Frequently Asked Questions

Building and deployment

Q: I built Wt and the examples, but many examples don't display correctly

If you are running the built example from within the build directory, then the examples will not find their resource bundles (.xml) files and the resources (CSS, images) will not be where they are to be expected.

The examples are designed so that you can run them without much hassle from the source directory.

For example, if you built Wt in a build subdirectory of Wt and want to run composer, you can do the following:

cd wt/examples/composer   # source directory for composer example
../../build/examples/composer/composer.wt --docroot . --http-listen --resources-dir ../../resources

Applications that have explicit approot and/or docroot directories (e.g. widgetgallery), must be started with the appropriate ---approot and ---docroot parameters:

cd wt/examples/widgetgallery # source directory for composer example
../../build/examples/widgetgallery/widgetgallery.wt --docroot docroot --approot approot --http-listen --resources-dir ../../resources

Q: How do I build my newly written "Hello World!" application?

Wt itself, and the examples, use CMake, but that is entirely a personal choice. You can use any build environment, like qmake, where you:

  • specify the library directory (Wt defaults to installing in /usr/local/lib)
  • specify the link libraries:
    • -lwt and -lwthttp or -lwtfcgi (for release build)
    • -lwtd and -lwthttpd or -lwtfcgid (for debug build)
  • specify the include directory (Wt defaults to installing in /usr/local/include)

Unlike Qt, there is no need for special features such as moc for starting a Wt project.

If you decide to use CMake, and have installed Wt in its default location (within /usr/local), you can use find_package in your CMakeLists.txt:

add_executable(myprog.wt PRIVATE Wt::Wt Wt::HTTP)

When you did not install the Wt libraries in /usr/local/lib/, you can use the -Wt_DIR argument. See the CMake documentation for more information.

The examples use a CMakeLists.txt which is customized for using the current build of Wt, and not that one that is already installed some place (with make install). Therefore, it is not really the recommended way to bootstrap your own Wt project.

The other methods are:

To handle many sourcefiles, dependencies... you need a makefile. Obviously, the way Wt is designed, you should have quickly many files for the many classes that will compose your app. Wt uses CMake: to make makefiles and that is probably a good choice. Just like many others, I switched to CMake (from hand-written makefiles) because of Wt and I am pretty happy with it.

make should produce the executable. At this point, you probably need to move the output of make to a directory available to your webserver; in practice you therefore need a script that is going to deploy the file. When you name the app, be sure the extension is recognized by the webserver. Also, you may need to kill active processes of your app and maybe copy the css and some other files (icons...) to the directory available to the webserver.

In the end, I bundled all that in a deploy file located in the build directory (the one that is usually created for CMake). After I have have finished changing the source files, I just type ./deploy on the command line and I can refresh my web page.

ps -A | grep app.wt | awk '{print $1}' | xargs kill
rm -f "~/${target_path}/${target_app}"
cp "${target_app}" ~/${target_path}/
cp ../app.css  ~/${target_path}/


You can use install command instead of cp, more or less like this:

install -m 0755 astariand.wt /var/www/game
install -m 0644 messages.xml /var/www/game
install -m 0644 astariand.css /var/www/game
install -m 0644 login.php /var/www/game
install -m 0644 includes.php /var/www/game
install -m 0755 -d /var/www/game/media
install -m 0755 -d /var/www/game/media/icons
install -m 0644 media/icons/* /var/www/game/media/icons
install -m 0755 -d /var/www/game/media/images
install -m 0644 media/images/* /var/www/game/media/images

Using install has two advantages. First, it allows you to set permissions on the fly (just as user and group, but I don't use this). Second, with

dedicated process session management you don't need to kill all processes beforehand - old connections will keep using the old binary and new

connections will use the new one, until all old connections "die from natural reasons".

Q: My browser shows a window with a message like 'Wt internal error: ReferenceError: Ext is not defined, code: undefined, description: undefined'. How do I resolve it?

Check your log for 404 messages regarding ExtJs. Download Ext 2.0.1 or 2.0.2 from the ExtJs homepage and install it as described here. ExtJs 2.0.2 is available for download here.

