Struct with Dynamic Layout

[Maurizio Cimadamore]

On 08/08/2020 05:08, Ardika Rommy Sanjaya wrote:
> In native c/c++ size of long is depends on the platform, ex "struct
> timeval" in Unix and Windows has a different size.
>
> // for unix
> @NativeStruct("[i64(tv_sec)i32(tv_usec)x32](timeval)")
>
> // for windows
> @NativeStruct("[i32(tv_sec)i32(tv_usec)](timeval)")
>
> Is it possible to create a new dynamic layout where I can use a single
> struct class with different sizes on a different platform?

I see that you are referring to NativeStruct annotations, which were 
part of the older annotation-driven Panama story.

You can find the latest documents explaining the current state of 
foreign interop here:

http://cr.openjdk.java.net/~mcimadamore/panama/foreign-memaccess.html

and

http://cr.openjdk.java.net/~mcimadamore/panama/ffi.html

To describe layouts we now use layout objects, with an API. The FFI 
support introduces a set of (basic, for now) platform-dependent layouts 
to work with C:

CSupport.C_INT, CSupport.C_DOUBLE

You can create a complex struct layouts where the leaf value layouts are 
those defined in CSupport; that might work.

That said, note that, as your example demonstrates, it's not just a 
matter of swapping 32 for 64 - in most cases padding and sizes are affected.

So I think the most robust way to do things would be to replicate what 
CSupport does:

static final MemoryLayout TIMEVAL =
                       switch (CSupport.getSystemLinker().name()) {
                             CSupport.Win64.NAME -> // win64 layout
                             CSupport.SysV.NAME -> // linux layout
                             CSupport.AArch64.NAME -> // aarch layout
                        };


Hope this helps

Maurizio

>
> Thanks,
> Rommy