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Source file src/sync/cond.go

Documentation: sync

  // Copyright 2011 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package sync
  
  import (
  	"sync/atomic"
  	"unsafe"
  )
  
  // Cond implements a condition variable, a rendezvous point
  // for goroutines waiting for or announcing the occurrence
  // of an event.
  //
  // Each Cond has an associated 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.
  type Cond struct {
  	noCopy noCopy
  
  	// L is held while observing or changing the condition
  	L Locker
  
  	notify  notifyList
  	checker copyChecker
  }
  
  // NewCond returns a new Cond with Locker l.
  func NewCond(l Locker) *Cond {
  	return &Cond{L: l}
  }
  
  // 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()
  	t := runtime_notifyListAdd(&c.notify)
  	c.L.Unlock()
  	runtime_notifyListWait(&c.notify, t)
  	c.L.Lock()
  }
  
  // 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 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)
  }
  
  // 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")
  	}
  }
  
  // noCopy may be embedded into structs which must not be copied
  // after the first use.
  //
  // See https://github.com/golang/go/issues/8005#issuecomment-190753527
  // for details.
  type noCopy struct{}
  
  // Lock is a no-op used by -copylocks checker from `go vet`.
  func (*noCopy) Lock() {}
  

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