wasmtime | A fast and secure runtime for WebAssembly | Binary Executable Format library
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QUESTION
Hey @all I was playing with WebAssembly Studio and created an empty Rust project.
In my Rust code, I'm returning the pointer and the length for my "Hello World" string. Compiling worked fine, but I was expecting the resulting WASM to have a function returning two i32. But I found a function taking one i32 and returning nothing.
Why is the function not having the signature fn () -> (i32,i32) ?
How am I supposed to extract the two values from the WASM module? (Using Rust-wasmtime)
Below you can find the code snippets I'm talking about. Thanks in advance!
...ANSWER
Answered 2022-Jan-18 at 09:48WebAssembly got the ability to return multiple values just recently. The compiler used doesn't seem to support this yet or doesn't use it for compatibility reasons, i.e. for runtimes that don't know this feature yet.
Therefore, the compiler rewrites the code as follows:
QUESTION
Let me explain a little bit about what I'm doing. I have an application that I'm embedding a wasm engine (specifically wasmtime) and writing the wasm in Rust. The target is not a browser so I'm not using wasm-bindgen. I'm trying to import two functions from the host environment. I've gotten it to work but with a bit of a hack and I'd like to remove the hack. Here's the problem I'm having. I've been able to figure out how to get Rust to generate the import statements in the wasm output. The problem is that if those functions aren't used the import statements in the wasm aren't included. Wasmtime seem to require that the number of imports passed to the wasm instance match the number of import statements. Makes sense that if the wasm module is expecting imports it's a problem if you don't provide them. The problem is, it's also an error if you provide imports and they're not used.
So I see two ways to fix that, either I need to figure out how to get Rust to include the import statements even if they're not used (right now the hack is to call the functions but ignore the results and it's basically a no-op) or to find a way to introspect the wasm an figure out what kind of imports it's expecting and only pass what it's asking for.
My question is is that the correct approach and if so how do you do that?
...ANSWER
Answered 2021-Oct-27 at 06:30You can convert between .wasm
(binary format) and .wat
(textual format) with the The WebAssembly Binary Toolkit. It is open source. Maybe it can help you to extract the needed imports programmatically.
The proper way, should be that your host environment gives you the import requirements before you initialize the instance. The Module gives you the imports. See the "Instance variables". You get that in between the module loading and the creation of the instance that itself needs these imports.
QUESTION
Given I want to sum the first n terms of a series 1,2,3,.. with the following function in Rust
...ANSWER
Answered 2021-Sep-03 at 20:35It seems to be because wasm does not support native u64
as a type, only the signed variants (notably, i64
), which is why it's using i64
as the type for the arithmetic operations. Since this then overflows a 64-bit integer (the correct output is n * (n+1) / 2
, or 50000000005000000000
, you're getting a negative value due to the overflow, which is then getting printed to console. This is due to a lack of type support in wasm.
Just for reference, a Σ n=0 to N := (N * (N+1) / 2
, which I use from here on out since it's much faster computationally, and correct for our purposes.
The result, 50000000005000000000
, takes ~65.4 bits in memory to accurately represent in memory, which is why you get wrapping behavior for x86_64 and wasm, just the types it wraps to are different.
Using NumPy, we can clearly confirm this:
QUESTION
I've been handwriting some WebAssembly modules to learn how they work. Right now I'm trying to understand how imports and exports work, and how to perform IO.
The module is as follows:
...ANSWER
Answered 2021-Sep-01 at 13:15At least in Javascript you can do something like
QUESTION
part of the talk of Lin Clark in https://hacks.mozilla.org/2019/03/standardizing-wasi-a-webassembly-system-interface/:
It also gives us sandboxing because the host can choose which wasi-core functions to pass in — so, which system calls to allow — on a program-by-program basis. This preserves security.
she says host can choose which system calls pass to each wasm
module. for example read()
system call passes to module A
and write()
system call to module B
.
is it implemented in wasmtime
or lucet
or other runtimes? or is it just a dream without implementation in real world?
ANSWER
Answered 2021-Apr-08 at 06:42Yes it is implemented in all runtimes implementing wasi
. The reason is that this feature is related to import/export mechanism of WebAssembly.
QUESTION
I have been trying to pass a string to a Rust function (compiled to Wasm), however for what I understood, right now there is no way to pass strings directy, because the "str" is not a type in the "FFI world" (at least that's what the rust compiler is saying):
= help: consider using `*const u8` and a length instead
So what I did was changing the function to this form (instead of using a simple &str type):
...ANSWER
Answered 2021-Jan-20 at 11:00Took me a little to understand exactly how the wasmtime Go-package worked, but in the end I resolved my problem doing this:
QUESTION
I would like to create a callback function that has access to the argument instanceContext
, but I am not sure how to fix the code.
The failing code is instanceContext.instances.len()
.
I have tried using lifetimes, move, Rc, Arc, Box, RefCell, but nothing seems to work.
The goal is to have multiple Instances which will have callbacks that will make it possible for one Instance to call/modify another Instance, through the InstanceContext.
During my experimentation I've been getting the following errors:
- may outlive borrowed value
- borrowed value does not live long enough
- closure is
FnOnce
because it moves the variable - this closure implements
FnOnce
, notFn
ANSWER
Answered 2020-Nov-27 at 18:32You don't provide the compiler error message or an actual reproduction case so it's hard to know whether the diagnostic achieved by just thinking about things is the correct one, but to me it looks like this would be the solution:
QUESTION
I'm trying to call a WASI function (fd_write) directly in a C program using wasi-sdk. This is the the library (lib.c):
...ANSWER
Answered 2020-Jul-25 at 07:33Found the answer in wasi-libc source code:
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