Accessing global arrays in the guest from host, when embedded?

[wasmerio]

Accessing global arrays in the guest from host, when embedded?

[bjfish]

@lachlansneff I think we are missing Export from the public runtime API, can you confirm?

@hrydgard I modified your example to inspect the global value:

Export::Global(global) => {
   println!("  global: {}, {:?}", name, global);
}

which resulting in the following:

global: SCRATCH_I, Global { desc: GlobalDescriptor { mutable: false, ty: I32 }, value: I32(1053432) }

I suspect this value 1053432 to be an index into the memory, maybe some space setup for statics like this. So, I setup the array to contain some values to confirm this suspicion:

#[no_mangle]
pub static mut SCRATCH_I : [u8; 5] = [1, 2, 3, 4, 5];

and then print this part of the memory:

let i = 1053432;
let memory = instance.context().memory(0);
for b in memory.view::<u8>()[i as usize..(i + 5) as usize].iter() {
    println!("{:?}", b);
}

Results in:

Cell { value: 1 }
Cell { value: 2 }
Cell { value: 3 }
Cell { value: 4 }
Cell { value: 5 }

So, it appears that the SCRATCH_I global contains an index into the memory to the first value in the array.

[hrydgard]

Ah, thanks! Good that it's there. Could be clearer in the documentation though...

GlobalDescriptor is even entirely missing from the docs due to a missing /// comment line.

Also it could be nice to add this stuff to the official example :) I'm sure other people will want to do similar things.

[bjfish]

@hrydgard I've added GlobalDescriptor to runtime with documentation and added Export as well in this PR: #320 Yes, we'll be adding more examples over time.

I'll close this issue as I think it is resolved now. Let us know if you have any more questions, thanks.

[hrydgard]

@bjfish I still believe you're missing documentation that you find the address of global arrays in the Value::I32, which I find somewhat unintuitive.

Apart from that, all good!