f func()) {
if atomic.LoadUint32(&o.done) == 1 {
// Slow-path.
defer o.m.Unlock()
if o.done == 0 {
defer atomic.StoreUint32(&o.done, 1)