tessi
commented
3 years ago Hey @sleipnir thanks for asking so nicely 💚
Passing and receiving memory really isn't as easy as I wish it was. So, sadly, I don't have an easy solution for you, but a lengthy post. Hope this still helps (and if helpful, could be something I point others to who have the same question). The good news is, that there is a silver lining (see the footnote).
Handling strings is not easy because WebAssembly does not know "strings" (as we understand them in elixir), but only sees "a bunch of bytes". This means, when looking at our WebAssembly memory, the whole memory is just "a big array of bytes". So we need to know where to start reading the string from memory (memory_position aka pointer) and how many bytes to read (length).
There is two ways to pass this information:
- The old-school "C language" way: Every string ends with a zero-byte (
"\0") and we read the memory starting from ourpointeruntil we discover a zero byte. The upside is that we only need to pass one argument to/from WebAssembly (thepointer). But the downside is that it opens the door wide for mistakes (what if we forgot a zero byte, what if our string contains zero-bytes, what if an attacker manages to sneak-in or remove some zero-bytes etc.). - We always pass two arguments, the
pointerandlength(in bytes) of a string. This has the upside of being way more secure (against attackers and programming mistakes) but the downside of us needing to always pass two variables to/from WebAssembly. This is, by the way, how Rust handlesStrings internally.
For wasmex, we decided to go with the second approach. This works well for passing Strings down to WebAssembly. Given we have this method in WebAssembly (implemented in Rust, but could be any language compiling to wasm):
#[no_mangle]
pub extern "C" fn do_something_with_a_string(bytes: *const u8, length: usize) -> u8 {
// do something with the given byte-array and string length
}We could call the method in Elixir like this:
{:ok, memory} = Wasmex.memory(instance, :uint8, 0)
string = "hello, world"
memory_position = 42 # aka "pointer"
Wasmex.Memory.write_binary(memory, memory_position, string) # copy the bytes to WASM memory, so our WASM function can see it
Wasmex.call_function(instance, :do_something_with_a_string, [memory_position, String.length(string)])The other way around (handing a string from WebAssembly back to Elixir) is more complicated, because we can currently only return one value from wasm functions, but we need two values (pointer and length). (see 👣 footnotes for details)
This is often solved by having two functions in WebAssembly: One function producing a string, and another function returning the string-length.
#[no_mangle]
pub extern "C" fn demo_string() -> *const u8 {
b"Hello, World!".as_ptr()
}
#[no_mangle]
pub extern "C" fn demo_string_len() -> u8 {
13
}We would use it in Elixir like this:
{:ok, [pointer]} = Wasmex.call_function(instance, :demo_string, [])
{:ok, [length]} = Wasmex.call_function(instance, :demo_string_len, [])
assert Wasmex.Memory.read_string(memory, pointer, length) == "Hello, World!"Now that you know how to pass strings down to WebAssembly and back to Elixir again, there should be nothing stopping you from combining both approaches.
If your specific use-case requires it, you can of course go the "C strings" way of ending strings with zero-bytes or invent any other custom protocol (e.g. starting a string, so the first byte is the string-length). These customs routes don't have helper methods, though, in wasmex.
👣 Footnote: "wasm can only return one value from a function call"
This is only half-true. In fact, the WebAssembly standard already allows returning multiple values. Wasmer, the web assembly execution engine we use, already partially implements that. Unfortunately, we don't have that feature yet in singlepass compilation (see https://docs.wasmer.io/ecosystem/wasmer/wasmer-features). Once wasmer supports multi-value returns everywhere, we can build better helper methods in elixir to make the whole process way easier.
sleipnir
First of all I would like to thank you for this excellent library. I need to call a function that takes a string as an argument and that returns a string as a result. In the examples I only saw the use of one or the other, never passing and receiving strings. It may seem trivial to expand, but I'm a little confused about the WASM API and the use of Memory to achieve this goal. Could someone help me?