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// 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 (
// 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() {
t := runtime_notifyListAdd(&c.notify)
runtime_notifyListWait(&c.notify, t)
// 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() {
// 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() {
// 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
// for details.
type noCopy struct{}
// Lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) Lock() {}
func (*noCopy) Unlock() {}