How do I access the File System API of emscripten when compiled with MODULARIZE option?

Question

If I compile my C++ code with emcc without -s MODULARIZE=1 -s 'EXPORT_NAME="createMyModule"', I can load my WebAssembly and access the File System API like this:

HTML:

<script type="text/javascript">
  var Module = {
    onRuntimeInitialized: function() {
      console.log('Module loaded!');
      if (FS.mkdir) {
        console.log('File System API available!');
      }
    }
  };
</script>
<script type="text/javascript" src="myWebAssemblyModule.js"></script>
<script type="module" src="my.js"></script>

Log:

Module loaded!
File System API available!

But if I add -s MODULARIZE=1 -s 'EXPORT_NAME="createMyModule"' as compiler options, and try to access the File System API based on a Promise, it doesn't work:

HTML:

<script type="text/javascript" src="myWebAssemblyModule.js"></script>
<script type="module" src="my.js"></script>

my.js:

createMyModule().then(MyModule => {
  console.log('Module loaded!');
  if (MyModule.FS.mkdir) {
    console.log('File System API available!');
  }
});

Log:

Module loaded!

So how can I access the File System API with the "MODULARIZED/Promise based" approach?

MyModule.FS is accessible btw., but not MyModule.FS.mkdir.

stackprotector · Accepted Answer · 2020-09-25 05:54:31Z

3

To make FS available in a modularized build, you have to export it explicitly:

emcc ... -s 'EXPORTED_RUNTIME_METHODS=["FS"]'

edited Sep 25, 2020 at 5:54

stackprotector

14.2k6 gold badges69 silver badges97 bronze badges

answered Sep 23, 2020 at 5:23

zakki

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