The following code is taken from the wasm-bindgen tutorial.
My question pertains to the code block that begins with
extern "C" and contains the functions log_u32 and log_many.
From what I understand, calling extern "C" lets you use C functions in Rust. For example, log here is simply the log function from C, if I understand correctly.
But, here's my issue: That code block also contains the functions log_u32 and log_many. But there are no C functions named log_u32 and log_many that I'm aware of. So where do these two functions get their definitions? How does the Rust compiler make sense of them?
#![allow(unused_variables)]
fn main() {
use wasm_bindgen::prelude::*;
#[wasm_bindgen(start)]
pub fn run() {
bare_bones();
using_a_macro();
using_web_sys();
}
// First up let's take a look of binding `console.log` manually, without the
// help of `web_sys`. Here we're writing the `#[wasm_bindgen]` annotations
// manually ourselves, and the correctness of our program relies on the
// correctness of these annotations!
#[wasm_bindgen]
extern "C" {
// Use `js_namespace` here to bind `console.log(..)` instead of just
// `log(..)`
#[wasm_bindgen(js_namespace = console)]
fn log(s: &str);
// The `console.log` is quite polymorphic, so we can bind it with multiple
// signatures. Note that we need to use `js_name` to ensure we always call
// `log` in JS.
#[wasm_bindgen(js_namespace = console, js_name = log)]
fn log_u32(a: u32);
// Multiple arguments too!
#[wasm_bindgen(js_namespace = console, js_name = log)]
fn log_many(a: &str, b: &str);
}
fn bare_bones() {
log("Hello from Rust!");
log_u32(42);
log_many("Logging", "many values!");
}
// Next let's define a macro that's like `println!`, only it works for
// `console.log`. Note that `println!` doesn't actually work on the wasm target
// because the standard library currently just eats all output. To get
// `println!`-like behavior in your app you'll likely want a macro like this.
macro_rules! console_log {
// Note that this is using the `log` function imported above during
// `bare_bones`
($($t:tt)*) => (log(&format_args!($($t)*).to_string()))
}
fn using_a_macro() {
console_log!("Hello {}!", "world");
console_log!("Let's print some numbers...");
console_log!("1 + 3 = {}", 1 + 3);
}
// And finally, we don't even have to define the `log` function ourselves! The
// `web_sys` crate already has it defined for us.
fn using_web_sys() {
use web_sys::console;
console::log_1(&"Hello using web-sys".into());
let js: JsValue = 4.into();
console::log_2(&"Logging arbitrary values looks like".into(), &js);
}
}
