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Source file src/sync/atomic/value.go

Documentation: sync/atomic

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package atomic
     6  
     7  import (
     8  	"unsafe"
     9  )
    10  
    11  // A Value provides an atomic load and store of a consistently typed value.
    12  // The zero value for a Value returns nil from Load.
    13  // Once Store has been called, a Value must not be copied.
    14  //
    15  // A Value must not be copied after first use.
    16  type Value struct {
    17  	v interface{}
    18  }
    19  
    20  // ifaceWords is interface{} internal representation.
    21  type ifaceWords struct {
    22  	typ  unsafe.Pointer
    23  	data unsafe.Pointer
    24  }
    25  
    26  // Load returns the value set by the most recent Store.
    27  // It returns nil if there has been no call to Store for this Value.
    28  func (v *Value) Load() (x interface{}) {
    29  	vp := (*ifaceWords)(unsafe.Pointer(v))
    30  	typ := LoadPointer(&vp.typ)
    31  	if typ == nil || uintptr(typ) == ^uintptr(0) {
    32  		// First store not yet completed.
    33  		return nil
    34  	}
    35  	data := LoadPointer(&vp.data)
    36  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    37  	xp.typ = typ
    38  	xp.data = data
    39  	return
    40  }
    41  
    42  // Store sets the value of the Value to x.
    43  // All calls to Store for a given Value must use values of the same concrete type.
    44  // Store of an inconsistent type panics, as does Store(nil).
    45  func (v *Value) Store(x interface{}) {
    46  	if x == nil {
    47  		panic("sync/atomic: store of nil value into Value")
    48  	}
    49  	vp := (*ifaceWords)(unsafe.Pointer(v))
    50  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    51  	for {
    52  		typ := LoadPointer(&vp.typ)
    53  		if typ == nil {
    54  			// Attempt to start first store.
    55  			// Disable preemption so that other goroutines can use
    56  			// active spin wait to wait for completion; and so that
    57  			// GC does not see the fake type accidentally.
    58  			runtime_procPin()
    59  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    60  				runtime_procUnpin()
    61  				continue
    62  			}
    63  			// Complete first store.
    64  			StorePointer(&vp.data, xp.data)
    65  			StorePointer(&vp.typ, xp.typ)
    66  			runtime_procUnpin()
    67  			return
    68  		}
    69  		if uintptr(typ) == ^uintptr(0) {
    70  			// First store in progress. Wait.
    71  			// Since we disable preemption around the first store,
    72  			// we can wait with active spinning.
    73  			continue
    74  		}
    75  		// First store completed. Check type and overwrite data.
    76  		if typ != xp.typ {
    77  			panic("sync/atomic: store of inconsistently typed value into Value")
    78  		}
    79  		StorePointer(&vp.data, xp.data)
    80  		return
    81  	}
    82  }
    83  
    84  // Disable/enable preemption, implemented in runtime.
    85  func runtime_procPin()
    86  func runtime_procUnpin()
    87  

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