Guidelines and HOWTOs/Debugging/Using Error Messages: Difference between revisions

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= Qt 4 / kdelibs 4 =
A screen recording version is available https://www.youtube.com/watch?v=oTAfNb9Pcj4


kdelibs provides a [http://api.kde.org/4.0-api/kdelibs-apidocs/kdecore/html/group__kdebug.html family of functions] that output information to <tt>stderr</tt>, meaning that if you run an application from the terminal, it will be displayed in that terminal window.  If you run the application from the desktop (using KRunner or the application menu, for example), the output will normally appear in {{path|~/.xsession-errors}} or {{path|~/.X.err}} if you use a login manager like KDM, or on the console you ran <tt>startx</tt> from if you started X that way.
This requires at least Qt version 5.


To use these functions in your code, you need to include the correct header file <syntaxhighlight lang="cpp-qt">
== Controlling Messages ==
#include <KDebug>
</syntaxhighlight>
and then you can use the functions <syntaxhighlight lang="cpp-qt">
kDebug() << "Something happened that only developers care about" << someVariable;
kWarning() << "Something bad happened that users (end-users, or application developers using this library) should be aware of";
kError() << "Something even worse happened";
kFatal() << "Something happened so bad we had to terminate the application";
</syntaxhighlight>


The syntax is much like <tt>cout</tt>, and most native C++ types, Qt-provided types and kdelibs-provided types can be passed directly (like with <tt>someVariable</tt> in the example).
<tt>kDebug()</tt> and friends have been deprecated in KDE Frameworks 5, and you should use Qt's built-in debugging instead.  We recommend that you use [https://doc.qt.io/qt-5/qloggingcategory.html QLoggingCategory], particularly for libraries and plugins.  Note that this is only available in Qt 5.2 and later.


Note that the <tt>kDebug</tt> calls will only do anything if the code was compiled with debugging enabled (and so will generally not work in packages from a distribution)This means <tt>cmake</tt> should be run with the <tt>-DCMAKE_BUILD_TYPE=debugfull</tt> argument.  The other functions, however, will produce output no matter how the code was compiled.
The source code for a library, plugin, or program named "Foo" may contain statements like
{{bc-hl|lang=cpp-qt|code=
qCDebug(LOG_FOO) << "Log something:" << someVariable;
}}
Here, <tt>LOG_FOO</tt> is a ''logging category''If debug-level messages have been enabled for that logging category, then the statement will write a message to <tt>stderr</tt>.


== Debug Areas ==
Some source file will define the logging category:
{{bc-hl|lang=cpp-qt|code=
Q_LOGGING_CATEGORY(LOG_FOO, "some.namespace.foo")
}}
Here, <tt>"some.namespace.foo"</tt> is the ''category name''.  Once you know the category's name, you can
set the <tt>QT_LOGGING_RULES</tt> environment variable to enable debug-level messages for the category:
{{bc-hl|lang=bash|code=
QT_LOGGING_RULES="some.namespace.foo.debug=true"
}}


The debugging output can be controlled at runtime using debugging areas.  This allows enabling debugging output for only certain libraries or plugins, for example.  Debugging areas are numbers, so the <tt>KStatusNotifierItemPrivate::registerToDaemon</tt> method in the kdeui library, for example, has the call <syntaxhighlight lang="cpp-qt">
You can also enable debug-level messages for the default category (when using <tt>qDebug</tt>), by setting:
kDebug(299) << "Registering a client interface to the KStatusNotifierWatcher";
{{bc-hl|lang=bash|code=
</syntaxhighlight>
QT_LOGGING_RULES="default.debug=true"
The file <tt>kdebug.areas</tt> in the <tt>kdecore</tt> directory of kdelibs indicates that the number 299 is associated with "kdeui (KNotification)".
}}


This information is used by the <tt>kdebugdialog</tt> utility (which you can just run from the commandline or using KRunner) to turn these areas on and off, enabling or disabling those <tt>kDebug</tt> statements.  It is also used by <tt>kDebug</tt>, <tt>kWarning</tt>, <tt>kError</tt> and <tt>kFatal</tt> to indicate which component output the line.  For example, the line in the above example, found in the method <tt>KStatusNotifierItemPrivate::registerToDaemon</tt>, would produce the line <pre>kwalletmanager(642)/kdeui (KNotification) KStatusNotifierItemPrivate::registerToDaemon: Registering a client interface to the KStatusNotifierWatcher</pre> when executed from within the application kwalletmanager, with PID 642.
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.


For applications, you can generally just omit the area number, and <tt>kDebug</tt> will use the default area.  If you are developing a library or a plugin, though, you should get a number assigned (via the kde-core-devel mailing list) for your library or plugin, and use it in your code. Rather than using the number directly in every call to <tt>kDebug</tt> and friends, you can just add<syntaxhighlight lang="cmake">
To specify several categories, separate them with semicolon:
add_definition(-DKDE_DEFAULT_DEBUG_AREA=<number>)'
{{bc-hl|lang=bash|code=
</syntaxhighlight>
QT_LOGGING_RULES="*.debug=false;driver.usb.debug=true"
to your <tt>CMakeLists.txt</tt> file.
QT_LOGGING_RULES="*.debug=true;qt.*.debug=false"
}}


== Improving Log Output ==
They can also be stored in various configuration files. Please see the [https://doc.qt.io/qt-5/qloggingcategory.html#logging-rules QLoggingCategory documentation] for details.


