Synchronous Virtual XHR Backed File System Usage

Emscripten supports lazily loading binary data from HTTP servers using XHR's (ala AJAX but no XML or JSON).

Restriction: only possible in Web Workers (due to browser limitations)

It is recommended to use a web server with byte serving support. This will enable Emscripten to only load the parts of the file that are actually read. If it is not available, the runtime will of course be forced to load the whole file, even if you only read a single byte from a 100 MB file.

Instructions for use

1. You will need a page that spawns the web worker. See the page inlined in tests/runner.py/test_chunked_synchronous_xhr. The prejs we use below will cause the program running in the Web Worker to postMessage it's stdout back. If you use that solution, the mother page should probably contain your handwritten glue code (not Emscriptened) to handle the stdout data.
2. The tests use checksummer.c as the actual Emscriptened program. As you can see, it is simply a vanilla C program using fopen/fread/fclose and no Emscripten specific code at all.
3. You will need at prejs that sets up the mapping between the file path in your equivalent of checksummer.c and the server to download from. Remember CORS! The test case also contains an HTTP server that shows some CORS headers that might need to be set. Of course, if the resources are hosted from the same domain Emscripten runs from, there is no issue.