libgo/go/sync/once.go

   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 }