Qt for WebAssembly

Qt for Webassembly lets you to run Qt applications on the web.

WebAssembly (abbreviated Wasm) is a binary instruction format intended to be executed in a virtual machine, for example in a web browser.

With Qt for WebAssembly, you can distribute your application as a web application that runs in a browser sandbox. This approach is suitable for web distributed applications that do not require full access to host device capabilities.

Getting Started with Qt for WebAssembly

Building Qt applications for WebAssembly is similar to building Qt for other platforms. You need to install an SDK (Emscripten), install Qt (or build Qt from source), and finally, build the application. Some differences exist, for example, Qt for WebAssembly supports fewer modules and less features than other Qt builds.

Installing Emscripten

Emscripten is a toolchain for compiling to WebAssembly. It lets you run Qt on the web at near-native speed without browser plugins.

Refer to the Emscripten documentation for more information about installing the Emscripten SDK.

After installation, you should have the Emscripten compiler in your path. Check this with the following command:

em++ --version

Each minor version of Qt targets a specific Emscripten version, which remains unchanged in patch releases. Qt's binary packages are built using the target Emscripten version. Applications should use the same version since Emscripten does not guarantee ABI compatibility between versions.

The Emscripten versions are:

• Qt 6.2: 2.0.14
• Qt 6.3: 3.0.0
• Qt 6.4: 3.1.14
• Qt 6.5: 3.1.25
• Qt 6.6: 3.1.37
• Qt 6.7: 3.1.50

Use emsdk to install specific Emscripten versions. For example, to install it for Qt 6.7 enter:

• ./emsdk install 3.1.50
• ./emsdk activate 3.1.50

On Windows, Emscripten is in your path after installation. On macOS or Linux you need to add it to your path, like this:

source /path/to/emsdk/emsdk_env.sh

Check this with the following command:

em++ --version

You can build Qt from source if you require more flexibility when selecting the Emscripten version. In this case the versions above are minimum versions. Later versions are expected to work but may introduce behavior changes which require making changes to Qt.

Installing Qt

Download Qt from the Downloads section of your Qt account. We provide builds for Linux, macOS, and Windows as development platforms.

The binary builds are designed to run on as many browsers as possible, and come in single-threaded and multi-threaded versions. Non-standard features such as Wasm SIMD and Wasm exceptions are not supported by the binary builds.

Building Qt from Source

Building from source lets you set Qt configuration options such as thread support, OpenGL ES level, or SIMD support. Download the Qt sources from the Downloads section of your Qt account.

Configure Qt as a cross-compile build for the wasm-emscripten platform. This sets the -static, -no-feature-thread, and -no-make examples configure options. You can enable thread support with the -feature-thread, configure option. Shared library builds are not supported.

You need a host build of the same version of Qt and specify that path in the QT_HOST_PATH CMake variable or by using the -qt-host-path configure argument.

Although it should be detected, you may optionally set the CMAKE_TOOLCHAIN_FILE CMake variable to the Emscripten.cmake toolchain file that comes with Emscripten SDK. This can be done by setting the environment variable CMAKE_TOOLCHAIN_FILE or by passing CMAKE_TOOLCHAIN_FILE=/path/to/Emscripten.cmake to configure.

./configure -qt-host-path /path/to/Qt -platform wasm-emscripten -prefix $PWD/qtbase

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

configure -qt-host-path C:\Path\to\Qt -no-warnings-are-errors -platform wasm-emscripten -prefix %CD%\qtbase

Then build the required modules:

cmake --build . -t qtbase -t qtdeclarative [-t another_module]

Building Applications on the Command Line

Qt for WebAssembly supports building applications using qmake and make, or CMake with ninja or make.

$ /path/to/qt-wasm/qtbase/bin/qt-cmake .
$ cmake --build .

Building the application generates several output files, including a .wasm file that contains the application and Qt code (statically linked), a .html file that can be opened in the browser to run the application.

Running Applications

Running the application requires a web server. The build output files are all static content, so any web server will do. Some use cases might require special server configuration, such as providing https certificates or setting http headers required to enable multithreading support.

