On Friday, July 7, 2017 at 8:42:28 PM UTC+8, Marvin Renich wrote:
>
> * T L <tapi...@gmail.com <javascript:>> [170707 01:32]:
> > Then how about this?
> >
> > package main
> >
> > import "fmt"
> > import "sync/atomic"
> >
> > func main() {
> > var x, y int32
> > go func() {
> > atomic.AddInt32(&x, 1)
> > atomic.AddInt32(&y, 1)
> > }()
> >
> > if atomic.LoadInt32(&y) == 1 {
> > fmt.Println("x =", atomic.LoadInt32(&x)) // always 1 if it is
> printed?
> > }
>
> Yes, the go routine establishes a happens-before relationship such that
> x is incremented before y, and the atomic Add of y and Load of y ensure
> that the Load either happens before or after, but not during, the Add.
>
But I remember that the "happens-before relationship" is never written down
in any official Go documentation.
>
> > if atomic.LoadInt32(&x) == 0 {
> > fmt.Println("y =", atomic.LoadInt32(&y)) // always 0 if it is
> printed?
> > }
>
No. If the Load of x returns 0, it must happen before the Add of 1 to x.
> But after the test and before the Load of y, the main go routine and the
>
func go routine are both runnable and there is nothing to establish a
> happens-before relationship between the Load of y and the Add of 1 to y.
>
> > }
>
> ...Marvin
>
>
Great explanation.
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