Demo Debugging WebAssembly

In this hands-on tutorial, we’ll explore WebAssembly (WASM) debugging workflows using Emscripten and Chrome DevTools. You’ll learn how to compile a simple C function to WASM, step through raw instructions in the browser, and then enable source-level debugging with DWARF symbols for C/C++ projects.

The image shows a Chrome Web Store page for the "C/C++ DevTools Support (DWARF)" extension, with a 3.7-star rating and 8,000 users. There's a screenshot of a code editor interface below.

Basic Debugging Experience

We start with a minimal C library that calculates the factorial of a non-negative integer:

#include <emscripten.h>

EMSCRIPTEN_KEEPALIVE
int factorial(int n) {
    if (n < 0) return -1;           // Undefined for negatives
    if (n <= 1) return 1;           // 0! and 1! are 1
    int result = 1;
    for (int i = 2; i <= n; ++i) {
        result *= i;
    }
    return result;
}

Add an HTML page with JS glue to invoke the WASM module:

<!DOCTYPE html>
<html>
  <body>
    <script type="module">
      import initModule from './debug_wasm_basic.js';

      initModule().then(module => {
        console.log(module._factorial(10)); // Outputs 3628800
      });
    </script>
  </body>
</html>

Compile using Emscripten:

emcc debug_wasm_basic.c \
  -s MODULARIZE \
  -s EXPORT_ES6 \
  -o debug_wasm_basic.js

When you open the HTML in Chrome and inspect DevTools > Sources, you’ll find both the generated JavaScript and the raw WebAssembly (.wasm):

(module
  (table $t_indirect_function_table (export "_indirect_f") (elem (i32.const 0) $factorial))
  (memory $mory 256 256)
  (global $gglobal (export "global") i32 (i32.const 66576))
  (func $factorial (export "factorial") (param $var0 i32) (result i32)
    (local $var1 i32) (local $var2 i32) (local $var3 i32)
    ;; function body omitted
  )
)

You can set breakpoints in JS, step into the WASM module, and inspect raw locals like var0. However, mapping these low-level variables back to your C source can be tedious.

Low-level WASM debugging displays raw opcodes (local.get, i32.mul, etc.) and generic local names. To see your original C/C++ code directly, you need DWARF debug symbols.

Source-Level Debugging with DWARF

For larger C++ projects—such as rendering a Julia set fractal with SDL2—inspecting raw assembly is cumbersome. A Julia set is generated by iterating a simple complex-numbers formula for each pixel. Here’s a concise renderer:

#include <SDL2/SDL.h>
#include <complex>
#include <cstdlib>
#include <ctime>

int main() {
    const int width = 600, height = 600;
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window* window;
    SDL_Renderer* renderer;
    SDL_CreateWindowAndRenderer(width, height, SDL_WINDOW_OPENGL, &window, &renderer);

    const int MAX_ITER = 256;
    SDL_Color palette[MAX_ITER];
    std::srand(static_cast<unsigned>(std::time(nullptr)));
    for (int i = 0; i < MAX_ITER; ++i) {
        palette[i] = {
            static_cast<uint8_t>(std::rand()),
            static_cast<uint8_t>(std::rand()),
            static_cast<uint8_t>(std::rand()),
            255
        };
    }

    std::complex<double> c(-0.7, 0.27015);
    double zoom = 1.0;

    for (int y = 0; y < height; ++y) {
        for (int x = 0; x < width; ++x) {
            double zx = 1.5 * (x - width / 2.0) / (0.5 * zoom * width);
            double zy = (y - height / 2.0) / (0.5 * zoom * height);
            std::complex<double> z(zx, zy);
            int iter = MAX_ITER;
            while (std::abs(z) < 2.0 && iter > 0) {
                z = z * z + c;
                --iter;
            }
            SDL_Color color = palette[iter];
            SDL_SetRenderDrawColor(renderer, color.r, color.g, color.b, color.a);
            SDL_RenderDrawPoint(renderer, x, y);
        }
    }

    SDL_RenderPresent(renderer);
    SDL_Quit();
    return 0;
}

Compiling with Debug Info

Install the C/C++ DevTools Support (DWARF) extension from the Chrome Web Store.
Compile with SDL2 support and DWARF symbols:

emcc debug_wasm_advanced.cc \
  -s ALLOW_MEMORY_GROWTH \
  -s USE_SDL=2 \
  -g \
  -o debug_wasm_advanced.html

Flag	Description
`-g`	Embed DWARF debug symbols
`-s ALLOW_MEMORY_GROWTH`	Enable dynamic memory expansion
`-s USE_SDL=2`	Link SDL2 for graphics and input

Open the resulting HTML in Chrome (with the DWARF extension enabled). In DevTools > Sources, you’ll find a C/C++ tab where you can:

Set breakpoints in your original .cpp files
Step through high-level C++ code, not raw WASM opcodes
Inspect variables by name and type

This workflow turns WebAssembly debugging into a familiar, source-level experience.

Introduction

Introduction to WebAssembly (WASM)

WebAssembly Core Concepts

Getting Started with WebAssembly

Compiling to WebAssembly

Future: WebAssembly in Cloud

Community & Resources

Demo Debugging WebAssembly

Basic Debugging Experience

Source-Level Debugging with DWARF

Compiling with Debug Info

Links and References

Watch Video

​Basic Debugging Experience

​Source-Level Debugging with DWARF

​Compiling with Debug Info

​Links and References

Watch Video

Basic Debugging Experience

Source-Level Debugging with DWARF

Compiling with Debug Info

Links and References