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Shraddha Vijay Inamdar
Shraddha Vijay Inamdar

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Let's do a "Hello world" program in Rust and executing in wasmtime runtime

#webassembly #rust #programming

Pre-requisites :

Please ensure that you have installed Rust and Cargo

 

Tool Setup

We need to install WASM Pack

cargo install wasm-pack
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Rust Code

First create a new binary crate using below command:

cargo new demo
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You can use any IDE of your choice and open up this project folder.

On Cargo.toml we are going to add the next:

[package]
name = "demo"
version = "0.1.0"
authors = ["Shraddha Inamdar <shraddha.inamdar95@gmail.com>"]
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]


[[bin]]
path = "src/main.rs"
name = "main"
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Writing Rust code

Writing a code for reading from file and writing to file using Rust.

  use std::env;
use std::fs;
use std::io::{Read, Write};

fn process(input_fname: &str, output_fname: &str) -> Result<(), String> {
    let mut input_file =
        fs::File::open(input_fname).map_err(|err| format!("Not Able To Open {}: {}", input_fname, err))?;
    let mut contents = Vec::new();
    input_file
        .read_to_end(&mut contents)
        .map_err(|err| format!("Not Able To Read: {}", err))?;

    let mut output_file = fs::File::create(output_fname)
        .map_err(|err| format!("Not Able To Open Output {}: {}", output_fname, err))?;
    output_file
        .write_all(&contents)
        .map_err(|err| format!("write error: {}", err))
}

fn main() {
    let args: Vec<String> = env::args().collect();
    let program = args[0].clone();

    if args.len() < 3 {
        eprintln!("usage: {} <from> <to>", program);
        return;
    }

    if let Err(err) = process(&args[1], &args[2]) {
        eprintln!("{}", err)
    }
}
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Compiling Rust Code

  1. Compile using cargo 
cargo build --target=wasm32-wasi
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  1. The wasm file created in debug folder of wasi32 
file target/wasm32-wasi/debug/main.wasm
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  1. Wasm runtime 

wasmtime target/wasm32-wasi/debug/main.wasm
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  1. Created test.txt and output.txt with test.txt containing input 
echo "Hi there, You made it! Cheers :)" > test.txt
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touch output.txt
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  1. Run using wasmtime 

wasmtime target/wasm32-wasi/debug/main.wasm test.txt output.txt
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  1. Solving the above error using –dir 

wasmtime --dir=. --dir=. target/wasm32-wasi/debug/main.wasm test.txt output.txt
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  1. Displaying the output 
cat output.txt
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