Support returning strings about wasmi HOT 5 CLOSED

yeah that could help, but at same time I actually wonder if we should put it in MemoryInstance. I'm afraid that it could become a swiss-knife. At the same time, this functionality could be implemented externally by using with_direct_access.

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Hello!

If you compile your code

$ echo 'use std::os::raw::c_char;
use std::ffi::CString;

#[no_mangle]
pub extern fn test3() -> *mut c_char {
    CString::new("ohai!").unwrap()
        .into_raw()
}
' | rustc --crate-type cdylib --target wasm32-unknown-unknown -

and look for the definition of test3 in the result file you will see:

$ wasm2wat rust_out.wasm | grep '(func $test3'
  (func $test3 (type 9) (result i32)
  (export "test3" (func $test3))

this means that your function indeed has a i32 return type. The number you see is an actual offset of the string in the linear memory of the wasm module. What you need to do is to get the linear memory from the module instance and then read bytes from it.

For example:

// Get the linear memory from the module instance.
// "memory" is the export name of the exported memory. It is usually called that, but YMMV.
let memory = instance.export_by_name("memory")
    .expect("`memory` export not found")
    .as_memory()
    .expect("export name `memory` is not of memory type");

// Read memory contents
let size = 5;
let contents = memory.get(offset, size).expect("OOB error");

as you've might spotted already there is a problem: we need to provide the size for reading the contents and *mut c_char doesn't specify it. Moreover, wasm doesn't support returning of multiple-results from functions. There are several workarounds, such as: read the string byte-by-byte until you reach the null-terminator, pass the size through memory or invert your API design (say, instead of returning the string, you call some function that receives the string).

Yes, some high-level bindings generator akin to wasm-bindgen would be useful although as I've said it's a priority right now.

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Maybe we want to have an export like memory.get_until_null(offset)

from wasmi.

See #256 for how to read strings from wasm

from wasmi.

I will close this issue as it is quite outdated, @stevemk14ebr gave a link to an answer and we will probably not introduce a specialized API to ready out strings to our linear memory abstractions.

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