Notes: WebAssembly by follower

Notes from working with WebAssembly / Emscripten / wasm / etc

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18 January 2018

  • The default .html page ends up with a FS object (see File System API — Emscripten documentation) for interacting with the in-memory filesystem which is automatically created.

    From the console you can do this to check the existence of the file system root:

        FS.stat("/")

    And check for the existence of particular files like this:

        FS.stat("/example.bin")

    You can extract the content of a file like this:

        file_content = FS.readFile("/example.bin")

    Clearly interacting with the "filesystem" isn't an ideal for passing data from your compiled WASM code but as a first step to porting it's quite handy.

  • In order to download (to your local computer) a copy of a file that has been created inside the WebAssembly in-memory file system this approach works (from within the console, for a binary file, at least):

        function download(filename, the_file_content, mime_type) {
      //
      // Based on: <https://ourcodeworld.com/articles/read/189/how-to-create-a-file-and-generate-a-download-with-javascript-in-the-browser-without-a-server>
      //
      // With modifications via: <https://stackoverflow.com/questions/44913341/octet-stream-data-uri-ignoring-a-download-attribute#44913497>
      //

      var element = document.createElement('a');
      element.setAttribute('href',
                           URL.createObjectURL(new Blob([the_file_content],
                                              {type: mime_type})));
      element.setAttribute('download', filename);

      element.style.display = 'none';
      document.body.appendChild(element);

      element.click();

      document.body.removeChild(element);
    }

    file_content = FS.readFile("/example.bin")

    download("example.bin", file_content, 'application/octet-stream')

    The second link also makes reference to download.js which I suspect is likely to be a more robust approach than this quick snippet. :)

  • In case you were interested in Qt for WebAssembly, apparently that might be an option: :D

6 February 2018