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Source file src/sync/once.go

  // 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 (
  	"sync/atomic"
  )
  
  // 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 {
  		return
  	}
  	// Slow-path.
  	o.m.Lock()
  	defer o.m.Unlock()
  	if o.done == 0 {
  		defer atomic.StoreUint32(&o.done, 1)
  		f()
  	}
  }
  

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