Re: [go-nuts] Re: Question about safe unsafe.Pointer conversion rules
Many thanks Ian!
On Friday, June 19, 2020 at 11:32:19 PM UTC+3, Ian Lance Taylor wrote:
>
> On Fri, Jun 19, 2020 at 12:29 PM Iskander Sharipov > wrote:
> >
> > I used this to write something like a runtime in Go.
> >
> > https://play.golang.org/p/6jYw5L3fy0B
> >
> > I was wondering whether this code can break at some point.
>
> I don't see any way that that code could break, but I agree that it
> does not follow the rules, so it is not guaranteed to always work.
>
> Ian
>
>
>
> > On Friday, June 19, 2020 at 10:04:01 PM UTC+3, Iskander Sharipov wrote:
> >>
> >> Hello!
> >>
> >> https://golang.org/src/runtime/symtab.go?s=7594:7745#L244
> >>
> >> type Func struct {
> >>
> >> opaque struct{} // unexported field to disallow conversions
> >>
> >> }
> >>
> >>
> >> func (f *Func) raw() *_func {
> >>
> >> return (*_func)(unsafe.Pointer(f))
> >>
> >> }
> >>
> >>
> >> An empty struct Func (T1) is converted to a non-zero struct _func (T2).
> >>
> >> From the unsafe.Pointer documentation (
> https://golang.org/pkg/unsafe/#Pointer):
> >>
> >> > T1->T2 conversion is safe if T2 is no larger than T1 ... (shortened
> for clarity)
> >>
> >> I'm guessing that documentation probably meant "allocation size", not
> the "static type size"?
> >> T1 pointer can be obtained from the T2 allocation, so it's allocation
> size can be different from the unsafe.Sizeof(T1{}).
> >>
> >> Or it could be that runtime can violate that particular rule.
> >>
> >> My question: is it same to convert T1<->T2 even if
> sizeof(T1) >>
> >> Thank you in advance.
> >>
> >>
> >>
> > --
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>
>
>
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Re: [go-nuts] Re: Question about safe unsafe.Pointer conversion rules
On Fri, Jun 19, 2020 at 12:29 PM Iskander Sharipov wrote:
>
> I used this to write something like a runtime in Go.
>
> https://play.golang.org/p/6jYw5L3fy0B
>
> I was wondering whether this code can break at some point.
I don't see any way that that code could break, but I agree that it
does not follow the rules, so it is not guaranteed to always work.
Ian
> On Friday, June 19, 2020 at 10:04:01 PM UTC+3, Iskander Sharipov wrote:
>>
>> Hello!
>>
>> https://golang.org/src/runtime/symtab.go?s=7594:7745#L244
>>
>> type Func struct {
>>
>> opaque struct{} // unexported field to disallow conversions
>>
>> }
>>
>>
>> func (f *Func) raw() *_func {
>>
>> return (*_func)(unsafe.Pointer(f))
>>
>> }
>>
>>
>> An empty struct Func (T1) is converted to a non-zero struct _func (T2).
>>
>> From the unsafe.Pointer documentation
>> (https://golang.org/pkg/unsafe/#Pointer):
>>
>> > T1->T2 conversion is safe if T2 is no larger than T1 ... (shortened for
>> > clarity)
>>
>> I'm guessing that documentation probably meant "allocation size", not the
>> "static type size"?
>> T1 pointer can be obtained from the T2 allocation, so it's allocation size
>> can be different from the unsafe.Sizeof(T1{}).
>>
>> Or it could be that runtime can violate that particular rule.
>>
>> My question: is it same to convert T1<->T2 even if sizeof(T1)> but the memory was allocated for the bigger object?
>>
>> Thank you in advance.
>>
>>
>>
> --
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[go-nuts] Re: Question about safe unsafe.Pointer conversion rules
I used this to write something like a runtime in Go.
https://play.golang.org/p/6jYw5L3fy0B
I was wondering whether this code can break at some point.
On Friday, June 19, 2020 at 10:04:01 PM UTC+3, Iskander Sharipov wrote:
>
> Hello!
>
> https://golang.org/src/runtime/symtab.go?s=7594:7745#L244
>
> type Func struct {
>
> opaque struct{} // unexported field to disallow conversions
>
> }
>
>
> func (f *Func) raw() *_func {
>
> return (*_func)(unsafe.Pointer(f))
>
> }
>
>
> An empty struct Func (T1) is converted to a non-zero struct _func (T2).
>
> From the unsafe.Pointer documentation (
> https://golang.org/pkg/unsafe/#Pointer):
>
> > T1->T2 conversion is safe if T2 is no larger than T1 ... (shortened for
> clarity)
>
> I'm guessing that documentation probably meant "allocation size", not the
> "static type size"?
> T1 pointer can be obtained from the T2 allocation, so it's allocation size
> can be different from the unsafe.Sizeof(T1{}).
>
> Or it could be that runtime can violate that particular rule.
>
> *My question*: is it same to convert T1<->T2 even if
> sizeof(T1)
> Thank you in advance.
>
>
>
>
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[go-nuts] Re: Question about safe unsafe.Pointer conversion rules
s/same/safe/
The question is about whether it's safe to do the same conversion outside
of the Go runtime.
On Friday, June 19, 2020 at 10:04:01 PM UTC+3, Iskander Sharipov wrote:
>
> Hello!
>
> https://golang.org/src/runtime/symtab.go?s=7594:7745#L244
>
> type Func struct {
>
> opaque struct{} // unexported field to disallow conversions
>
> }
>
>
> func (f *Func) raw() *_func {
>
> return (*_func)(unsafe.Pointer(f))
>
> }
>
>
> An empty struct Func (T1) is converted to a non-zero struct _func (T2).
>
> From the unsafe.Pointer documentation (
> https://golang.org/pkg/unsafe/#Pointer):
>
> > T1->T2 conversion is safe if T2 is no larger than T1 ... (shortened for
> clarity)
>
> I'm guessing that documentation probably meant "allocation size", not the
> "static type size"?
> T1 pointer can be obtained from the T2 allocation, so it's allocation size
> can be different from the unsafe.Sizeof(T1{}).
>
> Or it could be that runtime can violate that particular rule.
>
> *My question*: is it same to convert T1<->T2 even if
> sizeof(T1)
> Thank you in advance.
>
>
>
>
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