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 ¶
- func AddInt32(addr *int32, delta int32) (new int32)
- func AddInt64(addr *int64, delta int64) (new int64)
- func AddUint32(addr *uint32, delta uint32) (new uint32)
- func AddUint64(addr *uint64, delta uint64) (new uint64)
- func AddUintptr(addr *uintptr, delta uintptr) (new uintptr)
- func CompareAndSwapInt32(addr *int32, old, new int32) (swapped bool)
- func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool)
- func CompareAndSwapPointer(addr *unsafe.Pointer, old, new unsafe.Pointer) (swapped bool)
- func CompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)
- func CompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
- func CompareAndSwapUintptr(addr *uintptr, old, new uintptr) (swapped bool)
- func LoadInt32(addr *int32) (val int32)
- func LoadInt64(addr *int64) (val int64)
- func LoadPointer(addr *unsafe.Pointer) (val unsafe.Pointer)
- func LoadUint32(addr *uint32) (val uint32)
- func LoadUint64(addr *uint64) (val uint64)
- func LoadUintptr(addr *uintptr) (val uintptr)
- func StoreInt32(addr *int32, val int32)
- func StoreInt64(addr *int64, val int64)
- func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)
- func StoreUint32(addr *uint32, val uint32)
- func StoreUint64(addr *uint64, val uint64)
- func StoreUintptr(addr *uintptr, val uintptr)
- func SwapInt32(addr *int32, new int32) (old int32)
- func SwapInt64(addr *int64, new int64) (old int64)
- func SwapPointer(addr *unsafe.Pointer, new unsafe.Pointer) (old unsafe.Pointer)
- func SwapUint32(addr *uint32, new uint32) (old uint32)
- func SwapUint64(addr *uint64, new uint64) (old uint64)
- func SwapUintptr(addr *uintptr, new uintptr) (old uintptr)
- type Bool
- type Int32
- type Int64
- type Pointer
- type Uint32
- type Uint64
- type Uintptr
- type Value
- Bugs
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
func AddInt32 ¶
AddInt32 atomically adds delta to *addr and returns the new value. Consider using the more ergonomic and less error-prone Int32.Add instead.
func AddInt64 ¶
AddInt64 atomically adds delta to *addr and returns the new value. Consider using the more ergonomic and less error-prone Int64.Add instead (particularly if you target 32-bit platforms; see the bugs section).
func AddUint32 ¶
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)). Consider using the more ergonomic and less error-prone Uint32.Add instead.
func AddUint64 ¶
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)). Consider using the more ergonomic and less error-prone Uint64.Add instead (particularly if you target 32-bit platforms; see the bugs section).
func AddUintptr ¶
AddUintptr atomically adds delta to *addr and returns the new value. Consider using the more ergonomic and less error-prone Uintptr.Add instead.
func CompareAndSwapInt32 ¶
CompareAndSwapInt32 executes the compare-and-swap operation for an int32 value. Consider using the more ergonomic and less error-prone Int32.CompareAndSwap instead.
func CompareAndSwapInt64 ¶
CompareAndSwapInt64 executes the compare-and-swap operation for an int64 value. Consider using the more ergonomic and less error-prone Int64.CompareAndSwap instead (particularly if you target 32-bit platforms; see the bugs section).
func CompareAndSwapPointer ¶
CompareAndSwapPointer executes the compare-and-swap operation for a unsafe.Pointer value. Consider using the more ergonomic and less error-prone Pointer.CompareAndSwap instead.
func CompareAndSwapUint32 ¶
CompareAndSwapUint32 executes the compare-and-swap operation for a uint32 value. Consider using the more ergonomic and less error-prone Uint32.CompareAndSwap instead.
func CompareAndSwapUint64 ¶
CompareAndSwapUint64 executes the compare-and-swap operation for a uint64 value. Consider using the more ergonomic and less error-prone Uint64.CompareAndSwap instead (particularly if you target 32-bit platforms; see the bugs section).
func CompareAndSwapUintptr ¶
CompareAndSwapUintptr executes the compare-and-swap operation for a uintptr value. Consider using the more ergonomic and less error-prone Uintptr.CompareAndSwap instead.