Emrun

Emscripten provides the emrun utility for test-running applications. Emrun starts a web server, launches a browser, and will also capture and forward stdout/stderr (which will normally go to the JavaScript console).

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

Python http.server

Another option is to start a development web server and then launch the web browser separately. One of the simplest options is http.server from Python:

python -m http.server

Note that this is only a simple webserver and does not support SharedArrayBuffer required for threading, as the required COOP and COED headers mentioned below are not sent.

qtwasmserver

Qt provides a developer web server which uses mkcert to generate https certificates. This allows testing web features which require a secure context. Note that delivery over http://localhost is also considered secure, without requiring a certificate.

The web server also sets the COOP and COEP headers to values which enables support for SharedArrayBuffer and multi-threading.

The qtwasmserver script starts one server which binds to localhost by default. You may add additional addresses using the -a command-line argument, or use --all to bind to all available addresses.

python /path/to/qtbase/util/wasm/qtwasmserver/qtwasmserver.py --all

Building Applications using Qt Creator

Setting Up Qt Creator for WebAssembly.

Deploying Applications on the web

Building an application generates several files (substitute "app" with the application name in the following table).

You can deploy app.html as-is, or discard it in favor of a custom HTML file. Smaller adjustments, such as changing the splash screen image from the Qt logo to the app logo, is also possible. In both cases, qtloader.js provides a JavaScript API for loading the application.

Compress the Wasm file using either gzip or brotli before deploying, as they offer better compression ratio than the other tools. See Minimizing the size of binaries for more information.

Enabling certain features, such as multi-threading and SIMD, produces .wasm binaries that are incompatible with browsers that do not support the enabled feature. It is possible to work around this limitation by building multiple .wasm files and then use JavaScript feature detection to select the correct one, but note that Qt does not provide any functionality for doing this.

Using qtloader

Qt provides a JavaScript API for downloading, compiling, and instantiating Qt for WebAssembly applications. This loading API wraps loading functionality provided by Emscripten, and provides additional features useful for Qt-based applications. It is implemented in the qtloader.js file. A copy of this file is written to the build directoty at build time.

Typical usage looks like the following:

const app_container_element = ...;
const instance = await qtLoad({
    qt: {
        containerElements: [ app_container_element ],
        onLoaded: () => { /* handle application load completed */  },
        onExit: () => {  /* handle application exit */ },
    }
});

The code calls the qtLoad() loader function with a configuration object. This configuration object can contain any emscripten configuration options, as well as a special "qt" configuration object. The qt configuration object supports the following properties:

The containerElements array is the main interface between Qt and the web page, where the html elements in this array (typically <div> elements) specify the location of the application content on the web page.

The application sees each container element as a QScreen instance, and can place application windows on the screen instances as usual. Windows with the Qt::WindowFullScreen state set use the entire screen area, while non-"fullscreen" windows get window decorations.

The qtLoad() function returns a promise, which yelds an Emscripten instance when awaited. The instance provides access to Embind exported functions. Qt exports several such functions, and these functions make up the instance API.

Using the Qt instance API

Qt provides several instance functions. Currently, these support adding and removing container elements at runtime.

Porting to the Qt 6.6 qtloader

Qt 6.6 includes a new qtloader with a simplified implementation and a smaller scope. This includes API changes which may require porting application JavaScript code. Qt provides a compatibility API to ease the transition. Depending use case there are several ways forward:

• If you are using the generated app.html file directly then this file will be updated as well at build time. No action is needed.
• If you are using the basic qtloader feature set then you may use the compatibility API included in Qt 6.6 as a temporary maesure. This API will be removed in a future release; you should plan on updating to use the new qtloader. Porting step 1 below is needed.
• If you are using advanced features (such as adding container elements at runtime), then porting to the new loader or instance API required. Porting steps 1 and 2 below are needed.

