Question about safe unsafe.Pointer conversion rules
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Iskander Sharipov
Jun 19, 2020, 3:04:01 PM6/19/20
to golang-nuts
Hello!
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)<sizeof(T2), but the memory was allocated for the bigger object?
Thank you in advance.
Iskander Sharipov
Jun 19, 2020, 3:08:01 PM6/19/20
to golang-nuts
s/same/safe/
The question is about whether it's safe to do the same conversion outside of the Go runtime.
The question is about whether it's safe to do the same conversion outside of the Go runtime.
Ian Lance Taylor
Jun 19, 2020, 3:13:49 PM6/19/20
to Iskander Sharipov, golang-nuts
the rules. In this case it is safe because Func values are never
allocated and are never garbage collected. They are always part of
the read-only section of the program itself.
Ian
Iskander Sharipov
Jun 19, 2020, 3:29:07 PM6/19/20
to golang-nuts
I used this to write something like a runtime in Go.
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:
Ian Lance Taylor
Jun 19, 2020, 4:32:19 PM6/19/20
to Iskander Sharipov, golang-nuts
On Fri, Jun 19, 2020 at 12:29 PM Iskander Sharipov <quas...@gmail.com> 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
>
> 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.
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)<sizeof(T2), but the memory was allocated for the bigger object?
>>
>> Thank you in advance.
>>
>>
>>
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Iskander Sharipov
Jun 19, 2020, 6:46:49 PM6/19/20
to golang-nuts
Many thanks Ian!
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