code review 7583043: spec: clarify unsafe.Offsetof

spec: clarify unsafe.Offsetof

Fixes issue 4905.

[gri, 2013-03-07 18:42:46]

Hello rsc@golang.org, r@golang.org, iant@golang.org, ken@golang.org (cc:
golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go

[r, 2013-03-07 18:51:36]

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5737
doc/go_spec.html:5737: <code>s.f</code> denoting a struct field <code>f</code>
of any type and returns the field offset in
<code>s.f</code> denoting a field <code>f</code>, of struct value <code>s</code>
and of any type, and returns...

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5740
doc/go_spec.html:5740: it must not be embedded via a pointer indirection.
so i can't find the offset of the pointer?

[gri, 2013-03-07 19:11:55]

PTAL

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5737
doc/go_spec.html:5737: <code>s.f</code> denoting a struct field <code>f</code>
of any type and returns the field offset in
On 2013/03/07 18:51:36, r wrote:
> <code>s.f</code> denoting a field <code>f</code>, of struct value
<code>s</code>
> and of any type, and returns...

reformulated. note that s may be a struct _or_ a pointer to a struct (like for
any selector)

https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5740
doc/go_spec.html:5740: it must not be embedded via a pointer indirection.
On 2013/03/07 18:51:36, r wrote:
> so i can't find the offset of the pointer?

You can. But if the field is embedded _via_ a pointer Offsetof must fail:

type T0 struct {
   f int
}

type T1 struct {
   a, b, c int
   *T0
}

var s T1

unsafe.Offsetof(s.f) makes no sense; also the invariant stated below fails, of
course.

gc (unless fixed) returns the offset of f relative to T0 in this case, which
would be 0.

[gri, 2013-03-08 00:24:32]

ping


On Thu, Mar 7, 2013 at 11:11 AM, <gri@golang.org> wrote:

> PTAL
>
>
>
>
https://codereview.appspot.**com/7583043/diff/5001/doc/go_**spec.html<https:/...
> File doc/go_spec.html (right):
>
> https://codereview.appspot.**com/7583043/diff/5001/doc/go_**
>
spec.html#newcode5737<https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5737>
> doc/go_spec.html:5737: <code>s.f</code> denoting a struct field
> <code>f</code> of any type and returns the field offset in
> On 2013/03/07 18:51:36, r wrote:
>
>> <code>s.f</code> denoting a field <code>f</code>, of struct value
>>
> <code>s</code>
>
>> and of any type, and returns...
>>
>
> reformulated. note that s may be a struct _or_ a pointer to a struct
> (like for any selector)
>
>
> https://codereview.appspot.**com/7583043/diff/5001/doc/go_**
>
spec.html#newcode5740<https://codereview.appspot.com/7583043/diff/5001/doc/go_spec.html#newcode5740>
> doc/go_spec.html:5740: it must not be embedded via a pointer
> indirection.
> On 2013/03/07 18:51:36, r wrote:
>
>> so i can't find the offset of the pointer?
>>
>
> You can. But if the field is embedded _via_ a pointer Offsetof must
> fail:
>
> type T0 struct {
>    f int
> }
>
> type T1 struct {
>    a, b, c int
>    *T0
> }
>
> var s T1
>
> unsafe.Offsetof(s.f) makes no sense; also the invariant stated below
> fails, of course.
>
> gc (unless fixed) returns the offset of f relative to T0 in this case,
> which would be 0.
>
>
https://codereview.appspot.**com/7583043/<https://codereview.appspot.com/7583...
>

[r, 2013-03-08 00:44:40]

https://codereview.appspot.com/7583043/diff/10001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/7583043/diff/10001/doc/go_spec.html#newcode5740
doc/go_spec.html:5740: without pointer indirections.
but
var s *T
Offsetof(s.f)
requires indirecting through s.
still not quite right.

maybe 
without pointer indirections through fields of the struct.

[gri, 2013-03-08 00:47:33]

PTAL

https://codereview.appspot.com/7583043/diff/10001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/7583043/diff/10001/doc/go_spec.html#newcode5740
doc/go_spec.html:5740: without pointer indirections.
On 2013/03/08 00:44:41, r wrote:
> but
> var s *T
> Offsetof(s.f)
> requires indirecting through s.
> still not quite right.
> 
> maybe 
> without pointer indirections through fields of the struct.

Done.

[r, 2013-03-08 00:49:14]

LGTM

[iant, 2013-03-08 00:59:31]

LGTM

[rsc, 2013-03-08 04:05:56]

LGTM

[gri, 2013-03-08 04:12:03]

*** Submitted as https://code.google.com/p/go/source/detail?r=ed48ace1855b ***

spec: clarify unsafe.Offsetof

Fixes issue 4905.

R=rsc, r, iant, ken
CC=golang-dev
https://codereview.appspot.com/7583043