Porting steps

1. Include the app.js (JavaScript runtime generated by Emscripten) from the loading html file.

   <script src="app.js"></script>

   Before Qt 6.6, qtloader would load and evaluate this JavaScript file. This is no longer done, and the file must be included using a <script> tag.

2. Port to using the new JavaScript and instance API.

See documentation sections above.

Supported Browsers

Desktop

Qt for WebAssembly is developed and tested on the following browsers:

• Chrome
• Firefox
• Safari
• Edge

Qt should run if the browser supports WebAssembly. Qt has a fixed WebGL requirement, even if the application itself does not use hardware accelerated graphics. Browsers that support WebAssembly often support WebGL, though some browsers blacklist older or unsupported GPUs. s/qtloader.js provides APIs to check if WebGL is available.

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

Mobile

Qt for WebAssembly applications runs on mobile browsers such as mobile Safari and Android Chrome.

Supported Qt Modules

Qt for WebAssembly supports a subset of the Qt modules and features. Tested modules are listed below, other modules may or may not work.

• Qt Core
• Qt GUI
• Qt Network
• Qt Widgets
• Qt Qml
• Qt Quick
• Qt Quick Controls
• Qt Quick Layouts
• Qt 5 Core Compatibility APIs
• Qt Image Formats
• Qt OpenGL
• Qt SVG
• Qt WebSockets

In all cases, module support may not be complete and there may be additional limitations, either due to the browser sandbox or due to incompleteness of the Qt platform port. See Developing with Qt for WebAssembly for further info.

Qt for WebAssembly Technology Preview modules and features. These features may require to reconfigure and build Qt. They may contain features that are still experimental in the browsers or Emscripten.

• Qt Threading
• Qt Concurrent
• Emscripten Asyncify
• Qt Sockets

Developing with Qt for WebAssembly

Building with CMake

If there is a need for Emscripten-specific configuration in CMake, the following code can be utilized:

if(EMSCRIPTEN)
    # WebAssembly specific code
else()
    # other platforms
endif()

This code allows for the accommodation of Emscripten-specific configurations while ensuring compatibility with other platforms.

OpenGL and WebGL

Qt requires WebGL, also for applications which do not use OpenGL directly. 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.

Qt detects WebGL as OpenGL ES, with the following version mapping:

OpenGL ES 2 and OpenGL ES 3 are enabled by default, and can be selected through the QSurfaceFormat::setMajorVersion() function.

Web and Desktop OpenGL differences are documented in WebGL and OpenGL Differences. There are additional differences between WebGL 1.0 and WebGL 2.0, documented in the WebGL 2.0 Specification.

A WebGL-friendly subset of ES2 (and ES3) is used by default. If you need to use glDrawArrays and glDrawElements without bound buffers, you can enable full ES2 support by adding

target_link_options(<your target> PRIVATE -s FULL_ES2=1)

and/or full ES3 emulation by adding

target_link_options(<your target> PRIVATE -s FULL_ES3=1)

to your project's CMakeLists.txt.

Multithreading

Qt for WebAssembly supports multithreading using Emscripten's Pthreads support, where each thread is backed by a web worker. Enable multithreading by installing the "WebAssembly (multi-threaded)" component from Qt Maintenance Tool, or by building Qt from source and passing the "-feature-thread" flag to configure.

Existing threading code can generally be reused, but may need to be modified to work around specifics of the pthread implementation. Some Emscripten and Qt features are not supported, this includes the thread proxying feature and the Qt Quick threaded render loop.

Be aware that it is especially important to not block the main thread on Qt for WebAssembly, since the main thread might be required to service requests from secondary threads. For example, all timers in Qt are scheduled on the main thread, and will not fire if the main thread is blocked. Another example is that creating a new web worker (for a thread) can only be done from the main thread.

Emscripten provides some mitigations for this. Short-term waits such as acquiring a mutex lock is supported by busy-waiting and processing events while waiting for the lock. Longer waits on the main thread should be avoided. In particular, the common practice of calling QThread::wait() or pthread_join() to wait for a secondary thread will not work, unless the application can guarantee that the thread (and web worker) has already been started, and will be able to complete without assistance from the main thread at the time that the wait() or join() call is made.

