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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