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 must not be copied after first use. 21 type Cond struct { 22 noCopy noCopy 23 24 // L is held while observing or changing the condition 25 L Locker 26 27 notify notifyList 28 checker copyChecker 29 } 30 31 // NewCond returns a new Cond with Locker l. 32 func NewCond(l Locker) *Cond { 33 return &Cond{L: l} 34 } 35 36 // Wait atomically unlocks c.L and suspends execution 37 // of the calling goroutine. After later resuming execution, 38 // Wait locks c.L before returning. Unlike in other systems, 39 // Wait cannot return unless awoken by Broadcast or Signal. 40 // 41 // Because c.L is not locked when Wait first resumes, the caller 42 // typically cannot assume that the condition is true when 43 // Wait returns. Instead, the caller should Wait in a loop: 44 // 45 // c.L.Lock() 46 // for !condition() { 47 // c.Wait() 48 // } 49 // ... make use of condition ... 50 // c.L.Unlock() 51 // 52 func (c *Cond) Wait() { 53 c.checker.check() 54 t := runtime_notifyListAdd(&c.notify) 55 c.L.Unlock() 56 runtime_notifyListWait(&c.notify, t) 57 c.L.Lock() 58 } 59 60 // Signal wakes one goroutine waiting on c, if there is any. 61 // 62 // It is allowed but not required for the caller to hold c.L 63 // during the call. 64 func (c *Cond) Signal() { 65 c.checker.check() 66 runtime_notifyListNotifyOne(&c.notify) 67 } 68 69 // Broadcast wakes all goroutines waiting on c. 70 // 71 // It is allowed but not required for the caller to hold c.L 72 // during the call. 73 func (c *Cond) Broadcast() { 74 c.checker.check() 75 runtime_notifyListNotifyAll(&c.notify) 76 } 77 78 // copyChecker holds back pointer to itself to detect object copying. 79 type copyChecker uintptr 80 81 func (c *copyChecker) check() { 82 if uintptr(*c) != uintptr(unsafe.Pointer(c)) && 83 !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) && 84 uintptr(*c) != uintptr(unsafe.Pointer(c)) { 85 panic("sync.Cond is copied") 86 } 87 } 88 89 // noCopy may be embedded into structs which must not be copied 90 // after the first use. 91 // 92 // See https://golang.org/issues/8005#issuecomment-190753527 93 // for details. 94 type noCopy struct{} 95 96 // Lock is a no-op used by -copylocks checker from `go vet`. 97 func (*noCopy) Lock() {} 98
