Guidelines and HOWTOs/Debugging/Using Error Messages

This requires at least Qt version 5.

Controlling Messages

kDebug() and friends have been deprecated in KDE Frameworks 5, and you should use Qt's built-in debugging instead. We recommend that you use QLoggingCategory, particularly for libraries and plugins. Note that this is only available in Qt 5.2 and later.

The source code for a library, plugin, or program named "Foo" may contain statements like

qCDebug(LOG_FOO) << "Log something:" << someVariable;

Here, LOG_FOO is a logging category. If debug-level messages have been enabled for that logging category, then the statement will write a message to stderr.

Some source file will define the logging category:

Q_LOGGING_CATEGORY(LOG_FOO, "some.namespace.foo")

Here, "some.namespace.foo" is the category name. Once you know the category's name, you can set the QT_LOGGING_RULES environment variable to enable debug-level messages for the category:

QT_LOGGING_RULES="some.namespace.foo.debug=true"

You can also enable debug-level messages for the default category (when using qDebug), by setting:

QT_LOGGING_RULES="default.debug=true"

Logging rules can be more complex than the examples above. They can specify wildcards in the category name, enable or disable more than one message level, and control more than one logging category.

To specify several categories, separate them with semicolon:

QT_LOGGING_RULES="*.debug=false;driver.usb.debug=true"

They can also be stored in various configuration files. Please see the QLoggingCategory documentation for details.

If you run the application from within a terminal application, like Konsole, you will see the logging output in that terminal window. If you use an Integrated Development Environment like KDevelop it will display the output in its windows. In Qt Creator debug messages goes to systemd journal by default. You want to see them in Application Output, so you need to specify this variable in Run Environment:

QT_FORCE_STDERR_LOGGING=1

Also check the systemd journal, since the application could be its direct child process. Otherwise, if you happen to still use X11, the messages will usually appear in ~/.xsession-errors or ~/.X.err if you use a login manager like KDM, or on the console you ran startx from if you started X that way.

Adding Logging to Your Code

For a library or plugin called "Foo", you should have a common header that contains the following declaration (e.g. called "foo-debug.h")

#include <QLoggingCategory>
Q_DECLARE_LOGGING_CATEGORY(LOG_FOO)

and exactly one source file containing

#include "foo-debug.h"
Q_LOGGING_CATEGORY(LOG_FOO, "some.namespace.foo")

You should treat the category name ("some.namespace.foo" in the example) as something like reverse DNS; it cannot contain spaces, and dots indicate a hierarchy. For example, KDE PIM category names all start with "org.kde.pim.".

To simplify the setup, you can use the ECM macro ecm_qt_declare_logging_category, which generates the respective source files for you:

include(ECMQtDeclareLoggingCategory)
ecm_qt_declare_logging_category(FOO_SRCS
    HEADER foo-debug.h
    IDENTIFIER "LOG_FOO"
    CATEGORY_NAME "some.namespace.foo"
)

Logging lines then look like

#include "foo-debug.hpp"
qCDebug(LOG_FOO) << "Log something:" << someVariable;
qCWarning(LOG_FOO) << "Something bad happened that users (end-users, or application developers using this library) should be aware of";
qCCritical(LOG_FOO) << "Something happened so bad we had to terminate the application";

The syntax is much like cout, and most native C++ types, Qt-provided types and KF-provided types can be passed directly (like with someVariable in the example).

With Qt 5.2, the qCDebug line will not produce any output; this is because logging categories are disabled by default. You need to include the line

QLoggingCategory::setFilterRules(QStringLiteral("foo.debug = true"));

somewhere in the application code, generally in the main() function. Of course, you would typically disable this call in release versions.

Customize Logging Output

Qt provides a way of controlling the output of the logging methods via an QT_MESSAGE_PATTERN environment variable.

You can control the message pattern depending on message type:

QT_MESSAGE_PATTERN="[%{time hh:mm:ss.zzz} %{if-debug}D%{endif}%{if-info}I%{endif}%{if-warning}W%{endif}%{if-critical}C%{endif}%{if-fatal}F%{endif}] %{category} %{message}"

You can tell it to include the application name and PID, as well as the debugging category. See qSetMessagePattern documentation for the full list of placeholders.

You even can use escape sequences to colorize the text. For example, running the following lines in your shell will produce something that looks quite like kDebug's colored output:

c=`echo -e "\033"`  # escape symbol
export QT_MESSAGE_PATTERN="%{appname}(%{pid})/(%{category}) $c\[31m%{if-debug}$c\[34m%{endif}%{function}$c\[0m: %{message}"
unset c

It is possible to create hyperlinked text with OSC-8. For example, instead of printing the %{file} (a full path to file), you may want to print a class name, which is a hyperlink to the file and line of message.

If using CLion, keep in mind that colorizing the hyperlinked text is currently broken. See IDEA-218793.

Managing Lots of Output

If you have lots of debugging statements, they may appear too fast and leave the terminal window before you can read them. There are three main ways to deal with this:

1. Disable some logging categories to limit the amount of output generated
2. Increase the amount of scrollback in the terminal so that output is not lost; in Konsole, you can go to Settings > Edit Current Profile... and click on the Scrollback tab to change this. Konsole also has a useful search feature: just press Ctrl+Shift+F or click Find... on the Edit menu.
3. Save the output to a file. The UNIX command line tee is useful for this. For example, you can run application 2>&1 | tee ~/debug.log or application |& tee ~/debug.log to save the output to the file ~/debug.log while still viewing it in the terminal.