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.
12 // Cond implements a condition variable, a rendezvous point
13 // for goroutines waiting for or announcing the occurrence
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.
20 // A Cond must not be copied after first use.
22 // In the terminology of the Go memory model, Cond arranges that
23 // a call to Broadcast or Signal “synchronizes before” any Wait call
26 // For many simple use cases, users will be better off using channels than a
27 // Cond (Broadcast corresponds to closing a channel, and Signal corresponds to
28 // sending on a channel).
30 // For more on replacements for sync.Cond, see [Roberto Clapis's series on
31 // advanced concurrency patterns], as well as [Bryan Mills's talk on concurrency
34 // [Roberto Clapis's series on advanced concurrency patterns]: https://blogtitle.github.io/categories/concurrency/
35 // [Bryan Mills's talk on concurrency patterns]: https://drive.google.com/file/d/1nPdvhB0PutEJzdCq5ms6UI58dp50fcAN/view
39 // L is held while observing or changing the condition
46 // NewCond returns a new Cond with Locker l.
47 func NewCond(l Locker) *Cond {
51 // Wait atomically unlocks c.L and suspends execution
52 // of the calling goroutine. After later resuming execution,
53 // Wait locks c.L before returning. Unlike in other systems,
54 // Wait cannot return unless awoken by Broadcast or Signal.
56 // Because c.L is not locked while Wait is waiting, the caller
57 // typically cannot assume that the condition is true when
58 // Wait returns. Instead, the caller should Wait in a loop:
64 // ... make use of condition ...
66 func (c *Cond) Wait() {
68 t := runtime_notifyListAdd(&c.notify)
70 runtime_notifyListWait(&c.notify, t)
74 // Signal wakes one goroutine waiting on c, if there is any.
76 // It is allowed but not required for the caller to hold c.L
79 // Signal() does not affect goroutine scheduling priority; if other goroutines
80 // are attempting to lock c.L, they may be awoken before a "waiting" goroutine.
81 func (c *Cond) Signal() {
83 runtime_notifyListNotifyOne(&c.notify)
86 // Broadcast wakes all goroutines waiting on c.
88 // It is allowed but not required for the caller to hold c.L
90 func (c *Cond) Broadcast() {
92 runtime_notifyListNotifyAll(&c.notify)
95 // copyChecker holds back pointer to itself to detect object copying.
96 type copyChecker uintptr
98 func (c *copyChecker) check() {
99 // Check if c has been copied in three steps:
100 // 1. The first comparison is the fast-path. If c has been initialized and not copied, this will return immediately. Otherwise, c is either not initialized, or has been copied.
101 // 2. Ensure c is initialized. If the CAS succeeds, we're done. If it fails, c was either initialized concurrently and we simply lost the race, or c has been copied.
102 // 3. Do step 1 again. Now that c is definitely initialized, if this fails, c was copied.
103 if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
104 !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
105 uintptr(*c) != uintptr(unsafe.Pointer(c)) {
106 panic("sync.Cond is copied")
110 // noCopy may be added to structs which must not be copied
111 // after the first use.
113 // See https://golang.org/issues/8005#issuecomment-190753527
116 // Note that it must not be embedded, due to the Lock and Unlock methods.
119 // Lock is a no-op used by -copylocks checker from `go vet`.
120 func (*noCopy) Lock() {}
121 func (*noCopy) Unlock() {}