emscripten and qt5 for wasm reading files from the local file system

[Balázs Varga]

Hi!

I run into a problem with

QFileDialog::getOpenFileContent function, which are use to access to the local filesystem with emscripten:
https://doc.qt.io/qt-5/qfiledialog.html#getOpenFileContent

I tried to call it on a different thread then what on the wasm application is running:

std::thread th1 (QFileDialog::getOpenFileContent, "*.*", onFileReady);

th1.join();

But in that case, it froze at the emscripten part in the openFileDialog function and no file dialogue was opened.
emscripten::val document = emscripten::val::global("document");
emscripten::val input = document.call<emscripten::val>("createElement", std::string("input"));

I got the error message at creating the input value.

https://codereview.qt-project.org/c/qt/qtbase/+/250461/7/src/gui/platform/wasm/qwasmlocalfileaccess.cpp#129

Running on the main thread there is no error and works fine.

Any idea what could be the problem/solution here?

I got this error message:

worker.js onmessage() captured an uncaught exception: TypeError: Cannot read properties of undefined (reading 'createElement')
self.onmessage @ soffice.worker.js:207
soffice.worker.js:208 TypeError: Cannot read properties of undefined (reading 'createElement')
    at methodCaller_emscripten$$val_$std$$string$ (eval at new_ (47933e67-c694-46fe-af70-91dc8b50859c:8694:27), <anonymous>:5:20)
    at __emval_call_method (47933e67-c694-46fe-af70-91dc8b50859c:9267:14)
    at emscripten::internal::MethodCaller<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >::call(emscripten::internal::_EM_VAL*, char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> emscripten::internal::MethodCaller<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >::call(emscripten::internal::_EM_VAL*, char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) (val.h:278)
    at emscripten::val emscripten::val::call<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >(char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) emscripten::val emscripten::val::call<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >(char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) const (val.h:498)
    at QWasmLocalFileAccess::openFileDialog(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (qstdweb::FileList const&)> QWasmLocalFileAccess::openFileDialog(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (qstdweb::FileList const&)> const&) (qwasmlocalfileaccess.cpp:131)
    at QWasmLocalFileAccess::openFiles(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (int)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> QWasmLocalFileAccess::openFiles(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (int)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> const&) (qwasmlocalfileaccess.cpp:156)
    at QWasmLocalFileAccess::openFile(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, std::__2::function<void (bool)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> QWasmLocalFileAccess::openFile(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, std::__2::function<void (bool)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> const&) (qwasmlocalfileaccess.cpp:169)
    at QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2::operator()() (qfiledialog.cpp:2444)
    at decltype(std::__2::forward<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(fp)()) std::__2::__invoke<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> decltype(std::__2::forward<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(fp)()) std::__2::__invoke<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&) (type_traits:3694)
    at void std::__2::__invoke_void_return_wrapper<void, true>::__call<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> void std::__2::__invoke_void_return_wrapper<void, true>::__call<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&) (__functional_base:348)worker.js onmessage() captured an uncaught exception: TypeError: Cannot read properties of undefined (reading 'createElement')
self.onmessage @ soffice.worker.js:207
soffice.worker.js:208 TypeError: Cannot read properties of undefined (reading 'createElement')
    at methodCaller_emscripten$$val_$std$$string$ (eval at new_ (47933e67-c694-46fe-af70-91dc8b50859c:8694:27), <anonymous>:5:20)
    at __emval_call_method (47933e67-c694-46fe-af70-91dc8b50859c:9267:14)
    at emscripten::internal::MethodCaller<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >::call(emscripten::internal::_EM_VAL*, char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> emscripten::internal::MethodCaller<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >::call(emscripten::internal::_EM_VAL*, char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) (val.h:278)
    at emscripten::val emscripten::val::call<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >(char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) emscripten::val emscripten::val::call<emscripten::val, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > >(char const*, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >&&) const (val.h:498)
    at QWasmLocalFileAccess::openFileDialog(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (qstdweb::FileList const&)> QWasmLocalFileAccess::openFileDialog(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (qstdweb::FileList const&)> const&) (qwasmlocalfileaccess.cpp:131)
    at QWasmLocalFileAccess::openFiles(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (int)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> QWasmLocalFileAccess::openFiles(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, QWasmLocalFileAccess::FileSelectMode, std::__2::function<void (int)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> const&) (qwasmlocalfileaccess.cpp:156)
    at QWasmLocalFileAccess::openFile(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, std::__2::function<void (bool)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> QWasmLocalFileAccess::openFile(std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> > const&, std::__2::function<void (bool)> const&, std::__2::function<char* (unsigned long long, std::__2::basic_string<char, std::__2::char_traits<char>, std::__2::allocator<char> >)> const&, std::__2::function<void ()> const&) (qwasmlocalfileaccess.cpp:169)
    at QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2::operator()() (qfiledialog.cpp:2444)
    at decltype(std::__2::forward<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(fp)()) std::__2::__invoke<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> decltype(std::__2::forward<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(fp)()) std::__2::__invoke<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&) (type_traits:3694)
    at void std::__2::__invoke_void_return_wrapper<void, true>::__call<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> void std::__2::__invoke_void_return_wrapper<void, true>::__call<QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&>(QFileDialog::getOpenFileContent(QString const&, std::__2::function<void (QString const&, QByteArray const&)> const&)::$_2&) (__functional_base:348)

