atomic

package standard library
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Published: Sep 29, 2022 License: BSD-3-Clause Imports: 1 Imported by: 98,732

Documentation

Overview

Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.

These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don't communicate by sharing memory.

The swap operation, implemented by the SwapT functions, is the atomic equivalent of:

old = *addr
*addr = new
return old

The compare-and-swap operation, implemented by the CompareAndSwapT functions, is the atomic equivalent of:

if *addr == old {
	*addr = new
	return true
}
return false

The add operation, implemented by the AddT functions, is the atomic equivalent of:

*addr += delta
return *addr

The load and store operations, implemented by the LoadT and StoreT functions, are the atomic equivalents of "return *addr" and "*addr = val".

In the terminology of the Go memory model, if the effect of an atomic operation A is observed by atomic operation B, then A “synchronizes before” B. Additionally, all the atomic operations executed in a program behave as though executed in some sequentially consistent order. This definition provides the same semantics as C++'s sequentially consistent atomics and Java's volatile variables.

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

func AddInt32

func AddInt32(addr *int32, delta int32) (new int32)

AddInt32 atomically adds delta to *addr and returns the new value.

func AddInt64

func AddInt64(addr *int64, delta int64) (new int64)

AddInt64 atomically adds delta to *addr and returns the new value.

func AddUint32

func AddUint32(addr *uint32, delta uint32) (new uint32)

AddUint32 atomically adds delta to *addr and returns the new value. To subtract a signed positive constant value c from x, do AddUint32(&x, ^uint32(c-1)). In particular, to decrement x, do AddUint32(&x, ^uint32(0)).

func AddUint64

func AddUint64(addr *uint64, delta uint64) (new uint64)

AddUint64 atomically adds delta to *addr and returns the new value. To subtract a signed positive constant value c from x, do AddUint64(&x, ^uint64(c-1)). In particular, to decrement x, do AddUint64(&x, ^uint64(0)).

func AddUintptr

func AddUintptr(addr *uintptr, delta uintptr) (new uintptr)

AddUintptr atomically adds delta to *addr and returns the new value.

func CompareAndSwapInt32

func CompareAndSwapInt32(addr *int32, old, new int32) (swapped bool)

CompareAndSwapInt32 executes the compare-and-swap operation for an int32 value.

func CompareAndSwapInt64

func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool)

CompareAndSwapInt64 executes the compare-and-swap operation for an int64 value.

func CompareAndSwapPointer

func CompareAndSwapPointer(addr *unsafe.Pointer, old, new unsafe.Pointer) (swapped bool)

CompareAndSwapPointer executes the compare-and-swap operation for a unsafe.Pointer value.

func CompareAndSwapUint32

func CompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)

CompareAndSwapUint32 executes the compare-and-swap operation for a uint32 value.

func CompareAndSwapUint64

func CompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)

CompareAndSwapUint64 executes the compare-and-swap operation for a uint64 value.

func CompareAndSwapUintptr

func CompareAndSwapUintptr(addr *uintptr, old, new uintptr) (swapped bool)

CompareAndSwapUintptr executes the compare-and-swap operation for a uintptr value.

func LoadInt32

func LoadInt32(addr *int32) (val int32)

LoadInt32 atomically loads *addr.

func LoadInt64

func LoadInt64(addr *int64) (val int64)

LoadInt64 atomically loads *addr.

func LoadPointer

func LoadPointer(addr *unsafe.Pointer) (val unsafe.Pointer)

LoadPointer atomically loads *addr.

func LoadUint32

func LoadUint32(addr *uint32) (val uint32)

LoadUint32 atomically loads *addr.

func LoadUint64

func LoadUint64(addr *uint64) (val uint64)

LoadUint64 atomically loads *addr.

func LoadUintptr

func LoadUintptr(addr *uintptr) (val uintptr)

LoadUintptr atomically loads *addr.

func StoreInt32

func StoreInt32(addr *int32, val int32)

StoreInt32 atomically stores val into *addr.

func StoreInt64

func StoreInt64(addr *int64, val int64)

StoreInt64 atomically stores val into *addr.

func StorePointer

func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)

StorePointer atomically stores val into *addr.

func StoreUint32

func StoreUint32(addr *uint32, val uint32)

StoreUint32 atomically stores val into *addr.

func StoreUint64

func StoreUint64(addr *uint64, val uint64)

StoreUint64 atomically stores val into *addr.

func StoreUintptr

func StoreUintptr(addr *uintptr, val uintptr)

StoreUintptr atomically stores val into *addr.

func SwapInt32 added in go1.2

func SwapInt32(addr *int32, new int32) (old int32)

SwapInt32 atomically stores new into *addr and returns the previous *addr value.

func SwapInt64 added in go1.2

func SwapInt64(addr *int64, new int64) (old int64)

SwapInt64 atomically stores new into *addr and returns the previous *addr value.

func SwapPointer added in go1.2

func SwapPointer(addr *unsafe.Pointer, new unsafe.Pointer) (old unsafe.Pointer)

SwapPointer atomically stores new into *addr and returns the previous *addr value.

func SwapUint32 added in go1.2

func SwapUint32(addr *uint32, new uint32) (old uint32)

SwapUint32 atomically stores new into *addr and returns the previous *addr value.

func SwapUint64 added in go1.2

func SwapUint64(addr *uint64, new uint64) (old uint64)

SwapUint64 atomically stores new into *addr and returns the previous *addr value.

func SwapUintptr added in go1.2

func SwapUintptr(addr *uintptr, new uintptr) (old uintptr)

SwapUintptr atomically stores new into *addr and returns the previous *addr value.

