I was referring to the “special cases” in this section
https://golang.org/cmd/cgo/ - wouldn’t all of these uintptr be stable?
> On Jun 4, 2021, at 12:25 AM, Ian Lance Taylor <i...@golang.org> wrote:
>
> On Thu, Jun 3, 2021 at 9:13 PM Robert Engels <reng...@ix.netcom.com> wrote:
>>
>> Doesn’t that depend on what the uintptr refers to? Eg if it was allocated
>> off heap to begin with then it should be stable and computing a hash on it
>> is valid.
>
> That is true in current implementations, but Go, the language, does
> not guarantee that pointers will never move.
>
> Ian
>
>
>> On Jun 3, 2021, at 9:42 AM, 'Axel Wagner' via golang-nuts
>> <golang-nuts@googlegroups.com> wrote:
>>
>>
>>> On Thu, Jun 3, 2021 at 4:19 PM christoph...@gmail.com
>>> <christophe.mees...@gmail.com> wrote:
>>>
>>> The documentation doesn’t specify that we can cast an uintptr into a uint64.
>>
>>
>> Both are integer types, so they can be converted. You might be worried about
>> their respective sizes and you are correct that the spec does not guarantee
>> that a uintptr is at most 64 bit. However, at least for any implementation I
>> know on any supported architecture, that's the case. If you *really* want to
>> make sure, you can do something like this:
>>
>> func toBytes(v uintptr) []byte {
>> var out []byte
>> mask := ^uintptr(0)
>> for mask != 0 {
>> out = append(out, uint8(v))
>> v >>= 8
>> mask >>= 8
>> }
>> return out
>> }
>>
>> But really, you can just trust that a uintptr fits into a uint64. Or, if you
>> want to future-proof, assume it's uint64, but guard by build tags to known
>> architectures (so it at least fails to compile if that assumption ever
>> changes).
>>
>>> To give some context, the application is for a cache with the hash key
>>> computed over a pointer to a heap allocated struct. I need to store the
>>> pointer in the cache entry to test for matching, but this would prevent the
>>> struct to be garbage collected. Storing the pointer as an uintptr would do
>>> the trick.
>>
>>
>> This is problematic. The address of a value can change, so the uintptr would
>> change as well. So there is no guarantee that your hash stays the same, in
>> subsequent calls.
>> Today, that usually doesn't happen, but there is no guarantee it stays that
>> way. If you are willing to assume that it doesn't happen, you should
>> definitely also be willing to assume a uintptr fits into a uint64
>>
>>>
>>>
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