WebAssembly Error Handling

Why WebAssembly Lacks Native Error Handling?

WebAssembly is designed to be a low-level, portable binary instruction format optimized for performance. Its minimalistic design focuses on simplicity, and it delegates advanced features like error handling to the host environment. This decision ensures that WebAssembly remains lightweight and interoperable across platforms.

Common Sources of Errors in WebAssembly

Errors in WebAssembly can arise due to various reasons:

1. Invalid Memory Access:
   • Accessing memory beyond the allocated range.
   • Writing to protected memory areas.
2. Type Mismatches:
   • Incorrect data types passed to functions.
   • Operations incompatible with the data type.
3. Unresolved Imports:
   • Failure to provide an expected import during instantiation.
4. Trapped Execution:
   • Specific instructions leading to a “trap,” which is WebAssembly’s way of signaling an error.

Handling Errors in WebAssembly

WebAssembly errors are typically managed by the host environment. For example, when running WebAssembly in a browser, JavaScript is responsible for catching and processing errors. Below are some effective techniques:

1. Using JavaScript try-catch for Error Handling

Since WebAssembly doesn’t have a try-catch mechanism, errors that occur during execution can be caught in the host environment.

Example: Handling Memory Access Error

(async () => {
  const memory = new WebAssembly.Memory({ initial: 1 }); // 64 KB
  const importObject = {
    env: { memory },
  };

  try {
    const response = await fetch('example.wasm');
    const wasmModule = await WebAssembly.instantiateStreaming(response, importObject);
    const { faultyFunction } = wasmModule.instance.exports;

    faultyFunction(); // May cause a memory error
  } catch (error) {
    console.error("Error caught:", error.message);
  }
})();

2. Returning Error Codes from WebAssembly Functions

A common approach is to use error codes in WebAssembly functions. Instead of throwing errors, functions return codes indicating success or failure.

WebAssembly Text Format:

(module
  (func $divide (param $numerator i32) (param $denominator i32) (result i32)
    local.get $denominator
    i32.const 0
    i32.eq
    if (result i32)
      i32.const -1 ;; Error code for division by zero
      return
    end
    local.get $numerator
    local.get $denominator
    i32.div_s ;; Perform division
  )
  (export "divide" (func $divide))
)

JavaScript Code:

(async () => {
  const wasmCode = await fetch('divide.wasm').then((res) => res.arrayBuffer());
  const { instance } = await WebAssembly.instantiate(wasmCode);

  const divide = instance.exports.divide;
  const result = divide(10, 0); // Trying to divide by zero
  if (result === -1) {
    console.log("Error: Division by zero");
  } else {
    console.log("Result:", result);
  }
})();

3. Using Host Callbacks for Custom Error Handling

You can pass error-handling logic from the host environment to WebAssembly by importing a callback function. This approach ensures that errors are handled consistently.

WebAssembly Text Format:

(module
  (import "env" "errorHandler" (func $errorHandler (param i32)))

  (func $checkValue (param $value i32)
    local.get $value
    i32.const 0
    i32.lt_s
    if
      i32.const 1 ;; Error code
      call $errorHandler
    end
  )
  (export "checkValue" (func $checkValue))
)

JavaScript Code:

const importObject = {
  env: {
    errorHandler: (code) => {
      if (code === 1) {
        console.error("Error: Value cannot be negative.");
      }
    },
  },
};

(async () => {
  const wasmCode = await fetch('check.wasm').then((res) => res.arrayBuffer());
  const { instance } = await WebAssembly.instantiate(wasmCode, importObject);

  instance.exports.checkValue(-5); // Triggers the error handler
})();

WebAssembly Traps and Their Handling

A trap is WebAssembly’s built-in mechanism to signal runtime errors. Traps occur when instructions cannot be executed, such as:

• Division by zero.
• Stack overflows.
• Unreachable code execution.

Traps are not recoverable in WebAssembly, and execution stops immediately. Handling traps involves catching them in the host environment.

Example: Division Trap

(async () => {
  try {
    const response = await fetch('trap-example.wasm');
    const wasmModule = await WebAssembly.instantiateStreaming(response);
    const { divide } = wasmModule.instance.exports;

    console.log(divide(10, 0)); // Division by zero
  } catch (error) {
    console.error("Trap caught:", error.message);
  }
})();

Best Practices for Error Handling in WebAssembly

1. Validate Inputs:
   • Perform input validation in the host environment to avoid triggering traps.
2. Use Descriptive Error Codes:
   • Define meaningful error codes for all possible failure scenarios.
3. Document Function Contracts:
   • Clearly specify what each WebAssembly function expects and returns, including error codes.
4. Test for Edge Cases:
   • Ensure your WebAssembly code handles all edge cases gracefully.
5. Leverage the Host Environment:
   • Use the robust error-handling features of JavaScript or other host environments.