package sync

Import Path
	sync (on go.dev)

Dependency Relation
	imports 5 packages, and imported by 129 packages


Involved Source Files

	    cond.go
	    hashtriemap.go
	
		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.

	    once.go
	    oncefunc.go
	    pool.go
	    poolqueue.go
	    runtime.go
	    runtime2.go
	    rwmutex.go
	    waitgroup.go


Code Examples

	
		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
			}
		}

	
		package main
		
		import (
			"fmt"
			"sync"
		)
		
		func main() {
			once := sync.OnceValue(func() int {
				sum := 0
				for i := 0; i < 1000; i++ {
					sum += i
				}
				fmt.Println("Computed once:", sum)
				return sum
			})
			done := make(chan bool)
			for i := 0; i < 10; i++ {
				go func() {
					const want = 499500
					got := once()
					if got != want {
						fmt.Println("want", want, "got", got)
					}
					done <- true
				}()
			}
			for i := 0; i < 10; i++ {
				<-done
			}
		}

	
		package main
		
		import (
			"fmt"
			"os"
			"sync"
		)
		
		func main() {
			once := sync.OnceValues(func() ([]byte, error) {
				fmt.Println("Reading file once")
				return os.ReadFile("example_test.go")
			})
			done := make(chan bool)
			for i := 0; i < 10; i++ {
				go func() {
					data, err := once()
					if err != nil {
						fmt.Println("error:", err)
					}
					_ = data // Ignore the data for this example
					done <- true
				}()
			}
			for i := 0; i < 10; i++ {
				<-done
			}
		}

	
		package main
		
		import (
			"bytes"
			"io"
			"os"
			"sync"
			"time"
		)
		
		var bufPool = sync.Pool{
			New: func() any {
				// 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")
		}

	
		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.example.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()
		}




Package-Level Type Names (total 19, in which 8 are exported)


	/* sort exporteds by:  |  */
	
		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 [Cond.Wait] method.
		
		A Cond must not be copied after first use.
		
		In the terminology of [the Go memory model], Cond arranges that
		a call to [Cond.Broadcast] or [Cond.Signal] “synchronizes before” any Wait call
		that it unblocks.
		
		For many simple use cases, users will be better off using channels than a
		Cond (Broadcast corresponds to closing a channel, and Signal corresponds to
		sending on a channel).
		
		For more on replacements for [sync.Cond], see [Roberto Clapis's series on
		advanced concurrency patterns], as well as [Bryan Mills's talk on concurrency
		patterns].
		
		[the Go memory model]: https://go.dev/ref/mem
		[Roberto Clapis's series on advanced concurrency patterns]: https://blogtitle.github.io/categories/concurrency/
		[Bryan Mills's talk on concurrency patterns]: https://drive.google.com/file/d/1nPdvhB0PutEJzdCq5ms6UI58dp50fcAN/view

		
			
				L is held while observing or changing the condition

			
		
			
				Broadcast wakes all goroutines waiting on c.
				
				It is allowed but not required for the caller to hold c.L
				during the call.

			
				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() does not affect goroutine scheduling priority; if other goroutines
				are attempting to lock c.L, they may be awoken before a "waiting" goroutine.

			
				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 [Cond.Broadcast] or [Cond.Signal].
				
				Because c.L is not locked while Wait is waiting, 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 NewCond(l Locker) *Cond


	
		A Locker represents an object that can be locked and unlocked.

		
			( Locker) Lock()
			( Locker) Unlock()
		
			*Mutex
			*RWMutex
			*internal/sync.Mutex
			
		
			func (*RWMutex).RLocker() Locker
		
			func NewCond(l Locker) *Cond
			
		
			


	
		Map is like a Go map[any]any 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.
		
		In the terminology of [the Go memory model], Map arranges that a write operation
		“synchronizes before” any read operation that observes the effect of the write, where
		read and write operations are defined as follows.
		[Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap],
		and [Map.CompareAndDelete] are read operations;
		[Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations;
		[Map.LoadOrStore] is a write operation when it returns loaded set to false;
		[Map.CompareAndSwap] is a write operation when it returns swapped set to true;
		and [Map.CompareAndDelete] is a write operation when it returns deleted set to true.
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
		
			
				Clear deletes all the entries, resulting in an empty Map.

			
				CompareAndDelete deletes the entry for key if its value is equal to old.
				The old value must be of a comparable type.
				
				If there is no current value for key in the map, CompareAndDelete
				returns false (even if the old value is the nil interface value).

			
				CompareAndSwap swaps the old and new values for key
				if the value stored in the map is equal to old.
				The old value must be of a comparable type.

			
				Delete deletes the value for a key.

			
				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.

			
				LoadAndDelete deletes the value for a key, returning the previous value if any.
				The loaded result reports whether the key was present.

			
				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.

			
				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 (including by f), Range may reflect any
				mapping for that key from any point during the Range call. Range does not
				block other methods on the receiver; even f itself may call any method on m.
				
				Range may be O(N) with the number of elements in the map even if f returns
				false after a constant number of calls.

			
				Store sets the value for a key.

