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?
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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