Qt for WebAssembly

Qt for WebAssembly makes it possible to build Qt applications for the web platform.

Qt for WebAssembly is currently in development. Qt 5.12 featured Qt for WebAssembly as a tech preview. From Qt 5.13 onwards, Qt for WebAssembly is a supported platform.

Note: The Qt documentation is the authoritative source on Qt for WebAssembly. This page contains additional platform notes.

Licensing

Qt for WebAssembly is available for commercial usage under the Qt Commercial license, and for open source usage under the GNU General Public License version 3. For more information, please check the general licensing information of Qt.

Getting Started

Qt-based applications are built for WebAssembly using the Emscripten SDK. Supported host/build platforms are Linux, macOS, and Windows Subsystem for Linux . Qt 5.13 will in addition support Windows with MinGW.

The getting started steps are:

1. Install the Emscripten SDK
2. Build Qt from source
3. Build and run applications

Install the Emscripten SDK

Install Emscripten according to instructions.

Each Qt minor release supports a known-good Emscripten version. This version has been tested to work well with the given Qt minor version, and will continue to be supported for the lifetime of that Qt version. We aim to make Qt work with newer Emscripten versions as well. Older Emscripten versions are not supported.

The known-good versions are:

• Qt 5.12: 1.38.16
• Qt 5.13: 1.38.27 (multithreading: 1.38.30)
• Qt 5.14: it's complicated (1.38.27)

Use emsdk to install specific emscripten versions (example):

• ./emsdk install sdk-fastcomp-1.38.27-64bit or (was) sdk-1.38.16-64bit
• ./emsdk activate --embedded sdk-fastcomp-1.38.27-64bit or (was) sdk-1.38.16-64bit

You may want to install multiple versions of the SDK, in which case the "-embedded" option is useful. It locates all configuration and cache files inside the SDK dir so that multiple versions do not conflict. The "64bit" in the SDK version refers to the host bit-ness.

Emsdk version for Qt 5.14

The Qt 5.14 binary packagers are produced with emsdk 1.38.27: make sure to use that version when using the binary package.

Starting with emsdk 1.39.0, the emcc compiler no longer compiles via asm.js, but instead generates wasm bytecode directly. Support for the 1.39.x series is in scope for Qt 5.14. However, be aware that the build flags are still tailored for the previous compiler, where minimizing the size of the intermediate asm.js was important.

It is possible to install the previous compiler by appending "-fastcomp" to the version, e.g. "1.39.0-fastcomp". See output from "./emsdk list". Qt 5.15 will most likely drop support for fastcomp and require the new compiler.

Build Qt from source

The Qt sources can be downloaded from your Qt Account, in the Downloads section. For the latest revision of the code, you can also check it out from our git repositories directly: http://code.qt.io/cgit/qt/qt5.git/

Configure Qt as a cross-compile build for the "wasm-emscripten" platform. This will implicitly set the "-static" and "-no-feature-thread" options. The Qt examples are not built by default in order to minimize build size and time. Pass the "-compile-examples" option to enable examples.

./configure -xplatform wasm-emscripten -nomake examples -prefix $PWD/qtbase

On Windows, make sure you have MinGW as well as sed in your PATH and configure with the following:

configure -no-warnings-are-errors -xplatform wasm-emscripten -platform win32-g++ -nomake examples -prefix %CD%\qtbase

Build required modules:

make module-qtbase module-qtdeclarative [other modules]

Build and run applications

We support the qmake build system. Run qmake from the Qt for WebAssembly build in the directory which contains the applications .pro file:

/path/to/qtbase/bin/qmake && make

The build process will produce several files: (with "appname" substituted for the actual application name)

Name	Description
appname.wasm	Main application binary
appname.js	Emscripten JavaScript runtime
appname.html	Html container
qtloader.js	Qt JavaScript runtime

Run the application by loading the appname.html file in a web browser. The files must be served by a web server using http(s). Start a web server (e.g. "python3 -m http.server"), open e.g. localhost:8000/app.html in a web browser. We test on Chrome, Firefox, and Safari (all desktop). Wasm compiling and loading performance wary between the browsers; test several and find one which works for you.

You can also use:

/path/to/emscripten/emrun --browser=firefox appname.html

Debug and logging output go to the browser's debug console. If you are using Firefox, you can use CTRL+SHIFT+K to open the debug console.

