RPC

In this section, we will walk through the protocol example in Rust. It converts the case of alphabetic ASCII characters from its request to its response and switches the letters I and O (similarly for lower-case the letters i and o).

The platform protocol (used to install applets, update the platform, reboot the platform, etc) also provides a way to call into an applet by sending a request and reading the response. The applet can define a handler taking a Vec<u8> request as argument and returning a Vec<u8> response (which can reuse the backing storage of the request).

In this example, the handler simply converts all characters of the request, prints how many times it was called, and returns the updated request as its response¹.

    let mut counter = 0;
    let handler = move |mut data: Vec<u8>| {
        data.iter_mut().for_each(convert);
        counter += 1;
        debug!("Converted {counter} lines.");
        data
    };

A listener can be created and leaked as usual.

    rpc::Listener::new(&platform::protocol::RpcProtocol, handler).leak();

The conversion function is straightforward:

fn convert(x: &mut u8) {
    if x.is_ascii_alphabetic() {
        *x ^= 0x20; // switch case
    }
    if matches!(*x, b'I' | b'O' | b'i' | b'o') {
        *x ^= 0x6; // switch between I and O preserving case
    }
}

The final code looks like this:

#![no_std]
wasefire::applet!();

use alloc::vec::Vec;

fn main() {
    let mut counter = 0;
    let handler = move |mut data: Vec<u8>| {
        data.iter_mut().for_each(convert);
        counter += 1;
        debug!("Converted {counter} lines.");
        data
    };
    rpc::Listener::new(&platform::protocol::RpcProtocol, handler).leak();
}

fn convert(x: &mut u8) {
    if x.is_ascii_alphabetic() {
        *x ^= 0x20; // switch case
    }
    if matches!(*x, b'I' | b'O' | b'i' | b'o') {
        *x ^= 0x6; // switch between I and O preserving case
    }
}

Testing

You can use the wasefire applet-rpc command to send an RPC to an applet. By default it reads from standard input and write to standard output.

% echo HELLO ping | wasefire applet-rpc
helli PONG