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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 (
"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 can be created as part of other structures.
// A Cond must not be copied after first use.
type Cond struct {
// L is held while observing or changing the condition
L Locker
sema syncSema
waiters uint32 // number of waiters
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()
if raceenabled {
raceDisable()
}
atomic.AddUint32(&c.waiters, 1)
if raceenabled {
raceEnable()
}
c.L.Unlock()
runtime_Syncsemacquire(&c.sema)
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.signalImpl(false)
}
// 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.signalImpl(true)
}
func (c *Cond) signalImpl(all bool) {
c.checker.check()
if raceenabled {
raceDisable()
}
for {
old := atomic.LoadUint32(&c.waiters)
if old == 0 {
if raceenabled {
raceEnable()
}
return
}
new := old - 1
if all {
new = 0
}
if atomic.CompareAndSwapUint32(&c.waiters, old, new) {
if raceenabled {
raceEnable()
}
runtime_Syncsemrelease(&c.sema, old-new)
return
}
}
}
// 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")
}
}
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