Platform Notes

The Qt for WebAssembly platform port has limitations; these are due to the sandboxed nature of the web platform, the platform APIs provided (or not provided), and the incomplete nature of the Qt port. This section describes several topics which may be of interest for application developers.

Supported target browsers/devices

Desktop:

• Chrome
• FireFox
• Safari
• Edge (Chrome)

If the browser supports WebAssembly then Qt should run. Note that Qt has fixed WebGL requirement, also for apps that do not use WebGL directly. Browsers often blacklist WebGL for older/unsupported GPUs.

Mobile:

• Android Browser
• Mobile Safari

Note: There is currently no support for text input using the virtual keyboard. Safari currently does not support wasm modules of the size Qt produces.

In general, Qt does not make direct use of operating system features and it makes no difference if e.g. FireFox runs on Windows or macOS. Qt does use some operating system adaptions, for example for ctrl/cmd key handling on macOS.

Supported Qt Modules

Qt for WebAssembly supports a subset of the Qt modules. The currently supported modules are

• qtbase
• qtdeclarative
• qtquickcontrols2
• qtwebsockets
• qtsvg
• qtcharts
• qtmqtt

Unsupported:

• qtmultimedia
• qtwebview

Other modules are not tested and may or may not work.

Multithreading Support

Qt on WebAssembly supports multithreading, however support is disabled by default in order to be compatible with as many browsers as possible. Thread support can be enabled by building Qt from source and configuring with the "-feature-thread" flag. The WebAssembly binary packages do not support multithreading.

The minimum Emscripten SDK version is 1.38.30.

Threads are supported in some (but not all) browsers. Configuration changes may be required. The mandelbrot demo can be used to determine if thread support is available. Note that it is the build mode which determines if browser thread support is required, not whether the application starts a thread or not.

Current Browser Status:

• Chrome (and derivatives): supported
• Safari: not supported
• FireFox: can be enabled

On FireFox, open about:config and make sure javascript.options.shared_memory in about:config is enabled.

Firefox 71 and higher has additional requirements and the web server must set two additional headers:

• Cross-Origin-Opener-Policy: same-origin
• Cross-Origin-Embedder-Policy: require-corp

Alternatively, on Nightly and Beta channels only, set "dom.postMessage.sharedArrayBuffer.bypassCOOP_COEP.insecure.enabled" to "true"

QTBUG-79087 documents how to build a web server for development purposes which supports setting headers and secure serving serving over https.

The Emscripten pthreads documentation contains relevant documentation for multithreading.

Applications may need to add two settings to its .pro file when enabling threads:

Thread Pool Size Applications should set the expected number of concurrent threads at build time. This can be done by setting QMAKE_WASM_PTHREAD_POOL_SIZE in the .pro file (maps to Emscripten PTHREAD_POOL_SIZE). Applications can exceed PTHREAD_POOL_SIZE, provided they return main thread control to the browser before waiting on the new thread, for example by returning from the event handler that started the new thread. This allows the browser to start another web worker. Immediately waiting for the new thread on the main thread (using QThread::wait() or similar) will deadlock. Qt sets PTHREAD_POOL_SIZE to 4 by default.

Heap Memory Size Applications should set the heap memory size at build time, since growing the heap is not supported with pthreads enabled. This can be be done by setting QMAKE_WASM_TOTAL_MEMORY in the .pro file (maps to Emscripten TOTAL_MEMORY). Browsers typically limit the initial WASM memory allocation size to 1GB. Qt sets TOTAL_MEMORY to 1GB by default (for -feature-thread enabled builds)

Files and local file system access

File system access is sandboxed, which has implications for how the application works with files. The Web platform provides APIs for accessing the local file system and persisted storage. Emscripten and Qt wraps these features and provides APIs which are easier to access from C++ and Qt-based applications.

Web platform features:

• <input type="file"> for showing a native open-file dialog where the user can pick a file.
• IndexedDB for persistent local storage (not accessible outside the browser)

Emscripten provides file systems with a POSIX like API

• MEMFS ephemeral file system - files are stored in-memory
• IDBFS persistent file system - files are stored using IndexedDB

Emscripten mounts a MEMFS filesystem to "/" at app startup.