The multithreading feature requires browser support for the SharedArrayBuffer API. (Normally, Emscripten stores the heap in an ArrayBuffer object. For multithreading, the heap must be shared with web workers and a SharedArrayBuffer is needed) This API is generally available in all modern browsers, but may be disabled if certain security requirements are not met. WebAssembly binaries with thread support enabled will then fail to run, also if the binary does not actually start a thread.

Enabling SharedArrayBuffer requires a secure browsing context (where the page is served over https:// or http://localhost), and that the page is in cross-origin isolated mode. The latter can be done by setting the so called COOP and COEP headers on the web server:

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

SIMD

Emscripten supports WebAssembly SIMD, which provides 128-bit SIMD types and operations for WebAssembly.

Build Qt from source and configure with the -feature-wasm-simd128 flag to enable; this will pass the -msimd128 flag at compile and link time. Note that Qt does not contain wasm-simd optimized code paths at this point, however enabling wasm-simd will enable compiler auto-vectorization where the compiler can use the SIMD instructions.

You can target WebAssembly SIMD directly using either GCC/Clang SIMD Vector Extensions or WASM SIMD128 intrinsics. For more information, see the Emscripten SIMD documentation .

In addition, Emscripten supports emulating/translating x86 SSE instructions to Wasm SIMD instructions. Qt does not use this emulation, as the use of SSE SIMD instructions that have no native Wasm SIMD equivalent may cause reduced performance.

Note that SIMD-enabled binaries are incompatible with browsers that do not support WebAssembly SIMD, also if the SIMD code paths are not called at run-time. SIMD support may need to be enabled in the browsers advanced configurations, such as 'about:config' or 'chrome:flags'

Networking

Qt provides limited support for networking. In general, network protocols which are already in use on the web can be use also from Qt, while others are not directly available due to the web sandbox.

The following protocols are supported:

• QNetworkAccessManager http requests to the web page origin server, or to a server which supports CORS. This includes XMLHttpRequest from QML.
• QWebSocket connections to any host. Note that web pages served over the secure https protocol allows websockets connections over secure wss protocol only.
• Emulated POSIX TCP Sockets over WebSockets, using functionality provided by Emscripten. Note that this requires running a forwarding server which handles socket translation.

All other network protocols are not supported.

Local File Access

File system access is sandboxed on the web, and this has implications for how the application works with files. The Web platform provides APIs for accessing the local file system in a way which is under user control, as well as APIs for accessing persistent storage. Emscripten and Qt wraps these features and provides APIs which are easier to use from C++ and Qt-based applications.

The web platform provides features for accessing local files and persistent storage:

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

Emscripten provides several file systems with a POSIX like API. These include:

• the MEMFS ephemeral file system which stores files in-memory
• the IDBFS persistent file system which stores files using IndexedDB

Emscripten mounts a temporary MEMFS filesystem to "/" at app startup. This means that QFile can be used, and will read and write files to memory by default. Qt provides other API as well:

• QSettings has an IndexedDB-based backend; Note that QSettings is asynchronous on WebAssembly.
• QFileDialog::getOpenFileContent() opens a native file dialog where the user can pick a file
• QFileDialog::saveFileContent() saves a file to the local file system via file download

Clipboard Access

Qt supports copying and pasting text to the system clipboard, with some differences due to the web sandbox. In general clipboard access require user permission, which can be obtained by handling an input event (e.g. CTRL+c), or by using the Clipboard API.

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), and that some browsers my require changing configuration flags.

• Chrome version 66 and Safari version 13.1 support the Clipboard API
• Firefox version 90 supports the Clipboard API if you enable the following flags in 'about:config':

  dom.events.asyncClipboard.read
  dom.events.asyncClipboard.clipboardItem

Fonts

The Qt WASM module contains 3 embedded fonts: "Bitstream Vera Sans" (fallback font), "DejaVu Sans", "DejaVu Sans Mono".

