https://play.golang.org/p/6aQYNjojyBD
I'm clearly missing something about the way sync.Mutex and atomic.Value
work in Go.
I'm attempting to write a pair of concurrency safe methods, load() and
update(), for accessing a struct. The struct is stored as an atomic.Value
and accessed with atomic.Load and atomic.Value. I'm also wrapping the
accesses within a mutex Lock/Unlock. That's probably unneeded for my
load() method but I added it trying to figure out why it's not returning
updated info. In my minimal example below (also in the Go Playground link
above) the output of main() should be:
s={65535}
v={65535}
but I get
s={65535}
v={0}
indicating that the updated value is not available after the call to
update().
The only thing I'm doing that's a little different from the examples in the
doc for sync/atomic is passing a function that takes a pointer to a struct
instance to my update function. I do that to make it easy to write code
that updates just a few items in the state struct (which in my real
application has many members instead of just one as shown here.)
Apologies for wasting the group's time if I've overlooked a brain-dead
error, but I've been fooling with this for several hours now and can't see
why it shouldn't be working,
package main
import (
"fmt"
"sync"
"sync/atomic"
)
type state struct {
R4000 uint16
}
type guardedState struct {
v atomic.Value
}
var stateMutex = sync.Mutex{}
var gState guardedState
func init() {
gState.v.Store(state{})
}
func (g guardedState) load() state {
stateMutex.Lock()
defer stateMutex.Unlock()
s := gState.v.Load()
return s.(state)
}
func (g guardedState) update(f func(*state)) {
stateMutex.Lock()
defer stateMutex.Unlock()
s := g.v.Load().(state)
f(&s)
g.v.Store(s)
}
func main() {
f := func(s *state) {
s.R4000 = 65535
fmt.Printf("s=%v\n", *s)
}
gState.update(f)
v := gState.load()
fmt.Printf("v=%v\n", v)
}
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