Support for 64-bit pointers

[Ty Young]

Agreed, Java really needs unsigned types. Besides this specific issue, 
there is also a major issue representing unsigned types in a high-level 
API such as the previous Pointer API or my current API. In order to 
represent a 1-byte unsigned number(byte) you would have to use a 2-byte 
signed number(short), which distorts APIs and creates confusion as an 
API user. If a function header says that you need an unsigned long then 
Java should be able to pass an actual unsigned long type to it.


On 1/1/21 12:42 PM, leerho wrote:
> Hi Radosław & Florian,
>
> On a related topic, I have wished for years that Java would finally support
> unsigned integral types (byte, short, int, and long).  I'm sure that I'm
> not the first to mention this, so forgive me if this has been hashed out
> before.  But I do not understand why there would be resistance to this.
> For example, packing and unpacking "C-struct" -like data structures is a
> PITA in Java,  Bytes and shorts have to be masked when upcasting to ints
> and ints have to be masked when upcasting to longs all because of the
> automatic sign-extension.   Not having unsigned types creates a ripe area
> for bugs that can be hard to find.
>
> Is there any hope of Java finally getting full support of unsigned types?
>
> Cheers, and Happy New Year,
>
> Lee.
>
>
>
> On Fri, Jan 1, 2021 at 9:49 AM Radosław Smogura <mail at smogura.eu> wrote:
>
>> Hi Florian,
>>
>> That’s correct, even more there’s only 48 bits addressable.
>>
>> I’m not sure what I was thinking - I was doing some performance checks and
>> was concerned that unrestricted access looked like doing range checks
>> and... well found this unrelated topic.
>>
>> However just to mention, some Linux distribution uses vsyscalls (which is
>> going to be deprecated) and this is mapped to the tail of memory ([1] nice
>> asm code)
>>
>> So even it’s not addressable it’s usable. I’m not sure if it’s worth
>> handling.
>>
>> Kind regards,
>> Rado
>>
>> [1]
>> https://stackoverflow.com/questions/7266813/how-does-the-gettimeofday-syscall-work
>>
>> On Jan 1, 2021, at 2:04 PM, Florian Weimer <fw at deneb.enyo.de> wrote:
>>
>> * Radosław Smogura:
>>
>> In current version of implementation (many places), there’s a lot of
>> range checks. However longs in Java are signed, and C pointers are
>> unsigned so, at least for x86-64 architectures this should be taken
>> on account, otherwise we would not be able to address whole memory
>> in straightforward way (largest block size is 2^31, can directly
>> address upper half of memory).
>>
>> The x86-64 architecture actually has signed addresses in the sense
>> that some number of upper bits of pointers must match the sign bit.
>> On Linux, userspace addresses always have a zero sign bit.  I think
>> this is not true on Solaris.  I don't know about Windows.
>>
>> On some architectures, C struggles with similar issues because
>> ptrdiff_t is signed and does not cover the entire address space.  It's
>> therefore undefined to create objects whose size is greater than what
>> can be expressed as a ptrdiff_t value, despite the underlying
>> architecture supporting this.
>>