These fonts provide a limited character set. Qt provides several options for adding additional fonts:

One is using FontLoader in QML, which can either fetch a font by URL or using Qt Resource System (the same way the usual desktop apps work).

The other way to use font is to add it via QFontDatabase::addApplicationFontFromData.

Application Startup and the Event Loop

Qt for WebAssembly supports the standard Qt startup approach, where the application creates a QApplication object and calls the exec function:

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    QWindow appWindow;

    return app.exec();
}

The exec() call above normally blocks and processes events until application shutdown. Unfortunately this is not possible on the web platform where blocking the main thread is not allowed. Instead, control must be returned to the browser's event loop after processing each event.

Qt works around this by making exec() return main thread control to the browser, while preserving the stack. From the point of view of application code, the exec() function is entered and event processing happens as usual. However, the exec() call never returns, also not on application exit.

This behavior is usually acceptable since the browser will free up application memory at app shutdown time. It does mean that shutdown code does not run, since the application object is leaked and its destructor does not run.

You can avoid this by rewriting main() to be asynchronous, which is possible since Emscripten does not exit the runtime when main() returns. Application code then omits making the exec() call, and can shut down Qt cleanly by deleting the top-level window and application objects.

QApplication *g_app = nullptr;
AppWindow *g_appWindow = nullptr;

int main(int argc, char **argv)
{
    g_app = new QApplication(argc, argv);
    g_appWindow = new AppWindow();
    return 0;
}

Asyncify

The default build of Qt for WebAssembly does not support reentering the event loop, for example by calling QEventLoop::exec() or QDialog::exec(), due to restrictions of the web platform.

Emscripten's asyncify feature lifts these restrictions by allowing synchronous calls (like QEventLoop::exec() and QDialog::exec()) to yield to the event loop. Nested calls are not supported, and for this reason asyncify is not used for the top-level QApplication::exec() call.

Features that require asyncify are:

• QDialogs, QMessageBoxes with return values.
• Drag and drop (specifically drag).
• Nested/secondary event loops exec().

As of Qt 6.4, Asyncify support is enabled in the binary package, but needs to be enabled for applications by adding -sASYNCIFY -Os to linker options:

CMake:

target_link_options(<your target> PUBLIC -sASYNCIFY -Os)

qmake:

QMAKE_LFLAGS += -sASYNCIFY -Os

Enabling asyncify adds overhead in the form of increased binary sizes and increased CPU usage. Build with optimizations enabled to minimize the overhead.

Debugging and Profiling

Wasm debugging is done on browser JavaScript console, debugging applications on Wasm directly within Qt Creator is not possible.

• Qt debug and logging output is printed on the JavaScript console, which can be accessed via browser "Developer Tools" or similar.
• Source maps for stepping through code, can be created by re-configuring Qt with the –device-option QT_WASM_SOURCE_MAP=1, and building a debug build.
  • https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Use_a_source_map
  • https://developers.google.com/web/tools/chrome-devtools/javascript/source-maps#source_maps_in_devtools_sources_panel
• Debug symbols via DWARF are also enabled if the program is linked with the -g flag (tested with Chrome)
• This will require this extension: https://goo.gle/wasm-debugging-extension
• See also https://developer.chrome.com/blog/wasm-debugging-2020/
• Mobile browsers can use remote debugging
  • https://developer.mozilla.org/en-US/docs/Tools/Remote_Debugging
  • https://developers.google.com/web/tools/chrome-devtools/remote-debugging
  • https://developer.apple.com/safari/tools/
• To stop execution on a certain line and popup the browser debugger programmatically, you can add the function emscripten_debugger(); to the application source code.
• Profiling can be accomplished by using a debug build and the JavaScript console profiling features. Qt adds –profiling-funcs to the linker arguments in debug builds, which preserve function names in profiling

You can add more verbosity to help debug using Emscripten linker arguments:

• -s LIBRARY_DEBUG=1 (print out library calls)
• -s SYSCALL_DEBUG=1 (print out sys calls)
• -s FS_LOG=1 (print out filesystem operations)
• -s SOCKET_DEBUG (print out socket, network data transfer)

CMake:

target_link_options(<your target> PRIVATE -s LIBRARY_DEBUG=1)

qmake:

QMAKE_LFLAGS_DEBUG += -s LIBRARY_DEBUG=1

Optimizing

Qt for WebAssembly uses the Emscripten toolchain to generate binaries, and there are many flags that may impact performance and the size of binaries. See Emscripten: Optimizing Code for more information.