Types

type Bool added in go1.19

type Bool struct {
	// contains filtered or unexported fields
}

A Bool is an atomic boolean value. The zero value is false.

func (*Bool) CompareAndSwap added in go1.19

func (x *Bool) CompareAndSwap(old, new bool) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for the boolean value x.

func (*Bool) Load added in go1.19

func (x *Bool) Load() bool

Load atomically loads and returns the value stored in x.

func (*Bool) Store added in go1.19

func (x *Bool) Store(val bool)

Store atomically stores val into x.

func (*Bool) Swap added in go1.19

func (x *Bool) Swap(new bool) (old bool)

Swap atomically stores new into x and returns the previous value.

type Int32 added in go1.19

type Int32 struct {
	// contains filtered or unexported fields
}

An Int32 is an atomic int32. The zero value is zero.

func (*Int32) Add added in go1.19

func (x *Int32) Add(delta int32) (new int32)

Add atomically adds delta to x and returns the new value.

func (*Int32) CompareAndSwap added in go1.19

func (x *Int32) CompareAndSwap(old, new int32) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Int32) Load added in go1.19

func (x *Int32) Load() int32

Load atomically loads and returns the value stored in x.

func (*Int32) Store added in go1.19

func (x *Int32) Store(val int32)

Store atomically stores val into x.

func (*Int32) Swap added in go1.19

func (x *Int32) Swap(new int32) (old int32)

Swap atomically stores new into x and returns the previous value.

type Int64 added in go1.19

type Int64 struct {
	// contains filtered or unexported fields
}

An Int64 is an atomic int64. The zero value is zero.

func (*Int64) Add added in go1.19

func (x *Int64) Add(delta int64) (new int64)

Add atomically adds delta to x and returns the new value.

func (*Int64) CompareAndSwap added in go1.19

func (x *Int64) CompareAndSwap(old, new int64) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Int64) Load added in go1.19

func (x *Int64) Load() int64

Load atomically loads and returns the value stored in x.

func (*Int64) Store added in go1.19

func (x *Int64) Store(val int64)

Store atomically stores val into x.

func (*Int64) Swap added in go1.19

func (x *Int64) Swap(new int64) (old int64)

Swap atomically stores new into x and returns the previous value.

type Pointer added in go1.19

type Pointer[T any] struct {
	// contains filtered or unexported fields
}

A Pointer is an atomic pointer of type *T. The zero value is a nil *T.

func (*Pointer[T]) CompareAndSwap added in go1.19

func (x *Pointer[T]) CompareAndSwap(old, new *T) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Pointer[T]) Load added in go1.19

func (x *Pointer[T]) Load() *T

Load atomically loads and returns the value stored in x.

func (*Pointer[T]) Store added in go1.19

func (x *Pointer[T]) Store(val *T)

Store atomically stores val into x.

func (*Pointer[T]) Swap added in go1.19

func (x *Pointer[T]) Swap(new *T) (old *T)

Swap atomically stores new into x and returns the previous value.

type Uint32 added in go1.19

type Uint32 struct {
	// contains filtered or unexported fields
}

An Uint32 is an atomic uint32. The zero value is zero.

func (*Uint32) Add added in go1.19

func (x *Uint32) Add(delta uint32) (new uint32)

Add atomically adds delta to x and returns the new value.

func (*Uint32) CompareAndSwap added in go1.19

func (x *Uint32) CompareAndSwap(old, new uint32) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Uint32) Load added in go1.19

func (x *Uint32) Load() uint32

Load atomically loads and returns the value stored in x.

func (*Uint32) Store added in go1.19

func (x *Uint32) Store(val uint32)

Store atomically stores val into x.

func (*Uint32) Swap added in go1.19

func (x *Uint32) Swap(new uint32) (old uint32)

Swap atomically stores new into x and returns the previous value.

type Uint64 added in go1.19

type Uint64 struct {
	// contains filtered or unexported fields
}

An Uint64 is an atomic uint64. The zero value is zero.

func (*Uint64) Add added in go1.19

func (x *Uint64) Add(delta uint64) (new uint64)

Add atomically adds delta to x and returns the new value.

func (*Uint64) CompareAndSwap added in go1.19

func (x *Uint64) CompareAndSwap(old, new uint64) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Uint64) Load added in go1.19

func (x *Uint64) Load() uint64

Load atomically loads and returns the value stored in x.

