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Source file src/sync/atomic/value.go

Documentation: sync/atomic

  // Copyright 2014 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package atomic
  
  import (
  	"unsafe"
  )
  
  // A Value provides an atomic load and store of a consistently typed value.
  // Values can be created as part of other data structures.
  // 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 {
  	noCopy noCopy
  
  	v interface{}
  }
  
  // ifaceWords is interface{} internal representation.
  type ifaceWords struct {
  	typ  unsafe.Pointer
  	data unsafe.Pointer
  }
  
  // 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) Load() (x interface{}) {
  	vp := (*ifaceWords)(unsafe.Pointer(v))
  	typ := LoadPointer(&vp.typ)
  	if typ == nil || uintptr(typ) == ^uintptr(0) {
  		// First store not yet completed.
  		return nil
  	}
  	data := LoadPointer(&vp.data)
  	xp := (*ifaceWords)(unsafe.Pointer(&x))
  	xp.typ = typ
  	xp.data = data
  	return
  }
  
  // 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 (v *Value) Store(x interface{}) {
  	if x == nil {
  		panic("sync/atomic: store of nil value into Value")
  	}
  	vp := (*ifaceWords)(unsafe.Pointer(v))
  	xp := (*ifaceWords)(unsafe.Pointer(&x))
  	for {
  		typ := LoadPointer(&vp.typ)
  		if typ == nil {
  			// Attempt to start first store.
  			// Disable preemption so that other goroutines can use
  			// active spin wait to wait for completion; and so that
  			// GC does not see the fake type accidentally.
  			runtime_procPin()
  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
  				runtime_procUnpin()
  				continue
  			}
  			// Complete first store.
  			StorePointer(&vp.data, xp.data)
  			StorePointer(&vp.typ, xp.typ)
  			runtime_procUnpin()
  			return
  		}
  		if uintptr(typ) == ^uintptr(0) {
  			// First store in progress. Wait.
  			// Since we disable preemption around the first store,
  			// we can wait with active spinning.
  			continue
  		}
  		// First store completed. Check type and overwrite data.
  		if typ != xp.typ {
  			panic("sync/atomic: store of inconsistently typed value into Value")
  		}
  		StorePointer(&vp.data, xp.data)
  		return
  	}
  }
  
  // Disable/enable preemption, implemented in runtime.
  func runtime_procPin()
  func runtime_procUnpin()
  
  // noCopy may be embedded into structs which must not be copied
  // after the first use.
  //
  // See https://github.com/golang/go/issues/8005#issuecomment-190753527
  // for details.
  type noCopy struct{}
  
  // Lock is a no-op used by -copylocks checker from `go vet`.
  func (*noCopy) Lock() {}
  

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