You can pass linker and compiler flags just as for normal C++ applications: CMake qmake

target_compile_options(<your target> PRIVATE -oz -flto)
target_link_options(<your target> PRIVATE -flto)

QMAKE_CXXFLAGS += -oz -flto
QMAKE_LFLAGS += -flto

Minimizing the size of binaries

In order to provide a seamless user experience, it's important to reduce the time to download and load WebAssembly applications. Smaller application binary is one of the important aspects that enable faster download. Use the following alternatives to reduce the binary size:

• Make sure to distribute release builds. Debug builds contain debug symbols and are much bigger.
• Enable compression on a server. The most common algorithms like gzip and Brotli work well on Wasm binaries and can drastically reduce their size.
• Try compiler and linker flags that may results in generating smaller binaries (i.e. '-os', '-oz'). Results will vary depending on the particular application.
• Disable unsued features when compiling Qt for WebAssembly from source (see below).

Opting out of features

A WebAssembly application by default links statically to the Qt libraries, enabling the compiler to eliminate dead code. However, due to Qt's dynamic nature it's not always possible for the compiler to perform such optimizations.

If you build Qt for WebAssembly from source, you can disable features to reduce the size of Qt binaries, and — as a result — the size of .wasm binaries. Qt disables some features by default for the WebAssembly platform, but you can also disable features that your application does not use. See disabled features for more information.

You can disable the following features to reduce binary size (usually by 10-15%):

Wasm Exceptions

Qt is built without exception support by default, where throwing an exception will abort the program. WebAssembly exceptions can be enabled by building from source and passing the -feature-wasm-exceptions flag to Qt configure. This will pass the -fwasm-exceptions flag to the compiler at compile and link time. Qt does not support enabling Emscripten's support for the earlier JavaScript-based exception implementation.

Note that calling QApplication::exec() is not supported when exceptions are enabled, due to internal implementation details. Instead, write main() in the form where it returns early and does not call exec(), as described in Application Startup and the Event Loop.

Shared Libraries and Dynamic Linking Developer Preview

Qt for WebAssembly uses static linking by default, where the application is deployed as a single WebAssembly file which contains the Qt libraries and application code. Dynamic linking is an alternative build mode where each library and plugin is distributed individually.

For instance, an application which uses Qt Quick may make use of the following libraries and plugins:

• <qtpath>/lib/libQt6Core.so
• <qtpath>/lib/libQt6Gui.so
• <qtpath>/lib/libQt6Qml.so
• <qtpath>/lib/libQt6Quick.so
• <qtpath>/plugins/imageformats/libqjpeg.so
• <qtpath>/plugins/imageformats/libqjgif.so
• <qtpath>/qml/QtQuick/Window/libquickwindowplugin.so

Dynamic linking support is currently in developer preview. The implementation is suitable for prototyping and evaluation, but is not suitable for production use. Current limitations and restrictions include:

• The Emscripten SDK must be patched. Use emsdk 3.1.37, and apply this patch.
• The Chrome browser is not supported, due it not supporting synchronous loading of wasm files larger than 4K.
• Multithreading is not supported.
• Asyncify is not supported.

Quick Start

The build and deployment procedure is slightly different than that from static wasm and shared desktop builds. Consider starting with a small example before progressing to a full application build.

