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// Copyright 2009 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 time
// Sleep pauses the current goroutine for at least the duration d.
// A negative or zero duration causes Sleep to return immediately.
func Sleep(d Duration)
// Interface to timers implemented in package runtime.
// Must be in sync with ../runtime/time.go:/^type timer
type runtimeTimer struct {
pp uintptr
when int64
period int64
f func(interface{}, uintptr) // NOTE: must not be closure
arg interface{}
seq uintptr
nextwhen int64
status uint32
// when is a helper function for setting the 'when' field of a runtimeTimer.
// It returns what the time will be, in nanoseconds, Duration d in the future.
// If d is negative, it is ignored. If the returned value would be less than
// zero because of an overflow, MaxInt64 is returned.
func when(d Duration) int64 {
if d <= 0 {
return runtimeNano()
t := runtimeNano() + int64(d)
if t < 0 {
// N.B. runtimeNano() and d are always positive, so addition
// (including overflow) will never result in t == 0.
t = 1<<63 - 1 // math.MaxInt64
return t
func startTimer(*runtimeTimer)
func stopTimer(*runtimeTimer) bool
func resetTimer(*runtimeTimer, int64) bool
func modTimer(t *runtimeTimer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr)
// The Timer type represents a single event.
// When the Timer expires, the current time will be sent on C,
// unless the Timer was created by AfterFunc.
// A Timer must be created with NewTimer or AfterFunc.
type Timer struct {
C <-chan Time
r runtimeTimer
// Stop prevents the Timer from firing.
// It returns true if the call stops the timer, false if the timer has already
// expired or been stopped.
// Stop does not close the channel, to prevent a read from the channel succeeding
// incorrectly.
// To ensure the channel is empty after a call to Stop, check the
// return value and drain the channel.
// For example, assuming the program has not received from t.C already:
// if !t.Stop() {
// <-t.C
// }
// This cannot be done concurrent to other receives from the Timer's
// channel or other calls to the Timer's Stop method.
// For a timer created with AfterFunc(d, f), if t.Stop returns false, then the timer
// has already expired and the function f has been started in its own goroutine;
// Stop does not wait for f to complete before returning.
// If the caller needs to know whether f is completed, it must coordinate
// with f explicitly.
func (t *Timer) Stop() bool {
if t.r.f == nil {
panic("time: Stop called on uninitialized Timer")
return stopTimer(&t.r)
// NewTimer creates a new Timer that will send
// the current time on its channel after at least duration d.
func NewTimer(d Duration) *Timer {
c := make(chan Time, 1)
t := &Timer{
C: c,
r: runtimeTimer{
when: when(d),
f: sendTime,
arg: c,
return t
// Reset changes the timer to expire after duration d.
// It returns true if the timer had been active, false if the timer had
// expired or been stopped.
// For a Timer created with NewTimer, Reset should be invoked only on
// stopped or expired timers with drained channels.
// If a program has already received a value from t.C, the timer is known
// to have expired and the channel drained, so t.Reset can be used directly.
// If a program has not yet received a value from t.C, however,
// the timer must be stopped and—if Stop reports that the timer expired
// before being stopped—the channel explicitly drained:
// if !t.Stop() {
// <-t.C
// }
// t.Reset(d)
// This should not be done concurrent to other receives from the Timer's
// channel.
// Note that it is not possible to use Reset's return value correctly, as there
// is a race condition between draining the channel and the new timer expiring.
// Reset should always be invoked on stopped or expired channels, as described above.
// The return value exists to preserve compatibility with existing programs.
// For a Timer created with AfterFunc(d, f), Reset either reschedules
// when f will run, in which case Reset returns true, or schedules f
// to run again, in which case it returns false.
// When Reset returns false, Reset neither waits for the prior f to
// complete before returning nor does it guarantee that the subsequent
// goroutine running f does not run concurrently with the prior
// one. If the caller needs to know whether the prior execution of
// f is completed, it must coordinate with f explicitly.
func (t *Timer) Reset(d Duration) bool {
if t.r.f == nil {
panic("time: Reset called on uninitialized Timer")
w := when(d)
return resetTimer(&t.r, w)
func sendTime(c interface{}, seq uintptr) {
// Non-blocking send of time on c.
// Used in NewTimer, it cannot block anyway (buffer).
// Used in NewTicker, dropping sends on the floor is
// the desired behavior when the reader gets behind,
// because the sends are periodic.
select {
case c.(chan Time) <- Now():
// After waits for the duration to elapse and then sends the current time
// on the returned channel.
// It is equivalent to NewTimer(d).C.
// The underlying Timer is not recovered by the garbage collector
// until the timer fires. If efficiency is a concern, use NewTimer
// instead and call Timer.Stop if the timer is no longer needed.
func After(d Duration) <-chan Time {
return NewTimer(d).C
// AfterFunc waits for the duration to elapse and then calls f
// in its own goroutine. It returns a Timer that can
// be used to cancel the call using its Stop method.
func AfterFunc(d Duration, f func()) *Timer {
t := &Timer{
r: runtimeTimer{
when: when(d),
f: goFunc,
arg: f,
return t
func goFunc(arg interface{}, seq uintptr) {
go arg.(func())()