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golang sync.cond

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Cond implements a condition variable, a rendezvous(会和) point for goroutines waiting for or announcing(宣布，公告) the occurrence(发生) of an event.

Each Cond has an associated(关联) Locker L (often a *Mutex or *RWMutex), which must be held when changing the condition and when calling the Wait method.

A Cond must not be copied after first use.

函数以及用法和unix的条件变量很相似。代码路径/usr/local/Cellar/go/1.15.2/libexec/src/sync/cond.go

struct

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type Cond struct {
	noCopy noCopy   //禁止拷贝

	// L is held while observing or changing the condition
	L Locker

	notify  notifyList
	checker copyChecker
}

// Approximation of notifyList in runtime/sema.go. Size and alignment must
// agree.
type notifyList struct {
	wait   uint32
	notify uint32
	lock   uintptr // key field of the mutex
	head   unsafe.Pointer
	tail   unsafe.Pointer
}

NewCond

NewCond returns a new Cond with Locker l.

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// NewCond returns a new Cond with Locker l.
func NewCond(l Locker) *Cond {
	return &Cond{L: l}
}

Wait

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// Wait atomically unlocks c.L and suspends execution
// of the calling goroutine. After later resuming execution,
// Wait locks c.L before returning. Unlike in other systems,
// Wait cannot return unless awoken by Broadcast or Signal.
//
// Because c.L is not locked when Wait first resumes, the caller
// typically cannot assume that the condition is true when
// Wait returns. Instead, the caller should Wait in a loop:
//
//    c.L.Lock()
//    for !condition() {
//        c.Wait()
//    }
//    ... make use of condition ...
//    c.L.Unlock()
//
func (c *Cond) Wait() {
	c.checker.check()   //检查有没有被copy
	t := runtime_notifyListAdd(&c.notify)
	c.L.Unlock()
	runtime_notifyListWait(&c.notify, t)
	c.L.Lock()
}

check

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// copyChecker holds back pointer to itself to detect object copying.
type copyChecker uintptr

func (c *copyChecker) check() {
	if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
		!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
		uintptr(*c) != uintptr(unsafe.Pointer(c)) {
		panic("sync.Cond is copied")
	}
}

Signal

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// Signal wakes one goroutine waiting on c, if there is any.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Signal() {
	c.checker.check()
	runtime_notifyListNotifyOne(&c.notify)
}

Broadcast

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// Broadcast wakes all goroutines waiting on c.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Broadcast() {
	c.checker.check()
	runtime_notifyListNotifyAll(&c.notify)
}

基本比较简单，麻烦的是runtime系列函数，我在别的文章单独整理吧.

reference

[1]https://golang.org/pkg/sync/#Cond

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