Qt uses Emscripten's file system and also provides other APIs:

• QFile can be used as usual. Stored files go to MEMFS by default.
• QSettings has an IndexedDB-based backend.
• QFileDialog::getOpenFileContent() opens a native file dialog where the user can pick a file
• QFileDialog::saveFileContent() saves a file to the local file system

Known issues

• Recommended emsdk version: 1.38.27 (multithreading: 1.38.30)
• Supported development host systems: Linux, MacOS, and Windows (MinGW)
• Qt supports copying and pasting text to the system clipboard
  • Browsers that support the Clipboard API are preferred. Note that a requirement for this API is that the web page is served over a secure connection (e.g. https).
    • Chrome supports the Clipboard API
    • Firefox supports the Clipboard API behind a flag: dom.events.asyncClipboard.dataTransfer
  • Browsers that will send clipboard events to Qt's canvas element are also supported
    • This mode supports the CTRL+x/c/v keyboard shortcuts only
    • Ongoing work. Firefox works well, other browsers have some hiccups.

Browser configuration and extensions

Web browsers configurable and extendible, sometimes with surprising side effects for WebAssembly-based applications:

• JavaScript blockers may block JavaScript without enabling the NoScript. This means that <noscript> content is not displayed, and that the application appears to be stuck at the loading screen.
• Some ad-blockers block all .wasm files from github.com
• privacy.resistFingerprinting=true (FireFox) disables high-dpi support - the browser will appear to be running on a standard-dpi display.

General Notes

• Debugging: Qt debug and logging output is printed on the JavaScript console, which can be accessed via browser "Developer Tools" or similar.
• Nested event loops are not supported. Applications should not call e.g. QDialog::exec() or create a new QEventLoop object.
• Qt renders application content to a canvas element, and does not use (other) native DOM elements. This means accessibility (screen readers) are not supported and that text inputs won't trigger virtual keyboards.
• WebGL is required, even for applications which do not use OpenGL themselves. All relevant browsers support WebGL, but note that some browsers blacklist certain older GPUs. The Qt loader will detect this and display an error message.
• Mixing OpenGL and raster content is not supported
  • Child OpenGL windows are not supported. The window compositor (in the Qt for Wasm platform plugin) supports raster windows only.
  • QOpenGLWidget is not fully supported: https://bugreports.qt.io/browse/QTBUG-66944
• To target WebAssembly version specifically in qmake, use emscripten as the platform name: emscripten { message("Building for WebAssembly") }
• Qt will detect OpenGL support as OpenGL ES. In reality the browser will be providing WebGL. WebGL is based on ES and is very similar, but there are some incompatibilities. See WebGL and OpenGL Differences
• Applications do not have access to system fonts. Font files must be distributed with the application, for example in Qt resources. Qt for WebAssembly itself embeds one such font.
• High-DPI and scaling: High-DPI rendering is supported, and so is setting the overall UI visual size using the browser zoom feature. Browser font size (and type) settings have no effect on Qt applications.
• There may be artifacts of uninitialized graphics memory on some Qt Quick Controls 2 components, such as checkboxes.
• Network access: The web sandbox limits network access to a subset of what is available for native apps.
  • QNetworkAccessManager http requests to the web page origin server, or to a server which supports CORS.
  • QWebScoket connections to any host.
  • TCP and UDP socked tunneling using over WebSockets using a websockify server [implemented by Emscripten, not tested].
    • Websockify v0.8.0 can be used to tunnel TCP connections with QT5.12 but it is MANDATORY to specify the base64 or binary subprotocols before calling QWebSocket::open(). For example:

QWebSocket socket;

QUrl url{QString("ws://server:port")};

QNetworkRequest request{url};

request.setRawHeader("Sec-WebSocket-Protocol", "binary");

socket.open(request);

• Link-time warnings of the form: "cannot represent a NaN literal '0x7fdae4bde910' with custom bit pattern", are expected.
• Expected footprint (download size): Wasm modules as produced by the compiler can be large, but compress well.

Example	gzip	brotli
helloglwindow (QtCore + QtGui)	2.8M	2.1M
wiggly widget (QtCore + QtGui + QtWidgets)	4.3M	3.2M
SensorTag (QtCore + QtGui + QtWidgets + QtQuick + QtCharts)	8.6M	6.3M

Compression is typically handled on the web server side, using standard compression features: the server compresses automatically or picks up pre-compressed versions of the files. There's generally no need to have special handling of wasm files.

Qt Development

The main development tracking bug is https://bugreports.qt.io/browse/QTBUG-63917.