1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package sync 6 7 import ( 8 "sync/atomic" 9 ) 10 11 // Once is an object that will perform exactly one action. 12 type Once struct { 13 m Mutex 14 done uint32 15 } 16 17 // Do calls the function f if and only if Do is being called for the 18 // first time for this instance of Once. In other words, given 19 // var once Once 20 // if once.Do(f) is called multiple times, only the first call will invoke f, 21 // even if f has a different value in each invocation. A new instance of 22 // Once is required for each function to execute. 23 // 24 // Do is intended for initialization that must be run exactly once. Since f 25 // is niladic, it may be necessary to use a function literal to capture the 26 // arguments to a function to be invoked by Do: 27 // config.once.Do(func() { config.init(filename) }) 28 // 29 // Because no call to Do returns until the one call to f returns, if f causes 30 // Do to be called, it will deadlock. 31 // 32 // If f panics, Do considers it to have returned; future calls of Do return 33 // without calling f. 34 // 35 func (o *Once) Do(f func()) { 36 if atomic.LoadUint32(&o.done) == 1 { 37 return 38 } 39 // Slow-path. 40 o.m.Lock() 41 defer o.m.Unlock() 42 if o.done == 0 { 43 defer atomic.StoreUint32(&o.done, 1) 44 f() 45 } 46 } 47
