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// 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 the method is being called for the

// first time with this receiver.  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.

//

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 {

                f()

                atomic.CompareAndSwapUint32(&o.done, 0, 1)

        }

}