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I am looking for an API that let me read file provided via (or any other method that allows to access users files from browser) chunk at a time. I'm reading large files, so I don't want to load entire file to memory.
My use case is, I'm using ffmpeg library compiled with emcc and I wan't to process multimedia files by it. I could implement my own AVIOContext but to do that, I would need equivalent of C functions fread and fseek.
I was looking at FS API with WORKERFS file system type, but it is not clear to me that I could mount it from worker thread with File object from DOM.
877
asked Sep 21 '25 03:09
I was able to mount file with WORKERFS from worker thread.
Minimal example:
main.html:
<html>
<head>
<script>
const worker = new Worker("worker.js");
function onClick() {
const f = document.getElementById("in-file").files[0];
worker.postMessage([ f ]);
}
</script>
</head>
<body>
<input type="file" id="in-file" />
<input type="button" onClick="onClick()" value="ok" />
</body>
</html>
worker.js
onmessage = function(e) {
const f = e.data[0];
FS.mkdir('/work');
FS.mount(WORKERFS, { files: [f] }, '/work');
console.log(Module.read_file('/work/' + f.name));
}
self.importScripts('hello.js');
hello.js is compiled hello.cpp with
(emcc --bind -lworkerfs.js -o hello.js hello.cpp -s WASM=1):
#include <cstdio>
#include <string>
#include <iostream>
#include <fstream>
#include <emscripten/bind.h>
using namespace emscripten;
std::string read_file(const std::string &fn)
{
std::ifstream f(fn);
std::string line;
std::getline(f, line);
return line;
}
EMSCRIPTEN_BINDINGS(hello) {
function("read_file", &read_file);
}
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