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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 "runtime"
// 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.
type Cond struct {
L Locker // held while observing or changing the condition
m Mutex // held to avoid internal races
waiters int // number of goroutines blocked on Wait
sema *uint32
}
// 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.
//
// Because 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.m.Lock()
if c.sema == nil {
c.sema = new(uint32)
}
s := c.sema
c.waiters++
c.m.Unlock()
c.L.Unlock()
runtime.Semacquire(s)
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.m.Lock()
if c.waiters > 0 {
c.waiters--
runtime.Semrelease(c.sema)
}
c.m.Unlock()
}
// 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.m.Lock()
if c.waiters > 0 {
s := c.sema
n := c.waiters
for i := 0; i < n; i++ {
runtime.Semrelease(s)
}
// We just issued n wakeups via the semaphore s.
// To ensure that they wake up the existing waiters
// and not waiters that arrive after Broadcast returns,
// clear c.sema. The next operation will allocate
// a new one.
c.sema = nil
c.waiters = 0
}
c.m.Unlock()
}
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