cmem_helpers

cmem_helpers

This provides a few simple & light helpers for working with C-memory. It should be useful for FFI, native node-modules, and browser/node wasm, and has no dependencies. It should also work for other runtimes like bun, deno, or quickjs.

Use it to pass and work with strings, and structs. It is very light and intended for no-emscripten host-code, or situations where you want to do your own thing, a bit.

I also wrote a couple medium posts about how it works:

• structs
• strings

usage

installation

You can add it to your project like this:

npm i cmem_helpers

And then import or require it:

import memhelpers from 'cmem_helpers'

// OR

const memhelpers = require('cmem_helpers')

You can also use it on the web:

<script type=module>
import memhelpers from 'https://cdn.jsdelivr.net/npm/cmem_helpers/+esm'
</script>

You can also use an importmap to make your code look the same:

<script type="importmap">
{
  "imports": {
    "cmem_helpers": "https://cdn.jsdelivr.net/npm/cmem_helpers/+esm"
  }
}
</script>
<script type=module>
import memhelpers from 'cmem_helpers'
// YOUR CODE HERE
</script>

getting started

Here is an example with WASM, in the browser/nodejs:

import memhelpers from 'cmem_helpers'

// define this to pass functions to WASM
const env = {
  demo(namePtr){
    console.log(`Hello ${getString(namePtr)}!`)
  }
}

// load your bytes in wasmBytes however you do that
const wasmBytes = '...'

const mod = (await WebAssembly.instantiate(wasmBytes, { env })).instance.exports

// here is the actual setup
const { struct, structClass, setString, getString } = memhelpers(mod.memory.buffer, mod.malloc)

The first param is a buffer associated with the memory, and the second is optional, and it's a way to allocate bytes, and get a pointer. In this example, I exposed a function called malloc in my wasm, so I can allocate bytes, in the host. You can see an example in the test wasm.

strings

These are for basic C-style null-terminated UTF-8 strings.

// get a string from a pointer, using /0 termination (standard c-string)
getString(strPtr)

// explicitly tell it the length
getString(strPtr, 100)

// set a string in memory, with length (remember the last /0 char)
setString("Hello", address, 6)

// set a string in memory, without length
setString("Hello", address)

// get a pointer to a new string (if you setup malloc earlier)
const ptr = setString("Hello")

structs

This very simple helper uses DataView to interact directly with the memory.

Valid types are:

• BigInt64
• BigUint64
• Float32
• Float64
• Int16
• Int32
• Int8
• Uint16
• Uint32
• Uint8

You can define a struct like this:

const Color = struct({
  r: 'Uint8',
  g: 'Uint8',
  b: 'Uint8',
  a: 'Uint8'
})

And now you can make Color objects, with an address, and/or intiial value:

const color = Color({r: 0, g: 0, b: , a: 255}, address)

If you provided a malloc function earlier, when you set it up, you can also do this:

const color = Color()
const color = Color({r: 0, g: 0, b: , a: 255})

And it will allocate it for you. It will have a couple members: _size and _address that you can use in other things, for example to pass the pointer to a function:

mod.useMyColor(color._address)

You can also access the underlying bytes, if you need them:

console.log(color._bytes)

structClass

You can also use structClass, if you like to use them more like classes, and they will work the same:

const Color = structClass({
  r: 'Uint8',
  g: 'Uint8',
  b: 'Uint8',
  a: 'Uint8'
})
const color = new Color()

planned

I have a few ideas for the future:

• Nested struct fields as pointers (with param for bit-size to support wasm/ffi) or inline-bytes
• Tool to parse C header and pull out structs, in this format