You will receive similar error messages when you use a WTextEdit and TinyMCE is not properly deployed. Download TinyMCE from the TinyMCE homepage.

ExtJS and TinyMCE need to be available in the document root of your web server. By default, Wt expects ext-related files to be found in 'ext/' (relative to your application deployment location), and TinyMCE in 'resources/tiny_mce/'.

For example (Wt 2.2.1), to run the widgetgallery example (which needs both ExtJS and TinyMCE) from within its source directory, you need the following organisation of auxiliary files:

 $ pwd
 $ ls ext/
 ext-all.js  ext-base.js  resources
 $ ls resources/
 collapse.gif      items-ok.gif     nav-minus.gif              nav-plus-line-middle.gif  sort-arrow-down.gif  tab_l.gif
 expand.gif        line-last.gif    nav-minus-line-last.gif    orbited.js                sort-arrow-none.gif  tab_r.gif
 iframe.js         line-middle.gif  nav-minus-line-middle.gif  orbited_LICENSE.txt       sort-arrow-up.gif    tiny_mce
 items.gif         line-trunk.gif   nav-plus.gif               slider-thumb-h.gif        stripes              tv-line-last.gif
 items-not-ok.gif  loading.png      nav-plus-line-last.gif     slider-thumb-v.gif        tab_b.gif
 $ ls resources/tiny_mce/
 langs  license.txt  plugins  themes  tiny_mce.js  tiny_mce.js.gz  tiny_mce_popup.js  tiny_mce_src.js  utils

and then you can run the example using the following command line:

 $ ../../build/examples/widgetgallery/widgetgallery.wt --http-address= --http-port=8080 --docroot .

Q: My browser shows an empty window and I am disappointed now.

See the previous question.

Q: How does Wt organize sessions in processes and threads?

Wt makes a distinction in the conceptual organization (which is reflected in the API of WApplication) and the way the application is actually deployed.

Conceptually, every user session is completely isolated from each other. For each new session, Wt calls the function that is supplied to WRun(), to create a WApplication object for that session. As a programmer, you should program for the general case where different WApplication objects are in different processes, and thus if you wish to communicate between different session, you have the following options: (in increasing order of flexibility traded for convenience):

  • A database to which every session connects.
  • A dedicated server daemon, with socket based communication.
  • A combination of both, with possible peer-to-peer communication between different sessions.

Physically, Wt offers several choices for deployment, each of them with different trade-offs. If an application sticks strictly to the previous rules, you can freely change between different deployment options at deployment time.

The options that are available are:

  • Dedicated-process mode: mapping one session to one process. Advantages are:
    • Kernel-level isolation between processes (security and reliability!).
    • Kernel-based sharing of read-only memory segments (simply UNIX feature).
    • Development friendly: a new session uses the latest deployed binary, and valgrind may be used to debug one particular session, by modifying the URL request.

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  • Shared-process mode: mapping multiple sessions in a fixed number of processes. Advantages are:
    • No process and stack overhead per session.

Wt is capable of using multi-threading to improve performance for both situations. Threads are used for simultaneous handling of requests. Even in dedicated-process mode, several requests may be handled simultaneously, for example concurrent streaming of different WResource's. The multi-threading feature however is more important for shared-process mode, for handling concurrent requests for different sessions. In the latter case, however, the number of threads must not

equal the number of active sessions: threads are reused after every request is handled.

The shared-process mode has the notable disadvantage, inherent to C, that memory corruption may occur and can take down all sessions. It is however well suited for 'open' applications on the Internet (and the Wt homepage and all examples are deployed this way). If you design a restricted access application, or possibly a security sensitive application, or deploy the application on a private intranet, the dedicated process mode may be more suitable.

Q: How does it compare to Java servlets?

Differences with Java Servlets are mostly due to the Java Virtual Machine. Java has the benefit of automating pointer manipulation, and therefore eliminating unwanted interference between different sessions because of pointer bugs. On the other hand, because of the high costs (both run-time start up as well as memory usage) associated with a Java Virtual Machine instance, Java cannot afford kernel-level isolation between different Java sessions. If not programmed properly, two sessions can still interfere through for example the use of class static variables. Unfortunately, some servlet based frameworks, like the often-used struts framework, actually encourage sharing of for example form objects between different sessions for run-time efficiency reasons, making session cross talk readily an issue.