func LoadInt32 ¶
LoadInt32 atomically loads *addr. Consider using the more ergonomic and less error-prone Int32.Load instead.
func LoadInt64 ¶
LoadInt64 atomically loads *addr. Consider using the more ergonomic and less error-prone Int64.Load instead (particularly if you target 32-bit platforms; see the bugs section).
func LoadPointer ¶
LoadPointer atomically loads *addr. Consider using the more ergonomic and less error-prone Pointer.Load instead.
func LoadUint32 ¶
LoadUint32 atomically loads *addr. Consider using the more ergonomic and less error-prone Uint32.Load instead.
func LoadUint64 ¶
LoadUint64 atomically loads *addr. Consider using the more ergonomic and less error-prone Uint64.Load instead (particularly if you target 32-bit platforms; see the bugs section).
func LoadUintptr ¶
LoadUintptr atomically loads *addr. Consider using the more ergonomic and less error-prone Uintptr.Load instead.
func StoreInt32 ¶
StoreInt32 atomically stores val into *addr. Consider using the more ergonomic and less error-prone Int32.Store instead.
func StoreInt64 ¶
StoreInt64 atomically stores val into *addr. Consider using the more ergonomic and less error-prone Int64.Store instead (particularly if you target 32-bit platforms; see the bugs section).
func StorePointer ¶
StorePointer atomically stores val into *addr. Consider using the more ergonomic and less error-prone Pointer.Store instead.
func StoreUint32 ¶
StoreUint32 atomically stores val into *addr. Consider using the more ergonomic and less error-prone Uint32.Store instead.
func StoreUint64 ¶
StoreUint64 atomically stores val into *addr. Consider using the more ergonomic and less error-prone Uint64.Store instead (particularly if you target 32-bit platforms; see the bugs section).
func StoreUintptr ¶
StoreUintptr atomically stores val into *addr. Consider using the more ergonomic and less error-prone Uintptr.Store instead.
func SwapInt32 ¶ added in go1.2
SwapInt32 atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Int32.Swap instead.
func SwapInt64 ¶ added in go1.2
SwapInt64 atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Int64.Swap instead (particularly if you target 32-bit platforms; see the bugs section).
func SwapPointer ¶ added in go1.2
SwapPointer atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Pointer.Swap instead.
func SwapUint32 ¶ added in go1.2
SwapUint32 atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Uint32.Swap instead.
func SwapUint64 ¶ added in go1.2
SwapUint64 atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Uint64.Swap instead (particularly if you target 32-bit platforms; see the bugs section).
func SwapUintptr ¶ added in go1.2
SwapUintptr atomically stores new into *addr and returns the previous *addr value. Consider using the more ergonomic and less error-prone Uintptr.Swap instead.
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
CompareAndSwap executes the compare-and-swap operation for the boolean value x.
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) CompareAndSwap ¶ added in go1.19
CompareAndSwap executes the compare-and-swap operation for x.
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) CompareAndSwap ¶ added in go1.19
CompareAndSwap executes the compare-and-swap operation for x.
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
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.
type Uint32 ¶ added in go1.19
type Uint32 struct {
// contains filtered or unexported fields
}
A Uint32 is an atomic uint32. The zero value is zero.
func (*Uint32) CompareAndSwap ¶ added in go1.19
CompareAndSwap executes the compare-and-swap operation for x.
type Uint64 ¶ added in go1.19
type Uint64 struct {
// contains filtered or unexported fields
}
A Uint64 is an atomic uint64. The zero value is zero.
func (*Uint64) CompareAndSwap ¶ added in go1.19
CompareAndSwap executes the compare-and-swap operation for x.
type Uintptr ¶ added in go1.19
type Uintptr struct {
// contains filtered or unexported fields
}
A Uintptr is an atomic uintptr. The zero value is zero.
func (*Uintptr) CompareAndSwap ¶ added in go1.19
CompareAndSwap executes the compare-and-swap operation for x.
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
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
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
Store sets the value of the Value v to val. 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).
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.
Source Files