Jean Mertz (Apr 29 2020 at 22:08): Jean Mertz (Apr 29 2020 at 22:08):I'm trying to understand how I can direct the host to some piece of memory the guest has written some data to (both are implemented in Rust). I understand that a guest can't have direct access to the host memory, and all it knows is the offset of the memory it is assigned, but how can I go from doing something like Vec::with_capacity(10), to having an offset integer that I can pass to the host for it to use in a memory.data_unchecked()[offset + len] call? Basically, I know how to pass len (10), but I don't know how to get the offset u32 after allocating the memory in the guest.
Yury Delendik (Apr 29 2020 at 22:12):(assuming both sides are written in rust) was Vec::with_capacity(10) run on the wasm/guest side? if yes, then wasm's v.as_ptr() shall give you offset for host
Yury Delendik (Apr 29 2020 at 22:22):wasm pointer is an int32 and direct offset in host's allocated memory
Jean Mertz (Apr 29 2020 at 22:31):Ah interesting. Thanks @Yury Delendik. So something like this should work (some psuedo code): https://gist.github.com/rust-play/e790f19505cc19d2b1330f68f2e7321c?
Yury Delendik (Apr 29 2020 at 22:33):not so fast :) I bet the CString::new("foo") will be destroyed after run call
Yury Delendik (Apr 29 2020 at 22:34):but yeah, you'll get the offset
Jean Mertz (Apr 29 2020 at 22:39):Hmm, yeah, even with mem::forget I still get a NulError because I didn't pass the length. And since I can only return a single value from the guest function, I guess that means the right approach is for the guest to call a host function that takes the offset and length as an argument, right?
Yury Delendik (Apr 29 2020 at 22:39):there is also mem.data_ptr() which might be more intuitive in your case
Jean Mertz (Apr 29 2020 at 22:50):Thanks @Yury Delendik. One more question: how can I add the offset to the ptr from mem.data_ptr() to get the memory location of the string written by the guest? It seems data_ptr().add(offset as usize) won't work, as I get an empty string when I print the cstr from that pointer.
Yury Delendik (Apr 29 2020 at 22:54):.add() is right, so is .offset(). I'm skeptical about guest's code. Try: { static HELLO: &[u8] = b"Hello, world!\x00"; HELLO.as_ptr() as i32 }
Yury Delendik (Apr 29 2020 at 22:58):but normally, you would write something like fn run() -> *const u8 {in the function signature -- you don't have to cast that to i32
Jean Mertz (Apr 29 2020 at 22:58):Yep, that worked! I wonder why mem::forget doesn't work in this case though.
Yury Delendik (Apr 29 2020 at 22:59):yep, be careful, rust can trick you by moving and dropping your stuff
Jean Mertz (Apr 29 2020 at 23:00):Returning *const u8 won't work in this case, will it? Since WasmTy isn't implemented for that type?
Yury Delendik (Apr 29 2020 at 23:01):on host side... for guest side it is fine
Jean Mertz (Apr 29 2020 at 23:03):Aha, I see. Thanks again @Yury Delendik, I'm off to playing some more with this. It's definitely a lot of fun to work with.
Jean Mertz (Apr 30 2020 at 07:34):Quick update for anyone who reads this in the future, mem::forget does work (and so does ManuallyDrop), I was erroneously dropping the ptr instead of the value itself.
Last updated: Dec 06 2025 at 06:05 UTC