Thanks in advance,

Balazs

[Sam Clegg]

It looks like that API only works on the main thread (since it uses the DOM).

If you really need to call such APIs on a background thread there are various ways to proxy the work back to the main thread.  See https://emscripten.org/docs/porting/pthreads.html#proxying.   If the code in question is not a JS library (e.g. if its written in EM_JS or EM_ASM) you can use `MAIN_THREAD_EM_ASM` and/or `emscripten_sync_run_in_main_thread` helpers.

cheers,

sam

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[Balázs Varga]

Thanks for getting back to me. :) Yes, the code is a c++ and not a JS library, so MAIN_THREAD_EM_ASM can be useful. But I have no idea yet how to make it work with that.

Best,

Balázs

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[Balázs Varga]

Hi!

I would have a question about the FS.writeFile function.




I tried to use is in c++ similar like this: https://github.com/emscripten-core/emscripten/issues/17230





But didn't worked for me with this:




void Test::read()
{
    auto onFileReady = [=](const QString &fileName, const QByteArray &fileContents) {
        if (fileName.isEmpty()) {
            // Report error1
        } else {
            emscripten::val fileContentView = emscripten::val(emscripten::typed_memory_view(fileContents.size(), fileContents.constData()));
            emscripten::val fileContentCopy = emscripten::val::global("ArrayBuffer").new_(fileContents.size());
            emscripten::val fileContentCopyView = emscripten::val::global("Uint8Array").new_(fileContentCopy);
            fileContentCopyView.call<void>("set", fileContentView);
            //emscripten::val contentArray = emscripten::val::array();
            //contentArray.call<void>("push", fileContentCopyView);
            std::string path = fileName.toStdString();
            EM_ASM({
                FS.writeFile(path, new Uint8Array([ 1, 2, 3, 4 ]));
            });
        }
    };
   
    QFileDialog::getOpenFileContent("*.*", onFileReady);
}




If I tried to use a variable like 'path' in the example, it didn't work.


EM_ASM({
          FS.writeFile(path, new Uint8Array([ 1, 2, 3, 4 ]));
});




But If I used like this, it works:

EM_ASM({
          FS.writeFile("/write_file.txt", new Uint8Array([ 1, 2, 3, 4 ]));
});




So my question is, how can I use a variable inside the EM_ASM({ }) ?

Furthermore, how can I convert a char* to Uint8Array for the FS.writeFile() function? I tried a lot of ways (like in the example with emscripten::val) but it didn't work.





Thanks for the help in advance.




All the best,

Balázs


[Balázs Varga]

I think I figured it out.

It looked like the file content was copied to the wasm memory with this:

void Test::read()

{

    auto onFileReady = [=](const QString &fileName, const QByteArray &fileContents) {

        if (fileName.isEmpty()) {

            // Report error1

        } else {

            std::string aFileName = "/home/user/" + fileName.toStdString();

            EM_ASM({

                FS.writeFile(UTF8ToString($0), new Uint8Array(Module.HEAPU8.buffer, $1, $2));

            }, aFileName.c_str(), fileContents.data(), fileContents.size());

        }

    };

   

    QFileDialog::getOpenFileContent("*.*", onFileReady);

}

But I am still not sure, that it is correct, even if it is worked somehow:

EM_ASM({

                FS.writeFile(UTF8ToString($0), new Uint8Array(Module.HEAPU8.buffer, $1, $2));

            }, aFileName.c_str(), fileContents.data(), fileContents.size());

Best,

Balázs