1. Build Qt from source, pass the “-shared” option to the Qt configure script.
2. Build your application using Qt from step 1.
3. Deploy the Qt installation by copying or linking to a directory named "qt" in the application directory
   • ln -s <qtpath> qt
   • cp -r <qtpath> qt
4. Create plugin preloading lists by running deployment scripts.
   • <qtpath>/qtbase/util/wasm/preload/deploy_qt_plugins.py <qtpath>
   • <qtpath>/qtbase/util/wasm/preload/deploy_qml_imports.py <qthostpath> <qtpath>

Shared Libraries Deployment in Depth

The shared libraries build of Qt is deployed in two stages, where the first stage makes the Qt and application build available for download from the web server, and the second stage downloads required Qt plugins and Qt Quick imports at application startup.

In the first step, make the Qt installation available for download from the web server. Depending on the specifics of the web server setup there may be different ways to accomplish this. In common is that the Qt loader expects to find the Qt libraries and plugins in a directory name "qt", relative to the html file which loads the application.

If you are already copying the application to the web server as a part of deployment, then copying Qt as well is a possible option. If you are serving the application directly from its build directroy - often the case during development phases - then creating a symlink to Qt can work well.

Prepare for the second step by creating preload lists for Qt components such as plugins and Qt Quick imports. Preloading ensures that all required Qt components are availabe at application startup. Delayed loading, where components are dowloaded on demand, is also possbile but is not covered here.

Preloading is implemented by the Qt JavaScript loader, which downloads files from the web server to the in-memory file system provided by Emscripten. Which files to download is specified using json-formatted download lists. Qt provides two scripts for generating preload lists, see Quick Start section above.

Known Issues

• Nested event loops are not supported. Applications should not call API like QDialog::exec() and QEventLoop::exec(). Experimental feature Asyncify could be used.
• Printing is not supported
• QDnsLookup lookups, QTcpSocket, QSsl do not work and are not supported due to the web sandbox
• Fonts: Wasm sandbox does not allow access to system fonts. Font files must be distributed with the application, for example in Qt resources or downloading. Qt for WebAssembly itself embeds one such font.
• There may be artifacts of uninitialized graphics memory on some Qt Quick Controls 2 components, such as checkboxes. This can sometimes be seen on HighDPi displays.
• Native styles for Windows and macOS are not supported as Wasm as a platform is not providing that capability
• Link time error such as "wasm-ld: error: initial memory too small", requires adjustment of the initial memory size. Use QT_WASM_INITIAL_MEMORY to set the initial size in kb, which must be a multiple of 64KB (65536). Default is 50 MB. In CMakeLists.txt: set_target_properties(<target> PROPERTIES QT_WASM_INITIAL_MEMORY "150MB")
• add_executable in CMakeLists.txt does not produce <target>.html or copy qtloader.js. Use qt_add_executable instead.
• QWebSocket connections are supported by Emscripten only on the main thread.
• QWebSockets for WebAssembly does not support sending ping or pong frames, as the API available to web pages and browsers does not expose this functionality.
• Runtime error such as "RangeError: Out of memory" can be worked around by setting MAXIMUM_MEMORY to a value the device supports, for example

  target_link_options(<your target> PRIVATE -s MAXIMUM_MEMORY=1GB)

• To use QtWebsockets, the subprotocol might need to be set to 'mqtt' for using QtMqtt. Use QWebSocketHandshakeOptions when you open the QWebSocket.

Other Topics

Qt Configure Options Reference

The following configure options are relevant when building Qt for WebAssembly from source.

Qt disables some features by default for the WebAssembly platform, to reduce the binary size. You can explicitly enable a feature when you configure Qt for WebAssembly:

Typical Download Sizes

Expected footprint (download size): Wasm modules as produced by the compiler can be large, but compress well:

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.

For more information, see Minimizing the size of binaries.

Examples

• Industrial Panel Demo
• QMainWindow app
• A gallery of available controls in Qt Quick Controls
• Web app for ordering pizzas

External resources

• WebAssembly Resource site

License

Qt for WebAssembly is available under commercial licenses from The Qt Company. In addition, it is available under the GNU General Public License, version 3. See Qt Licensing for further details.