On Tuesday, April 21, 2020 at 8:13:17 PM UTC-4, Ian Lance Taylor wrote:
>
> On Tue, Apr 21, 2020 at 8:17 AM T L <tapi...@gmail.com <javascript:>>
> wrote:
> >
> > And is the runtime.KeepAlive call in the following program essential to
> keep the correctness?
> >
> > package main
> >
> > import (
> > "fmt"
> > "unsafe"
> > "reflect"
> > "runtime"
> > )
> >
> > func main() {
> > a := [6]byte{'G', 'o', 'o', 'g', 'l', 'e'}
> > bs := []byte("Golang")
> > hdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
> > hdr.Data = uintptr(unsafe.Pointer(&a))
> >
> > runtime.KeepAlive(&a) // Is this line essential here?
> >
> > hdr.Len = 2
> > hdr.Cap = len(a)
> > fmt.Printf("%s\n", bs) // Go
> > bs = bs[:cap(bs)]
> > fmt.Printf("%s\n", bs) // Google
> > }
>
> I do not think the runtime.KeepAlive is required. Setting the Data
> field using a *SliceHeader should keep the value alive.
>
> Ian
>
Many thanks for the answers.
I have another question, which doesn't matter with or without the
runtime.KeepAlive call.
We know that assignment in Go is not atomic.
Is it possible that when the assignment "hdr.Data =
uintptr(unsafe.Pointer(&a))" is half done,
the stack is resized, so the address of a might be changed (assume a is
allocated on stack),
the hdr.Data will be assigned the old invalid value?
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