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Source file src/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	}
    47	

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