wasmtime ruby memory by pointer · wasmtime

Can you access the WASM memory by pointer instead of just offset/length via memory.read/write in Ruby?

Alex Crichton (Nov 28 2023 at 17:07):

Noah Everett (Nov 28 2023 at 17:22):

Thanks @Alex Crichton ! The WASM code is allocating a buffer which returns a pointer back to the host to write to:

fun allocateBuffer(size : Int32) : Pointer(UInt8)
  ptr = Pointer(UInt8).malloc(size)
end

    bufPtr = instance.exports.allocateBuffer.(100)
    $memory = instance.export("memory")
    view = $memory.uint8_view bufPtr

But maybe there is a better way to do this using Wasmtime's read/write methods, just not for sure how to access the "memory" in my WASM code which is written in Crystal lang which is why the previous code was using pointers.

Alex Crichton (Nov 28 2023 at 17:24):

oh sorry I know very little about Ruby bindings myself, but I think Saul should know more

Noah Everett (Nov 28 2023 at 17:25):

No prob! just putting the relevant code here just in case, I appreciate the cc :)

Noah Everett (Nov 28 2023 at 20:00):

Hmm so I found "to_memory_view" but unsure how to use it in this context. Basically trying to figure out how to pass string data between the host and guest using an allocated buffer and pointer

Saúl Cabrera (Nov 28 2023 at 20:24):

Hi @Noah Everett -- if I'm understanding your question correctly, I think that you could achieve what you want by doing something like:

ptr  = instance.invoke("allocateBuffer",  100)
memory = instance.exports["memory"].to_memory
memory.write(ptr, <data>)
# use memory.read if needed

That said -- the bindings don't currently expose an API analogous to Memory::data_ptrhttps://docs.wasmtime.dev/api/wasmtime/struct.Memory.html#method.data_ptr, which could be another avenue to getting a mutable pointer to the memory

Noah Everett (Nov 28 2023 at 20:33):

@Saul Shanabrook oh wow I didn't know it could take a pointer (feel a little stupid now)! I just assumed it was just writing at the byte offset...trying it now, thank you!

Noah Everett (Nov 28 2023 at 20:37):

@Saúl Cabrera yup it works, I was making that way more complicated than it had to be, my fault...I really appreciate your help on the func_new and now this...cheers!

Stream: wasmtime

Topic: wasmtime ruby memory by pointer

Noah Everett (Nov 28 2023 at 16:52):