embedding the dart vm: part one

The Dart VM can be run as a standalone tool from command-line, and is embedded in a branch of Chromium named Dartium, but all of the public instructions I could find only discuss how to extend the VM with native methods. Definitely useful, but there are some applications when it would be useful to directly embed the VM in a native executable. For instance, if a game wanted to use Dart as an alternative to Lua.

The instructions for adding native extensions recommend building the extension as a shared library that the VM can load. We need to flip that model around and build the VM as a static library that our native executable can link against. This, part one of a series of who-knows-how-many, will focus on getting the source and building a native executable with the VM embedded. We won’t expect to actually run a script just yet, but by the end of this blog we will be loading and compiling a script.

Getting and Building the VM

The instructions for getting all of the Dart source (including the editor, VM, and runtime libraries) are here.

Once we have the source, we need to build the VM as a static library instead of the default. gyp is used to generate the Makefile, and it already includes a static library target for the VM. Sadly make doesn’t seem to be fully supported on OSX, given the pile of compiler errors that I hit, but opening the xcode project and building the dart-runtime project directly does work. I needed to tweak a few of the settings in the xcodeproject as it was set up to use GCC 4.2, which I don’t have, and I wanted the Debug build to be unoptimized for easy debugging, but eventually I had a few shiny static libs to play with.

Quite a bit is hidden in that ‘eventually’. There are a number of subtleties around linking in generated source files to get a snapshot buffer loaded that contains much of the core libraries. There’s also the issue of initializing the built-in libraries; the VM source contains some files that handle this, but it’s unclear which should be included and which shouldn’t. Similarly, some libraries are named with a _withcore suffix, and it’s not clear whether they are the ones that should be linked in or not.

Loading a script

To load a script we need an Isolate and some core libraries loaded to do things like resolve the path to the script, read the source, and compile it. Eventually, when we come to run, we’ll have to register some native methods, but for now just loading the core, io and uri libraries is a good start.

It took a while, but the end result is both exciting, and suggests the next step that’s required:

    Dart Initialized
    LoadScript: helloworld.dart
    CreateIsolate: helloworld.dart, main, 1
    Created isolate
    Loaded builtin libraries
    About to load helloworld.dart
    LoadScript: helloworld.dart, 1
    Script loaded into 0x100702590
    Invoking 'main'
    Invoke: main
    Failed to invoke main: Unhandled exception:
    UnsupportedOperationException: 'print' is not supported
    #0 _unsupportedPrint._unsupportedPrint (dart:core-patch:2525:3)
    #1 print (dart:core-patch:2521:16)
    #2 main (file:////~/git/embed-dart-vm/helloworld.dart:4:8)Done

There was one rather nice step where I was getting an error compiling the test Dart script. After spending a couple of hours debugging the code it turned out that I actually had a syntax error in the Dart script. I should have trusted my code after all!

Next time we’ll figure out those native extensions, built-in libraries, and get the script running.

Full source for this project can be found here and the version that this post describes is here.