Documentation
¶
Overview ¶
Package sync provides basic synchronization primitives such as mutual exclusion locks. Other than the Once and WaitGroup types, most are intended for use by low-level library routines. Higher-level synchronization is better done via channels and communication.
Values containing the types defined in this package should not be copied.
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Cond ¶
type Cond struct {
// L is held while observing or changing the condition
L Locker // 关联的 Locker
// contains filtered or unexported fields
}
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.
A Cond must not be copied after first use. Cond 实现条件变量，每一个 Cond 有一个关联的 Locker L （通常是 *Mutex ，*RWMutex）
func (*Cond) Broadcast ¶
func (c *Cond) Broadcast()
Broadcast wakes all goroutines waiting on c.
It is allowed but not required for the caller to hold c.L during the call. Broadcast 唤醒等待 c 的所有 goroutine ，调用时它可以（不必须）持久有个 c.L
func (*Cond) Signal ¶
func (c *Cond) Signal()
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. Signal 唤醒一个等待 c 的 goroutine（如果存在），在调用时它可以（不必须）持有一个 c.L
func (*Cond) Wait ¶
func (c *Cond) Wait()
Wait atomically unlocks c.L and suspends execution of the calling goroutine. After later resuming execution, Wait locks c.L before returning. Unlike in other systems, Wait cannot return unless awoken by Broadcast or Signal.
Because c.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()
Wait 原子式的 unlock c.L， 并暂停执行调用的 goroutine。
type Locker ¶
type Locker interface {
Lock()
Unlock()
}
A Locker represents an object that can be locked and unlocked.
type Map ¶
type Map struct {
// contains filtered or unexported fields
}
Map is like a Go map[interface{}]interface{} but is safe for concurrent use by multiple goroutines without additional locking or coordination. Loads, stores, and deletes run in amortized constant time.
The Map type is specialized. Most code should use a plain Go map instead, with separate locking or coordination, for better type safety and to make it easier to maintain other invariants along with the map content.
The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.
The zero Map is empty and ready for use. A Map must not be copied after first use. Map 是一种并发安全的 map[interface{}]interface{}，在多个 goroutine 中没有额外的锁条件。 读取、存储和删除操作的时间复杂度平均为常量。 Map 类型是专用的。大多数代码使用分段锁或协调的纯 Go map ，以提高类型安全性，并使其更易于维护 map 中的不变量。 Map 类型非常特殊，大部分代码应该使用原始的 Go map。它具有单独的锁或协调以获得类型安全且更易维护。 Map 类型针对两种常见的用例进行优化： 1. 给定 key 只会产生写一次但是却会多次读，类似乎只增的缓存 2. 多个 goroutine 读、写以及覆盖不同的 key 这两种情况下，与单独使用 Mutex 或 RWMutex 的 map 相比，会显著降低竞争情况
func (*Map) Delete ¶
func (m *Map) Delete(key interface{})
Delete deletes the value for a key. Delete 删除 key 对应的 value
func (*Map) Load ¶
Load returns the value stored in the map for a key, or nil if no value is present. The ok result indicates whether value was found in the map. Load 返回了存储在 map 中对应于 key 的值 value，如果不存在则返回 nil ok 表示了值能否在 map 中找到
func (*Map) LoadOrStore ¶
LoadOrStore returns the existing value for the key if present. Otherwise, it stores and returns the given value. The loaded result is true if the value was loaded, false if stored. LoadOrStore 在 key 已经存在时，返回存在的值，否则存储当前给定的值。 loaded 为 true 表示 actual 读取成功，否则为 false 表示 value 存储成功
func (*Map) Range ¶
Range calls f sequentially for each key and value present in the map. If f returns false, range stops the iteration.
Range does not necessarily correspond to any consistent snapshot of the Map's contents: no key will be visited more than once, but if the value for any key is stored or deleted concurrently, Range may reflect any mapping for that key from any point during the Range call.
Range may be O(N) with the number of elements in the map even if f returns false after a constant number of calls.
Range 为每个 key 顺序的调用 f。如果 f 返回 false，则 range 会停止迭代。 Range 的时间复杂度可能会是 O(N) 即便是 f 返回 false。
type Mutex ¶
type Mutex struct {
// contains filtered or unexported fields
}
A Mutex is a mutual exclusion lock. The zero value for a Mutex is an unlocked mutex.