func (*Uint64) Store added in go1.19

func (x *Uint64) Store(val uint64)

Store atomically stores val into x.

func (*Uint64) Swap added in go1.19

func (x *Uint64) Swap(new uint64) (old uint64)

Swap atomically stores new into x and returns the previous value.

type Uintptr added in go1.19

type Uintptr struct {
	// contains filtered or unexported fields
}

An Uintptr is an atomic uintptr. The zero value is zero.

func (*Uintptr) Add added in go1.19

func (x *Uintptr) Add(delta uintptr) (new uintptr)

Add atomically adds delta to x and returns the new value.

func (*Uintptr) CompareAndSwap added in go1.19

func (x *Uintptr) CompareAndSwap(old, new uintptr) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for x.

func (*Uintptr) Load added in go1.19

func (x *Uintptr) Load() uintptr

Load atomically loads and returns the value stored in x.

func (*Uintptr) Store added in go1.19

func (x *Uintptr) Store(val uintptr)

Store atomically stores val into x.

func (*Uintptr) Swap added in go1.19

func (x *Uintptr) Swap(new uintptr) (old uintptr)

Swap atomically stores new into x and returns the previous value.

type Value added in go1.4

type Value struct {
	// contains filtered or unexported fields
}

A Value provides an atomic load and store of a consistently typed value. The zero value for a Value returns nil from Load. Once Store has been called, a Value must not be copied.

A Value must not be copied after first use.

Example (Config)

The following example shows how to use Value for periodic program config updates and propagation of the changes to worker goroutines.

package main

import (
	"sync/atomic"
	"time"
)

func loadConfig() map[string]string {
	return make(map[string]string)
}

func requests() chan int {
	return make(chan int)
}

func main() {
	var config atomic.Value // holds current server configuration
	// Create initial config value and store into config.
	config.Store(loadConfig())
	go func() {
		// Reload config every 10 seconds
		// and update config value with the new version.
		for {
			time.Sleep(10 * time.Second)
			config.Store(loadConfig())
		}
	}()
	// Create worker goroutines that handle incoming requests
	// using the latest config value.
	for i := 0; i < 10; i++ {
		go func() {
			for r := range requests() {
				c := config.Load()
				// Handle request r using config c.
				_, _ = r, c
			}
		}()
	}
}
Output:

Example (ReadMostly)

The following example shows how to maintain a scalable frequently read, but infrequently updated data structure using copy-on-write idiom.

package main

import (
	"sync"
	"sync/atomic"
)

func main() {
	type Map map[string]string
	var m atomic.Value
	m.Store(make(Map))
	var mu sync.Mutex // used only by writers
	// read function can be used to read the data without further synchronization
	read := func(key string) (val string) {
		m1 := m.Load().(Map)
		return m1[key]
	}
	// insert function can be used to update the data without further synchronization
	insert := func(key, val string) {
		mu.Lock() // synchronize with other potential writers
		defer mu.Unlock()
		m1 := m.Load().(Map) // load current value of the data structure
		m2 := make(Map)      // create a new value
		for k, v := range m1 {
			m2[k] = v // copy all data from the current object to the new one
		}
		m2[key] = val // do the update that we need
		m.Store(m2)   // atomically replace the current object with the new one
		// At this point all new readers start working with the new version.
		// The old version will be garbage collected once the existing readers
		// (if any) are done with it.
	}
	_, _ = read, insert
}
Output:

func (*Value) CompareAndSwap added in go1.17

func (v *Value) CompareAndSwap(old, new any) (swapped bool)

CompareAndSwap executes the compare-and-swap operation for the Value.

All calls to CompareAndSwap for a given Value must use values of the same concrete type. CompareAndSwap of an inconsistent type panics, as does CompareAndSwap(old, nil).

func (*Value) Load added in go1.4

func (v *Value) Load() (val any)

Load returns the value set by the most recent Store. It returns nil if there has been no call to Store for this Value.

func (*Value) Store added in go1.4

func (v *Value) Store(val any)

Store sets the value of the Value to x. All calls to Store for a given Value must use values of the same concrete type. Store of an inconsistent type panics, as does Store(nil).

func (*Value) Swap added in go1.17

func (v *Value) Swap(new any) (old any)

Swap stores new into Value and returns the previous value. It returns nil if the Value is empty.

All calls to Swap for a given Value must use values of the same concrete type. Swap of an inconsistent type panics, as does Swap(nil).

Notes

Bugs

  • On 386, the 64-bit functions use instructions unavailable before the Pentium MMX.

    On non-Linux ARM, the 64-bit functions use instructions unavailable before the ARMv6k core.

    On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically via the primitive atomic functions (types Int64 and Uint64 are automatically aligned). The first word in an allocated struct, array, or slice; in a global variable; or in a local variable (because the subject of all atomic operations will escape to the heap) can be relied upon to be 64-bit aligned.

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