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".
func AddInt32(addr *int32, delta int32) (new int32)
AddInt32 atomically adds delta to *addr and returns the new value.
func AddInt64(addr *int64, delta int64) (new int64)
AddInt64 atomically adds delta to *addr and returns the new value.
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(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(addr *uintptr, delta uintptr) (new uintptr)
AddUintptr atomically adds delta to *addr and returns the new value.
func CompareAndSwapInt32(addr *int32, old, new int32) (swapped bool)
CompareAndSwapInt32 executes the compare-and-swap operation for an int32 value.
func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool)
CompareAndSwapInt64 executes the compare-and-swap operation for an int64 value.
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(addr *uint32, old, new uint32) (swapped bool)
CompareAndSwapUint32 executes the compare-and-swap operation for a uint32 value.
func CompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
CompareAndSwapUint64 executes the compare-and-swap operation for a uint64 value.
func CompareAndSwapUintptr(addr *uintptr, old, new uintptr) (swapped bool)
CompareAndSwapUintptr executes the compare-and-swap operation for a uintptr value.
func LoadInt32(addr *int32) (val int32)
LoadInt32 atomically loads *addr.
func LoadInt64(addr *int64) (val int64)
LoadInt64 atomically loads *addr.
func LoadPointer(addr *unsafe.Pointer) (val unsafe.Pointer)
LoadPointer atomically loads *addr.
func LoadUint32(addr *uint32) (val uint32)
LoadUint32 atomically loads *addr.
func LoadUint64(addr *uint64) (val uint64)
LoadUint64 atomically loads *addr.
func LoadUintptr(addr *uintptr) (val uintptr)
LoadUintptr atomically loads *addr.
func StoreInt32(addr *int32, val int32)
StoreInt32 atomically stores val into *addr.
func StoreInt64(addr *int64, val int64)
StoreInt64 atomically stores val into *addr.
func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)
StorePointer atomically stores val into *addr.
func StoreUint32(addr *uint32, val uint32)
StoreUint32 atomically stores val into *addr.
func StoreUint64(addr *uint64, val uint64)
StoreUint64 atomically stores val into *addr.
func StoreUintptr(addr *uintptr, val uintptr)
StoreUintptr atomically stores val into *addr.
func SwapInt32(addr *int32, new int32) (old int32)
SwapInt32 atomically stores new into *addr and returns the previous *addr value.
func SwapInt64(addr *int64, new int64) (old int64)
SwapInt64 atomically stores new into *addr and returns the previous *addr value.
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(addr *uint32, new uint32) (old uint32)
SwapUint32 atomically stores new into *addr and returns the previous *addr value.
func SwapUint64(addr *uint64, new uint64) (old uint64)
SwapUint64 atomically stores new into *addr and returns the previous *addr value.
func SwapUintptr(addr *uintptr, new uintptr) (old uintptr)
SwapUintptr atomically stores new into *addr and returns the previous *addr value.
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.
type Value struct {
// contains filtered or unexported fields
}
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
}
}()
}
}
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
}
func (v *Value) Load() (x interface{}) 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 (v *Value) Store(x interface{}) 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).
On x86-32, 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, x86-32, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned.
© Google, Inc.
Licensed under the Creative Commons Attribution License 3.0.
https://golang.org/pkg/sync/atomic/