Re: [go-nuts] Is the following funciton always safe to convert a string to a slice?

[T L]

For the first example, I always thought the "runtime.KeepAlive" call is 
essential before.
But after reading the explanations from Bryan C. Mills on Slack, I changed 
my mind now.

Bryan C. Mills said: The GC tracks pointers through their original 
allocations — the pointer scan does not care about lexical types, only 
allocation types.

On Tuesday, April 21, 2020 at 8:11:49 PM UTC-4, Ian Lance Taylor wrote:
>
> On Tue, Apr 21, 2020 at 6:49 AM T L > 
> wrote: 
> > 
> > func String2ByteSlice(str string) (bs []byte) { 
> > strHdr := (*reflect.StringHeader)(unsafe.Pointer()) 
> > sliceHdr := (*reflect.SliceHeader)(unsafe.Pointer()) 
> > 
> > // Is it possible that the str value is allocated on stack 
> > // and the stack grows at this moment, so the address value 
> > // stored in strHdr.Data will become obsoleted? 
> > 
> > sliceHdr.Data = strHdr.Data 
> > sliceHdr.Len = strHdr.Len 
> > sliceHdr.Cap = strHdr.Len 
> > 
> > runtime.KeepAlive() 
> > 
> > return 
> > } 
>
> Since you are correctly using *reflect.StringHeader, it doesn't matter 
> whether the stack is copied at that moment.  You are pointing at the 
> actual string value, so the pointer will be adjusted.  As far as I can 
> see, this is safe.  (Of course, it's not safe if somebody modifies the 
> contents of the returned []byte.) 
>
> Ian 
>

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Re: [go-nuts] Is the following funciton always safe to convert a string to a slice?

[Ian Lance Taylor]

On Tue, Apr 21, 2020 at 6:49 AM T L  wrote:
>
> func String2ByteSlice(str string) (bs []byte) {
> strHdr := (*reflect.StringHeader)(unsafe.Pointer())
> sliceHdr := (*reflect.SliceHeader)(unsafe.Pointer())
>
> // Is it possible that the str value is allocated on stack
> // and the stack grows at this moment, so the address value
> // stored in strHdr.Data will become obsoleted?
>
> sliceHdr.Data = strHdr.Data
> sliceHdr.Len = strHdr.Len
> sliceHdr.Cap = strHdr.Len
>
> runtime.KeepAlive()
>
> return
> }

Since you are correctly using *reflect.StringHeader, it doesn't matter
whether the stack is copied at that moment.  You are pointing at the
actual string value, so the pointer will be adjusted.  As far as I can
see, this is safe.  (Of course, it's not safe if somebody modifies the
contents of the returned []byte.)

Ian

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[go-nuts] Is the following funciton always safe to convert a string to a slice?

[T L]

func String2ByteSlice(str string) (bs []byte) {
strHdr := (*reflect.StringHeader)(unsafe.Pointer())
sliceHdr := (*reflect.SliceHeader)(unsafe.Pointer())

// Is it possible that the str value is allocated on stack
// and the stack grows at this moment, so the address value
// stored in strHdr.Data will become obsoleted?

sliceHdr.Data = strHdr.Data
sliceHdr.Len = strHdr.Len
sliceHdr.Cap = strHdr.Len

runtime.KeepAlive()

return
}

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