Go语言：sync 模块

sync模块下定义的所有struct及其方法如下：

type Cond struct {
    // L is held while observing or changing the condition
    L Locker
    // contains filtered or unexported fields
}
    func NewCond(l Locker) *Cond
    func (c *Cond) Broadcast()
    func (c *Cond) Signal()
    func (c *Cond) Wait()

type Locker interface {
    Lock()
    Unlock()
}

type Map
    func (m *Map) Delete(key interface{})
    func (m *Map) Load(key interface{}) (value interface{}, ok bool)
    func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool)
    func (m *Map) Range(f func(key, value interface{}) bool)
    func (m *Map) Store(key, value interface{})
type Mutex
    func (m *Mutex) Lock()
    func (m *Mutex) Unlock()
type Once
    func (o *Once) Do(f func())
type Pool
    func (p *Pool) Get() interface{}
    func (p *Pool) Put(x interface{})
type RWMutex
    func (rw *RWMutex) Lock()
    func (rw *RWMutex) RLock()
    func (rw *RWMutex) RLocker() Locker
    func (rw *RWMutex) RUnlock()
    func (rw *RWMutex) Unlock()
type WaitGroup
    func (wg *WaitGroup) Add(delta int)
    func (wg *WaitGroup) Done()
    func (wg *WaitGroup) Wait()

(来自 https://golang.org/pkg/sync)

Once is an object that will perform exactly one action.
eg:

func main() {
    var once sync.Once
    onceBody := func() {
        fmt.Println("Only once")
    }
    done := make(chan bool)
    for i := 0; i < 10; i++ {
        go func() {
            once.Do(onceBody)
            done <- true
        }()
    }
    for i := 0; i < 10; i++ {
        <-done
    }
}

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.

If f panics, Do considers it to have returned; future calls of Do return without calling f.