Many thanks. This is quite informative.
On Wednesday, April 22, 2020 at 4:44:18 PM UTC-4, Ian Lance Taylor wrote:
>
> On Wed, Apr 22, 2020 at 7:08 AM T L <tapi...@gmail.com <javascript:>>
> wrote:
> >
> > I understand that, by the case 6 in the unsafe package docs,
> > the second program should be safe with or without the runtime.KeepAlive
> call.
> > I just wonder how Go runtime achieves this goal.
> > I have an impression that pointer assignments in Go by gc is atomically,
> > but is this also true for uintptr value assignments?
>
> Using the meaning of "atomic" from my previous message, then, yes,
> both pointer assignments and uintptr assignments are done in a way
> that can not be interrupted. That more or less happens automatically,
> as it's a single machine instruction either way. And in fact it's the
> same machine instruction, since pointers and uintptr are the same size
> and use the same registers.
>
> Ian
>
>
> > On Wednesday, April 22, 2020 at 7:33:22 AM UTC-4, T L wrote:
> >>
> >>
> >>
> >> 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> 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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>
>
>
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