06

Example 6: Passing a string to a function

In example 5 we have seen how to return a string from a function which was implemented in C. In this example, we will do the opposite: pass a string from the WebAssembly runtime to a function implemented in C.

Given access to the memory, it is quite easy to encode the string, write it into the memory, and pass the numerical address to the function. However, without full knowledge of the program, this is not safe; we may overwrite used (now or in the future) memory. So, we need to use the same memory allocator/manager library as the program itself uses: for example malloc which ships with the C Standard Library. It needs to be exported by adding -export malloc to the linker arguments.

Then a pointer to enough free memory can be gotten:

const { malloc, free } = instance.exports;
const ptr = malloc(string.length + 1); // plus one byte for the terminating null character

Thanks to modern Web Platform APIs, in particular the TextEncoder interface, encoding a string directly into a specific location of a given WebAssembly memory is quite easy and concise:

const stats = new TextEncoder().encodeInto(string, memory.subarray(ptr));

However, we need to manually append a null byte to complete the encoding into a proper C string:

if (ptr + stats.written < memory.length)
  memory[ptr + stats.written] = 0; // Append null character

After the string is no longer needed, don't forget to free the memory again:

free(ptr);

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