Similarities between Wt and Java Servlets are the use of a thread pool to serve concurrent requests, and the abstraction of actual deployment details from the API, allowing easy scalability.

Note: you should consider using JWt if you would like to develop in Java.


Q: How do I deal with look and layout? Does Wt support CSS?

Wt provides you with options for layout.

  • You can use CSS for layout, and CSS may be either specified in CSS style sheets, or manipulated programmatorically. Tomasz Mazurek contributed a tutorial about it.

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  • You can use WTemplate for composing Wt widgets together in a layout. Within the templates, you can use CSS, but also Twitter Bootstrap for layouting.

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  • Wt's Twitter Bootstrap theme is a themable theme. Wt's widgets support rendering in bootstrap style, so if you compose your own program with WTemplate in bootstrap's style, you can benefit from Bootstrap and the Bootstrap themes you can find online (+ most design agencies know how to design a Bootstrap theme).

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  • You can use Wt's layout managers (e.g. WBoxLayout, WGridLayout, WBorderLayout). These have the advantage over CSS-based layouts that you also have control over vertical layout, but require JavaScript to work properly.

Q: How do I pass an additional argument from a signal to a slot?

Frequently, you may want to connect many different signals to a single slot, and identify the original sender in the slot.

For example:

 void Test::createWidgets()
   // create text1, text2, text3 widgets

   text1->clicked.connect(this, &Test::onClick);
   text2->clicked.connect(this, &Test::onClick);
   text3->clicked.connect(this, &Test::onClick);

 void Test::onClick()
   // How to know which widget?

Boost's boost.bind allows you to pass a 'generalized' function pointer where some (or all) of its arguments can be bound to a value of your choice:

 void Test::createWidgets()
   // Actually you can write onClick to accept any kind of argument
   // (int, enum, object pointer, ...). We chose to pass the sending widget
   // here.
   text1->clicked().connect(boost::bind(&Test::onClick, this, text1));
   text2->clicked().connect(boost::bind(&Test::onClick, this, text2));
   text3->clicked().connect(boost::bind(&Test::onClick, this, text3));

 void Test::onClick(Wt::WText* source)
   // source is where it is coming from
   // ...

The solution based on WSignalMapper is considered deprecated.

Q: How do I update my application window from another thread, or from a socket notifier?


See the documentation for Wt::WApplication::enableUpdates().


Q: Building web applications in a low-level language like C? Have you never heard of buffer overruns??

We are well aware of the hostile environment that is the Internet. We believe that Wt provides some unique benefits compared to other solutions to handle the most common attacks:

  • Cross-Site scripting attacks (XSS): an attacker forces the display of some script by letting the application render it to the browser of a victim that is also using the web application.
    • Unlike other web technologies, Wt does not require any effort from the programmer to avoid XSS attacks. Instead, any 'rich' XHTML text that needs to be displayed (for example in a WText using XHTMLFormatting) is filtered by a built-in XML parser for any potentially malicious tags or attributes (which is anything that may execute some JavaScript code). Unlike other (low-level) frameworks, Wt can provide this protection because there is no raw 'print' command. Instead, Wt generates all HTML/JavaScript from widgets and therefore it knows that rich text should only be "passive" rich text and not contain any "active" content.

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  • Cross-site request forgery attacks (CSRF): an attacker tricks a user into sending a request to a trusted website passing its credentials in a cookie.
    • This kind of attack is eliminated since Wt does not use cookies for session identification, but instead uses URL rewriting (within a (pseudo-) single page application). It uses a secure random number generator for the session ID (on platforms that provide this kernel-level service, such as Linux and Win32 platforms), and even when using cookies for session tracking, the session ID is always sent within the request as well, and verified within Wt (since Wt 2.2.0).