A Mutex must not be copied after first use. Mutex 互斥锁
func (*Mutex) Lock ¶
func (m *Mutex) Lock()
Lock locks m. If the lock is already in use, the calling goroutine blocks until the mutex is available. Lock 对申请锁的情况分为三种： 1. 无冲突，通过 CAS 操作把当前状态设置为加锁状态 2. 有冲突，开始自旋，并等待锁释放，如果其他 goroutine 在这段时间内释放该锁，直接获得该锁；如果没有释放则为下一种情况 3. 有冲突，且已经过了自旋阶段，通过调用 semrelease 让 goroutine 进入等待状态
func (*Mutex) Unlock ¶
func (m *Mutex) Unlock()
Unlock unlocks m. It is a run-time error if m is not locked on entry to Unlock.
A locked Mutex is not associated with a particular goroutine. It is allowed for one goroutine to lock a Mutex and then arrange for another goroutine to unlock it. 解锁， 与特定的 goroutine 没有关联，可以一个 goroutine 加锁，另一个解锁
type Once ¶
type Once struct {
// contains filtered or unexported fields
}
Once is an object that will perform exactly one action. Once 对象可以保证一个动作的绝对一次执行。
Example ¶
package main
import (
"fmt"
"sync"
)
func main() {
var once sync.Once
onceBody := func() {
fmt.Println("Only once")
}
done := make(chan bool)
for i := 0; i < 10; i++ {
go func() {
once.Do(onceBody)
done <- true
}()
}
for i := 0; i < 10; i++ {
<-done
}
}
Output: Only once
func (*Once) Do ¶
func (o *Once) Do(f func())
Do calls the function f if and only if Do is being called for the first time for this instance of Once. In other words, given
var once Once
if once.Do(f) is called multiple times, only the first call will invoke f, even if f has a different value in each invocation. A new instance of Once is required for each function to execute.
Do is intended for initialization that must be run exactly once. Since f is niladic, it may be necessary to use a function literal to capture the arguments to a function to be invoked by Do:
config.once.Do(func() { config.init(filename) })
Because no call to Do returns until the one call to f returns, if f causes Do to be called, it will deadlock.
If f panics, Do considers it to have returned; future calls of Do return without calling f.
type Pool ¶
type Pool struct {
// New optionally specifies a function to generate
// a value when Get would otherwise return nil.
// It may not be changed concurrently with calls to Get.
New func() interface{}
// contains filtered or unexported fields
}
A Pool is a set of temporary objects that may be individually saved and retrieved.
Any item stored in the Pool may be removed automatically at any time without notification. If the Pool holds the only reference when this happens, the item might be deallocated.
A Pool is safe for use by multiple goroutines simultaneously.
Pool's purpose is to cache allocated but unused items for later reuse, relieving pressure on the garbage collector. That is, it makes it easy to build efficient, thread-safe free lists. However, it is not suitable for all free lists.
An appropriate use of a Pool is to manage a group of temporary items silently shared among and potentially reused by concurrent independent clients of a package. Pool provides a way to amortize allocation overhead across many clients.
An example of good use of a Pool is in the fmt package, which maintains a dynamically-sized store of temporary output buffers. The store scales under load (when many goroutines are actively printing) and shrinks when quiescent.
On the other hand, a free list maintained as part of a short-lived object is not a suitable use for a Pool, since the overhead does not amortize well in that scenario. It is more efficient to have such objects implement their own free list.
A Pool must not be copied after first use.
Example ¶
package main
import (
"bytes"
"io"
"os"
"sync"
"time"
)
var bufPool = sync.Pool{
New: func() interface{} {
// The Pool's New function should generally only return pointer
// types, since a pointer can be put into the return interface
// value without an allocation:
return new(bytes.Buffer)
},
}
// timeNow is a fake version of time.Now for tests.
func timeNow() time.Time {
return time.Unix(1136214245, 0)
}
func Log(w io.Writer, key, val string) {
b := bufPool.Get().(*bytes.Buffer)
b.Reset()
// Replace this with time.Now() in a real logger.
b.WriteString(timeNow().UTC().Format(time.RFC3339))
b.WriteByte(' ')
b.WriteString(key)
b.WriteByte('=')
b.WriteString(val)
w.Write(b.Bytes())
bufPool.Put(b)
}
func main() {
Log(os.Stdout, "path", "/search?q=flowers")
}
Output: 2006-01-02T15:04:05Z path=/search?q=flowers
func (*Pool) Get ¶
func (p *Pool) Get() interface{}
Get selects an arbitrary item from the Pool, removes it from the Pool, and returns it to the caller. Get may choose to ignore the pool and treat it as empty. Callers should not assume any relation between values passed to Put and the values returned by Get.
