1 // Copyright 2011 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package sync 6 7 import ( 8 "sync/atomic" 9 "unsafe" 10 ) 11 12 // Cond implements a condition variable, a rendezvous point 13 // for goroutines waiting for or announcing the occurrence 14 // of an event. 15 // 16 // Each Cond has an associated Locker L (often a *Mutex or *RWMutex), 17 // which must be held when changing the condition and 18 // when calling the Wait method. 19 // 20 // A Cond can be created as part of other structures. 21 // A Cond must not be copied after first use. 22 type Cond struct { 23 noCopy noCopy 24 25 // L is held while observing or changing the condition 26 L Locker 27 28 notify notifyList 29 checker copyChecker 30 } 31 32 // NewCond returns a new Cond with Locker l. 33 func NewCond(l Locker) *Cond { 34 return &Cond{L: l} 35 } 36 37 // Wait atomically unlocks c.L and suspends execution 38 // of the calling goroutine. After later resuming execution, 39 // Wait locks c.L before returning. Unlike in other systems, 40 // Wait cannot return unless awoken by Broadcast or Signal. 41 // 42 // Because c.L is not locked when Wait first resumes, the caller 43 // typically cannot assume that the condition is true when 44 // Wait returns. Instead, the caller should Wait in a loop: 45 // 46 // c.L.Lock() 47 // for !condition() { 48 // c.Wait() 49 // } 50 // ... make use of condition ... 51 // c.L.Unlock() 52 // 53 func (c *Cond) Wait() { 54 c.checker.check() 55 t := runtime_notifyListAdd(&c.notify) 56 c.L.Unlock() 57 runtime_notifyListWait(&c.notify, t) 58 c.L.Lock() 59 } 60 61 // Signal wakes one goroutine waiting on c, if there is any. 62 // 63 // It is allowed but not required for the caller to hold c.L 64 // during the call. 65 func (c *Cond) Signal() { 66 c.checker.check() 67 runtime_notifyListNotifyOne(&c.notify) 68 } 69 70 // Broadcast wakes all goroutines waiting on c. 71 // 72 // It is allowed but not required for the caller to hold c.L 73 // during the call. 74 func (c *Cond) Broadcast() { 75 c.checker.check() 76 runtime_notifyListNotifyAll(&c.notify) 77 } 78 79 // copyChecker holds back pointer to itself to detect object copying. 80 type copyChecker uintptr 81 82 func (c *copyChecker) check() { 83 if uintptr(*c) != uintptr(unsafe.Pointer(c)) && 84 !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) && 85 uintptr(*c) != uintptr(unsafe.Pointer(c)) { 86 panic("sync.Cond is copied") 87 } 88 } 89 90 // noCopy may be embedded into structs which must not be copied 91 // after the first use. 92 // 93 // See https://github.com/golang/go/issues/8005#issuecomment-190753527 94 // for details. 95 type noCopy struct{} 96 97 // Lock is a no-op used by -copylocks checker from `go vet`. 98 func (*noCopy) Lock() {} 99