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  • Attacks against the application logic: an attacker issues a request to some page or service that is only accessible after authorization.
    • Wt protects the application logic because all incoming requests are interpreted in one central, well-tested routine. The request is parsed and only exposed event signals may be triggered. Exposed event signals are attached to widgets that are currently rendered on the screen. For example, a button click on a button that is currently shown on the screen. In this way, the logic of the application (such as for example: you need to first log in, and then only you may request for a payment) is automatically validated: only code in slots connected to exposed signals can be invoked by the user.

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  • Session cross-talk: sensitive data from one session spills in another session because of a programming error, where data is shared.
    • Wt is the only solution which may eliminate any cross-talk between sessions by deploying each session in a dedicated process, and thus using kernel-level protection (Dedicated Process mode of deployment). In the case of a bug, data from other sessions cannot be accessed and this is guaranteed by the kernel. This feature is especially valuable in sensitive areas such as financial transactions.
    • In other web application frameworks, such as Python/PHP/Java solutions, cross-talk between sessions is always a risk since sessions run within the same process for performance reasons since virtual machines and byte interpreters take their time to load. Cross-talk can be the consequence of a programming mistake where data structures are shared between sessions. In fact, many popular Java servlet-based frameworks encourage sharing of data structures, again for performance reasons, to avoid (expensive) object creation. For example, in struts form beans should be shared, and be reused by reinitialization rather than reconstruction.

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  • Buffer over-runs: A low-level C programming mistake is abused by an attacker to execute arbitrary code.
    • While it is true that C applications may suffer this problem, this is no longer a valid concern for modern C code. The main source of these programming mistakes was string manipulation in C, relying on careful memory management of the string buffers. In C, std::string avoids this issues entirely by automated memory management and buffer sizing. Furthermore, Wt is developed using the highest standards for code clarity (so we believe), and is thoroughly checked for memory-related problems by running it through memory checking tools such as valgrind.

All these attacks (except for the last one) are commonly exploited against current-day web applications which are vulnerable by the simple fact that too many web-related details are in the hands and responsibility of the developer. In contrast, Wt actively helps in avoiding programming mistakes which may lead to these exploits.

Q: How do I use the built-in HTTPS server in wthttpd?

You will need a private server key that is signed by a certificate authority, and a temporary file containing random Diffie-Hellman parameters. If you are simply experimenting with the feature, then you can create and sign a key yourself, or use the one that comes with the OpenSSL distribution (server.pem, which has the password 'test'). If you are about to set up a secure https server in a production environment, do not use the commands outlined in this section to generate the cryptographic files but get informed on up to date good practices.

The file with Diffie-Hellman parameters can be created using the command:

$ openssl dhparam -check -text -out dh2048.pem 2048

Then start Wt using:

$ ./app.wt --https-address= \
    --ssl-certificate=server.pem \
    --ssl-private-key=server.key \
    --ssl-tmp-dh=dh2048.pem \

Noe that the ssl-cipherlist is necessary because many flows have been discovered in recent years regarding weaknesses in specific ciphers. With this list (which follows the recommendation of you will avoid the use of weak ciphers.

Provide the password at the prompt.

To generate your own self-signed certificate:

# This sequence is found all over the internet:
openssl genrsa -des3 -out server.key 1024
openssl req -new -key server.key -out server.csr
cp server.key
openssl rsa -in -out server.key # removes the passphrase
openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt
# The PEM file is a combination of what is above:
cat server.crt server.key server.crt > server.pem

Alternatively, a reverse proxy that terminates https can be deployed. This wiki page explains how this can be configured, and also explains how "let's encrypt" (free certificates for everyone) can be used in this configuration:

Trouble shooting

Q: My application crashes, and my apache error log shows no information.

There is a known problem with mod_fcgid: STDERR (including everything printed to std::cerr) is not saved to the apache error log.

Wt uses STDERR by default for all error reporting. You can use a different log file in your wt_config.xml file ().

You may also consider using mod_fastcgi or the built-in web server (wthttpd) during development. The latter is especially convenient for development as it allows you to start from within a debugger, or diagnose memory-related problems with valgrind.

Updated by Roel Standaert 3 months ago ยท 42 revisions