			
				Swap swaps the value for a key and returns the previous value if any.
				The loaded result reports whether the key was present.

		
			


	
		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.
		
		In the terminology of [the Go memory model],
		the n'th call to [Mutex.Unlock] “synchronizes before” the m'th call to [Mutex.Lock]
		for any n < m.
		A successful call to [Mutex.TryLock] is equivalent to a call to Lock.
		A failed call to TryLock does not establish any “synchronizes before”
		relation at all.
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
		
			
				Lock locks m.
				If the lock is already in use, the calling goroutine
				blocks until the mutex is available.

			
				TryLock tries to lock m and reports whether it succeeded.
				
				Note that while correct uses of TryLock do exist, they are rare,
				and use of TryLock is often a sign of a deeper problem
				in a particular use of mutexes.

			
				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.

		
			*Mutex : Locker
		
			


	
		Once is an object that will perform exactly one action.
		
		A Once must not be copied after first use.
		
		In the terminology of [the Go memory model],
		the return from f “synchronizes before”
		the return from any call of once.Do(f).
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
		
			
				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.

			
		
			


	
		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.
		
		In the terminology of [the Go memory model], a call to Put(x) “synchronizes before”
		a call to [Pool.Get] returning that same value x.
		Similarly, a call to New returning x “synchronizes before”
		a call to Get returning that same value x.
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
				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.

			
		
			
				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 [Pool.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.

			
				Put adds x to the pool.

			
		
			
		
			


	
		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 any goroutine calls [RWMutex.Lock] while the lock is already held by
		one or more readers, concurrent calls to [RWMutex.RLock] will block until
		the writer has acquired (and released) the lock, to ensure that
		the lock eventually becomes available to the writer.
		Note that this prohibits recursive read-locking.
		A [RWMutex.RLock] cannot be upgraded into a [RWMutex.Lock],
		nor can a [RWMutex.Lock] be downgraded into a [RWMutex.RLock].
		
		In the terminology of [the Go memory model],
		the n'th call to [RWMutex.Unlock] “synchronizes before” the m'th call to Lock
		for any n < m, just as for [Mutex].
		For any call to RLock, there exists an n such that
		the n'th call to Unlock “synchronizes before” that call to RLock,
		and the corresponding call to [RWMutex.RUnlock] “synchronizes before”
		the n+1'th call to Lock.
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
		
			
				Lock locks rw for writing.
				If the lock is already locked for reading or writing,
				Lock blocks until the lock is available.

			
				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.

			
				RLocker returns a [Locker] interface that implements
				the [Locker.Lock] and [Locker.Unlock] methods by calling rw.RLock and rw.RUnlock.

			
				RUnlock undoes a single [RWMutex.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.

			
				TryLock tries to lock rw for writing and reports whether it succeeded.
				
				Note that while correct uses of TryLock do exist, they are rare,
				and use of TryLock is often a sign of a deeper problem
				in a particular use of mutexes.

			
				TryRLock tries to lock rw for reading and reports whether it succeeded.
				
				Note that while correct uses of TryRLock do exist, they are rare,
				and use of TryRLock is often a sign of a deeper problem
				in a particular use of mutexes.

			
				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 [RWMutex.RLock] ([RWMutex.Lock]) a RWMutex and then
				arrange for another goroutine to [RWMutex.RUnlock] ([RWMutex.Unlock]) it.

			
		
			*RWMutex : Locker
		
			
		
			  var syscall.ForkLock
			


	
		A WaitGroup waits for a collection of goroutines to finish.
		The main goroutine calls [WaitGroup.Add] to set the number of
		goroutines to wait for. Then each of the goroutines
		runs and calls [WaitGroup.Done] when finished. At the same time,
		[WaitGroup.Wait] can be used to block until all goroutines have finished.
		
		A WaitGroup must not be copied after first use.
		
		In the terminology of [the Go memory model], a call to [WaitGroup.Done]
		“synchronizes before” the return of any Wait call that it unblocks.
		
		[the Go memory model]: https://go.dev/ref/mem

		
			
		
			
				Add adds delta, which may be negative, to the [WaitGroup] counter.
				If the counter becomes zero, all goroutines blocked on [WaitGroup.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.

			
				Done decrements the [WaitGroup] counter by one.

			
				Wait blocks until the [WaitGroup] counter is zero.

		
			


	


Package-Level Functions (total 28, in which 4 are exported)


	
		NewCond returns a new Cond with Locker l.


	
		OnceFunc returns a function that invokes f only once. The returned function
		may be called concurrently.
		
		If f panics, the returned function will panic with the same value on every call.


	

		Type Parameters:
			T: any

		OnceValue returns a function that invokes f only once and returns the value
		returned by f. The returned function may be called concurrently.
		
		If f panics, the returned function will panic with the same value on every call.


	

		Type Parameters:
			T1: any
			T2: any

		OnceValues returns a function that invokes f only once and returns the values
		returned by f. The returned function may be called concurrently.
		
		If f panics, the returned function will panic with the same value on every call.


	


Package-Level Variables (total 4, none are exported)

	


Package-Level Constants (total 3, none are exported)