If Get would otherwise return nil and p.New is non-nil, Get returns the result of calling p.New.
type RWMutex ¶
type RWMutex struct {
// contains filtered or unexported fields
}
A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.
A RWMutex must not be copied after first use.
If a goroutine holds a RWMutex for reading and another goroutine might call Lock, no goroutine should expect to be able to acquire a read lock until the initial read lock is released. In particular, this prohibits recursive read locking. This is to ensure that the lock eventually becomes available; a blocked Lock call excludes new readers from acquiring the lock. RWMutex 读写锁
func (*RWMutex) Lock ¶
func (rw *RWMutex) Lock()
Lock locks rw for writing. If the lock is already locked for reading or writing, Lock blocks until the lock is available. Lock 获取写锁
func (*RWMutex) RLock ¶
func (rw *RWMutex) RLock()
RLock locks rw for reading.
It should not be used for recursive read locking; a blocked Lock call excludes new readers from acquiring the lock. See the documentation on the RWMutex type. RLock 获取读锁
func (*RWMutex) RLocker ¶
RLocker returns a Locker interface that implements the Lock and Unlock methods by calling rw.RLock and rw.RUnlock. RLocker 返回 rw.RLock 和 rw.RUnlock 实现的 Locker 接口。
func (*RWMutex) RUnlock ¶
func (rw *RWMutex) RUnlock()
RUnlock undoes a single RLock call; it does not affect other simultaneous readers. It is a run-time error if rw is not locked for reading on entry to RUnlock. RUnlock 释放读锁
func (*RWMutex) Unlock ¶
func (rw *RWMutex) Unlock()
Unlock unlocks rw for writing. It is a run-time error if rw is not locked for writing on entry to Unlock.
As with Mutexes, a locked RWMutex is not associated with a particular goroutine. One goroutine may RLock (Lock) a RWMutex and then arrange for another goroutine to RUnlock (Unlock) it. Unlock 释放写锁
type WaitGroup ¶
type WaitGroup struct {
// contains filtered or unexported fields
}
A WaitGroup waits for a collection of goroutines to finish. The main goroutine calls Add to set the number of goroutines to wait for. Then each of the goroutines runs and calls Done when finished. At the same time, Wait can be used to block until all goroutines have finished.
A WaitGroup must not be copied after first use. WaitGroup 用于等待一组 goroutines 执行完。 主 Goroutine 调用 Add 来设置需要等待的 Goroutine 的数量， 然后每个 Goroutine 运行并调用 Done 来确认已经执行网完毕，同时，Wait 可以用于阻塞并等待所有 Goroutine 完。
Example ¶
This example fetches several URLs concurrently, using a WaitGroup to block until all the fetches are complete.
package main
import (
"sync"
)
type httpPkg struct{}
func (httpPkg) Get(url string) {}
var http httpPkg
func main() {
var wg sync.WaitGroup
var urls = []string{
"http://www.golang.org/",
"http://www.google.com/",
"http://www.somestupidname.com/",
}
for _, url := range urls {
// Increment the WaitGroup counter.
wg.Add(1)
// Launch a goroutine to fetch the URL.
go func(url string) {
// Decrement the counter when the goroutine completes.
defer wg.Done()
// Fetch the URL.
http.Get(url)
}(url)
}
// Wait for all HTTP fetches to complete.
wg.Wait()
}
Output:
func (*WaitGroup) Add ¶
Add adds delta, which may be negative, to the WaitGroup counter. If the counter becomes zero, all goroutines blocked on Wait are released. If the counter goes negative, Add panics.
Note that calls with a positive delta that occur when the counter is zero must happen before a Wait. Calls with a negative delta, or calls with a positive delta that start when the counter is greater than zero, may happen at any time. Typically this means the calls to Add should execute before the statement creating the goroutine or other event to be waited for. If a WaitGroup is reused to wait for several independent sets of events, new Add calls must happen after all previous Wait calls have returned. See the WaitGroup example. Add 将 delta（可能为负）加到 WaitGroup 的计数器上，如果计数器归零，则所有阻塞在 Wait 的 Goroutine 被释放， 如果计数器为负，则 panic 。
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