Important knowledge of golang: sync Once explanation

sync.Once Introduce

As mentioned before Go Concurrent helper for ：WaitGroup. alike , sync.Once It's also Go An official concurrency helper , It enables functional methods to execute only once , To achieve a similar init The effect of the function . Let's take a brief look at its usage ：

func main() {
    
	var once sync.Once
	onceFunc := func() {
    
		fmt.Println("Only once")
	}

	for i := 0; i < 10; i++ {
    
		once.Do(onceFunc)
	}
}

After execution here, we will only see once Only once Print information for , This is it. sync.Once One time effect of .

sync.Once Source code

Let's see sync.Once Source code ：

type Once struct {
	done uint32
	m    Mutex
}

func (o *Once) Do(f func()) {
    //  Atomic load identification value , Judge whether it has been executed 
	if atomic.LoadUint32(&o.done) == 0 { 
		o.doSlow(f)
	}
}

func (o *Once) doSlow(f func()) { //  I haven't executed a function yet 
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 { //  Determine again whether the function has been executed 
		defer atomic.StoreUint32(&o.done, 1) //  Atomic manipulation ： Modify the identification value 
		f() //  Execute function 
	}
}

It can be concluded from the above that ,sync.Once Is through the identification of a value , Atomic modification and loading , To reduce lock competition .

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