Using Memory to Share Data

Wasm only understands basic integer and float primitives. So passing more complex types across the boundary involves passing pointers. To read, write, or allocate memory in a module, Chicory provides the Memory class. Let's look at an example where we have a module count_vowels.wasm, written in Rust, that takes a string input and counts the number of vowels in the string:

curl https://raw.githubusercontent.com/dylibso/chicory/main/wasm-corpus/src/main/resources/compiled/count_vowels.rs.wasm > count_vowels.wasm

Build and instantiate this module:

import com.dylibso.chicory.runtime.ExportFunction;
import com.dylibso.chicory.runtime.Instance;
import com.dylibso.chicory.wasm.Parser;

Instance instance = Instance.builder(Parser.parse(new File("./count_vowels.wasm"))).build();
ExportFunction countVowels = instance.export("count_vowels");

To pass it a string, we first need to write the string into the module's memory. To make this easier and safe, the module gives us some extra exports to allow us to allocate and deallocate memory:

ExportFunction alloc = instance.export("alloc");
ExportFunction dealloc = instance.export("dealloc");

Let's allocate Wasm memory for a string and write it into the instance memory. We can do this with Memory#put:

import com.dylibso.chicory.runtime.Memory;
Memory memory = instance.memory();
String message = "Hello, World!";
int len = message.getBytes().length;
// allocate {len} bytes of memory, this returns a pointer to that memory
int ptr = (int) alloc.apply(len)[0];
// We can now write the message to the module's memory:
memory.writeString(ptr, message);

Now we can call countVowels with this pointer to the string. It will do its job and return the count. We will call dealloc to free that memory in the module. Though the module could do this itself if you want:

var result = countVowels.apply(ptr, len)[0];
dealloc.apply(ptr, len);
assert(3L == result); // 3 vowels in Hello, World!