JS/C Interop with zig cc and wasm

Question

I am writing a webassembly demo in C to match a similar demo I wrote in zig.

I am currently able to call C functions from JS, and interact with shared memory on either side. However, I can't seem to be able to expose JS variables and functions to the C program.

Is anybody familiar with writing C this way? I wonder if the undefined symbols are getting optimized out somehow.

I'm using zig cc (i.e. clang) to compile to wasm on zig version 0.13.0-dev.351+64ef45eb0, using clang 16.0.0 on macos Sequoia 15.0.1 (aarch64).

This is the code I've written thus far:

resource.c

#include <stdlib.h>

void print_test(void); // the function I want to use

const unsigned char heap[4096];

int __attribute__((export_name("memstart")))
memstart() {
    return (int)&heap;
}

int __attribute__((export_name("return5")))
return5(int p) {
    return 5 * p;
}

int __attribute__((export_name("entryAt")))
entryAt(int p) {
    print_test();
    return heap[p];
}

index.html


<!DOCTYPE html>
<html lang="en">
    <head>
        <title>For Stephen</title>
    </head>

    <body>
        <input id="input" type="number" placeholder="int param..."></input>
        <input id="bytes" placeholder="bytes..."></input>
        <pre id="output">output goes gere</pre>
        <button id="button_r5">Return 5</button>
        <button id="button_nth">Get nth byte</button>
        <canvas id="my_canvas"></canvas>
        <script>
            const c = {};
            const encoder = new TextEncoder();
            const decoder = new TextDecoder();

            const importObject = {
                "env": {
                    "print_test": () => console.log("test print")
                }
            };

            WebAssembly.instantiateStreaming(fetch("resource.wasm", importObject))
            .then(result => {
                const {memory, memstart, return5, entryAt} = result.instance.exports;
                console.log(memstart);
                c.buffer  = new Uint8Array(memory.buffer, memstart());
                c.return5 = return5;
                c.entryAt = entryAt;
            });

            const button_r5  = document.getElementById("button_r5");
            const button_nth = document.getElementById("button_nth");
            const input  = document.getElementById("input");
            const bytes  = document.getElementById("bytes");
            const output = document.getElementById("output");

            button_r5.addEventListener("click", ()=>{
                output.textContent = c.return5(input.value);
            });

            button_nth.addEventListener("click", ()=>{
                c.buffer.set(encoder.encode(bytes.value));
                output.textContent = c.entryAt(input.value);
            });
        </script>
    </body>

</html>

Build command

zig cc -target wasm32-freestanding -g resource.c -lc -Wl,--no-entry -o resource.wasm

Maybe use __attribute__((import_module("env"), import_name("print_test"))) void print_test();? — Inkeliz
– Inkeliz, Commented Feb 17 at 6:51
@Inkeliz this works! Add it as an answer and I'll accept it. — J. B
– J. B, Commented Feb 17 at 19:47

Inkeliz · Accepted Answer · 2025-02-18 09:44:14Z

1

You can use some attributes to import functions from JavaScript, that will depends on the compiler.

Using Emscripten you can set EM_IMPORT(NAME). That is equivalent of __attribute__((import_module("env"), import_name(#NAME))) (source-code).

Considering that Zig also uses LLVM, you can also use the same attribute:

__attribute__((import_module("env"), import_name("print_test"))) void print_test();

However, if it doesn't work (maybe it's not using Zig compiler), you might have some other flavours:

void print_test() __attribute__((
    __import_module__("env"),
    __import_name__("print_test")
));

__attribute__((__import_name__("print_test"))) void print_test();

That are based on other languages/compilers, such as SwiftWasm and .NET-NativeLLVM (all based on LLVM).

edited Feb 18 at 9:44

answered Feb 18 at 7:42

Inkeliz

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