How can I access WASM exported memory?

[itsvks19]

How can I access WASM exported memory?

I want to read and write some values in WASM memory, how can I do this?

[itsvks19]

I found it.

val export = instance.exports.find { it.name == "memory" }
requireNotNull(export) { "Export 'memory' not found" }

val memory = requireNotNull(export.value as? Memory) { "Export '${export.name}' is not a memory" }

[CharlieTap]

You might be better off with:

val memory = instance.exports.firstNotNullOf { it.value as? Memory }

Whilst most modules will export a memory with the name "memory", theres nothing in the spec (wasm) that tells a compiler to emit the export with that name.

It's worth noting a couple of other edge cases:

• Some binaries may not expose a memory at all
• Some binaries may expose more than one memory, and thus you'd need to use the one associated with the functions you're calling.

What I would recommend doing:

1. Convert your wasm binary to wat format using wasm-tools

wasm-tools print ./foo.wasm -o ./foo.wat

2. Look for exports of memories, you can just search for "export" and try to find one that references a memory

Exports can be inline like this

(module
  (memory $exported_memory (export "mem") 2)

Or on their own line with a reference

(module
  (memory $exported_memory 2)
  (export "mem" (memory $exported_memory))
)

If there is only one export pointing to a memory you can use this

val memory = instance.exports.firstNotNullOf { it.value as? Memory }

If there is many its worth picking the one you want by name

val memory = instance.exports.firstNotNullOf { it.name == "mem" }