Go语言——sync.Once分析

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()
   }
}

package main

import (
        "fmt"
        "sync"
        "time"
)

var once sync.Once
var onceBody = func() {
    fmt.Println("Only once")
}

func main() {
        for i := 0; i < 10; i++ {
                go func(i int) {
                        once.Do(onceBody)
                        fmt.Println("i=",i)
                }()
        }
        time.Sleep(time.Second) //睡眠1s用于执行go程，注意睡眠时间不能太短
}

Only once
i= 3
i= 6
i= 4
i= 5
i= 7
i= 0
i= 1
i= 2
i= 9
i= 8