05

Example 5: Returning a string

This example returns the string Hello World! 😀 from a function called say() to the WebAssembly runtime, the web browser.

WebAssembly function return values can only be numerical types -- no strings. However, a string is just a numerical pointer to a location in memory -- a memory address -- and a C string is terminated by a zero value. When the WebAssembly runtime can read the memory, it can also read and decode the string.

Thanks to modern Web Platform APIs, in particular the TextDecoder interface, decoding strings from raw bytes is quite easy and concise:

new TextDecoder().decode(memory.subarray(strPtr, strPtr + strLen)); // returns "Hello World! 😀"

Here, strPtr is the numerical address of the beginning of the string as returned by the say() function. The tricky part is to find strLen, which is the offset to the terminating null character in the string. Again, Web Platform APIs save the day. The string length can simply and concisely be found:

const strLen = memory.subarray(strPtr).indexOf(0); // find end of string (null character)

In the accompanying index.js, all this is implemented by the function stringFromMemory.

This example can be implemented in just ~10 lines of JavaScript, and a WebAssembly component of just 81 bytes:

(module
  (type (;0;) (func (result i32)))
  (import "env" "memory" (memory (;0;) 2))
  (func (;0;) (type 0) (result i32)
    i32.const 1024)
  (export "say" (func 0))
  (data (;0;) (i32.const 1024) "Hello World! \f0\9f\98\80\00"))