Compile mruby with emscripten (or: how to include compiler in cross build)

[sasq64]

I am trying to build mruby for the web using emscripten, but I get two undefined symbols;

wasm-ld: error: CMakeFiles/toy.html.dir/src/toy.cpp.o: undefined symbol: mrb_load_string
wasm-ld: error: CMakeFiles/toy.html.dir/src/toy.cpp.o: undefined symbol: mrb_load_file

Emscripten works differently than normal c-compilers so errors like this are not too uncommon, but I just wonder;

Is there anything special about those two functions compared to other mrb-functions, since those are the only ones giving me trouble ?

[sasq64]

Examining the contents og librmuby.a and libmruby_core.a I see that they contain the same files, whereas in the PC build libmruby.a contains a lot more files, including y.tab.o.

And checking the build output, it seems y.tab.c is still compiled using normal gcc, even though I specified

MRuby::Toolchain.new('emscripten') do |conf|

  toolchain :clang
  
  conf.cc.command = 'emcc'
  conf.cxx.command = 'em++'
  conf.linker.command = 'em++'
  conf.archiver.command = 'emar'

end

... so I guess I have missed something in the config ?

[sasq64]

Ok so it seems this use case is no really supported ? A cross build does not include the compiler and can not load code dynamically ?
I think I solved it by building a non-cross build and copying over the bin tools from my non emscripten build.

What would be involved with supporting a cross build that included the compiler ?

[dearblue]

Is #4486 useful to you?
The build config file is https://github.com/mruby/mruby/pull/4486/files#diff-2cf92ce08f99cccec9dfb77b4e322228fef3e4d0624ebab89dad1cd08b7cefe4

[sasq64]

No, that PR does not include the compiler, like I noticed.

By the way, I am now running successfully running an mruby REPL in the browser, so the compiler in included, but I have to do the manual step mentioned above.

[zzak]

I think compiler has to be built using host environment, but it should be possible to build only the compiler as a separate build target and copy them like you suggested.

[sasq64]

Yes, and I wonder what needs to change in the build environment to allow this behavior.

[YunzheZJU]

Hi, Have you solved your problem?

Several months ago I successfully compiled the mruby compiler into WASM. It can execute ruby scripts which are dynamically inputted by the user (like p 'Hello world' which eventually log a string in the console) in the browser env.

Basically it first

• cross-builds mruby library targeting WebAssembly(which outputs lib/libmruby.a),
• then compiles mruby library (included with my own c source files) to wasm(which outputs mruby.mjs and mruby.wasm)
• and finally bundles mruby wasm and Javascript sources files of my own.

I don't quite understand the meaning of your include the compiler but if you are going to achieve a similar target as mine, I'd like to share more details about my build config.

[sasq64]

Right now I my mruby.cfg contains both a Build and a CrossBuild, and will fail when it tries to run the binaries in the host/bin dir (like mrbc) because it was built for emscripten. So then I just copy over the binaries from the PC build and build again, and I get a complete emscripten build.

If you have a config that allows me to skip this hack that wold be great.

But it's working, you can test it at http://apone.org/toy/

(It even supports blocking IO, like
name = gets() ; puts "hello #{name}"
... can you guess how that works ? :)

[matz]

Sorry for being late to reply.

mrb_load_string and mrb_load_file are functions provided by mruby-compiler gem. So you have to specify the gem for the target side as well. So your config file should be like:

MRuby::Toolchain.new('emscripten') do |conf|

  toolchain :clang
  
  conf.cc.command = 'emcc'
  conf.cxx.command = 'em++'
  conf.linker.command = 'em++'
  conf.archiver.command = 'emar'

  conf.gem :core => 'mruby-compiler'   ## << add this line
end

[sasq64]

Thank you, now it builds.
But I am missing some other core things. Like 'p()' so I guess there are some other gems I should add also ?

[sasq64]

I just replaced the gem line with
conf.gembox 'default'
which game me a full build

[matz]

FYI, 'p' is in the mruby-print gem.