There are a couple of useful environment variables to control the output of <tt>kDebug</tt> and friends. <syntaxhighlight lang="bash">
If you run the application from within a terminal application, like [http://www.kde.org/applications/system/konsole/ Konsole], you will see the logging output in that terminal window.  If you use an [https://develop.kde.org/docs/getting-started/building/ide/ Integrated Development Environment] like [https://kde.org/applications/en/development/org.kde.kdevelop 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:
export KDE_COLOR_DEBUG=1
{{bc-hl|lang=bash|code=
</syntaxhighlight> will make them produce colored output, and <syntaxhighlight lang="bash">
QT_FORCE_STDERR_LOGGING=1
export KDE_DEBUG_TIMESTAMP=1
}}
</syntaxhighlight> will include timestamps in the output. <syntaxhighlight lang="bash">
export KDE_DEBUG_TIMESTAMP=2
</syntaxhighlight> can be used to include milliseconds in the timestamps.


== Managing Lots of Output ==
Also check the [https://wiki.archlinux.org/title/Systemd/Journal 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 {{path|~/.xsession-errors}} or {{path|~/.X.err}} if you use a login manager like KDM, or on the console you ran <tt>startx</tt> 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")
 
{{bc-hl|lang=cpp-qt|code=
#include <QLoggingCategory>
Q_DECLARE_LOGGING_CATEGORY(LOG_FOO)
}}


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:
and exactly one source file containing
# Use <tt>kdebugdialog</tt> to disable some logging areas to limit the amount of output generated
# Increase the amount of scrollback in the terminal so that output is not lost; in Konsole, you can go to <tt>Settings > Edit Current Profile...</tt> and click on the <tt>Scrollback</tt> tab to change this.  Konsole also has a useful search feature: just press <tt>Ctrl+Shift+F</tt> or click <tt>Find...</tt> on the <tt>Edit</tt> menu.
# Save the output to a file; <tt>tee</tt> is useful for this.  For example, you can run <pre>application 2&gt;&amp;1 | tee debug.log</pre> to save the output to the file <tt>debug.log</tt> while still viewing it in the terminal.  This can also be used to capture output from <tt>startx</tt>.


= Qt 5 / KDE Frameworks 5 =
{{bc-hl|lang=cpp-qt|code=
#include "foo-debug.h"
Q_LOGGING_CATEGORY(LOG_FOO, "some.namespace.foo")
}}


<tt>kDebug()</tt> and friends have been deprecated in KDE Frameworks 5, and you should use Qt's built-in debugging insteadWe recommend that you use [http://doc-snapshot.qt-project.org/qt5-stable/qloggingcategory.html QLoggingCategory], particularly for libraries and plugins. Note that this is only available in Qt 5.2 and later.
You should treat the category name (<tt>"some.namespace.foo"</tt> in the example) as something like reverse DNS; it cannot contain spaces, and dots indicate a hierarchyFor example, KDE PIM category names all start with <tt>"org.kde.pim."</tt>.


In particular, for a library or plugin called "Foo", you should have a common header that contains the following declaration
To simplify the setup, you can use the ECM macro <tt>[https://api.kde.org/ecm/module/ECMQtDeclareLoggingCategory.html ecm_qt_declare_logging_category]</tt>, which generates the respective source files for you:
#include <QLoggingCategory>
Q_DECLARE_LOGGING_CATEGORY(LOG_FOO)
and exactly one source file containing
Q_LOGGING_CATEGORY(LOG_FOO, "foo")


You should treat the string as something like reverse DNS; it cannot contain spaces, and dots indicate a heirarchy. For example, a Plasma dataengine called "Foo" might use the category <tt>"plasma.engine.foo"</tt>.
{{bc-hl|lang=cmake|code=
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
Logging lines then look like
qCDebug(LOG_FOO) << "Log something:" << stuff();
 
qCWarning(LOG_FOO) << "Something bad happened that users (end-users, or application developers using this library) should be aware of";
{{bc-hl|lang=cpp-qt|code=
qCCritical(LOG_FOO) << "Something happened so bad we had to terminate the application";
#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 <tt>cout</tt>, and most native C++ types, Qt-provided types and KF-provided types can be passed directly (like with <tt>someVariable</tt> in the example).


With Qt 5.2, the <tt>qCDebug</tt> line will not produce any output; this is because logging categories are disabled by default.  You need to include the line
With Qt 5.2, the <tt>qCDebug</tt> 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 <tt>main()</tt> function.  Of course, you would typically disable this call in release versions.  Qt 5.3 will hopefully include a way to change this configuration externally, without recompiling.


If you run your application from within a terminal application, like [http://www.kde.org/applications/system/konsole/ Konsole], you will see the logging output in that terminal window.  Otherwise, it will usually appear in <tt>~/.xsession-errors</tt>.
{{bc-hl|lang=cpp-qt|code=
QLoggingCategory::setFilterRules(QStringLiteral("foo.debug = true"));
}}


== Improving Logging Output ==
somewhere in the application code, generally in the <tt>main()</tt> function.  Of course, you would typically disable this call in release versions.


Qt provides a way of controlling the output of the logging methods via an environment variable. You can tell it to include the application name and PID, as well as the debugging category, and color-code the text. For example, running the following lines in your shell will produce something that looks quite like <tt>kDebug</tt>'s colored output:
== Customize Logging Output ==
c=`echo -e "\033"`
 
export QT_MESSAGE_PATTERN="%{appname}(%{pid})/(%{category}) $c\[31m%{if-debug}$c\[34m%{endif}%{function}$c\[0m: %{message}"
Qt provides a way of controlling the output of the logging methods via an {{ic|QT_MESSAGE_PATTERN}} environment variable.  
unset c
 
You can control the message pattern depending on message type:
 
{{bc-hl|lang=bash|code=
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 [https://doc.qt.io/qt-6/qtglobal.html#qSetMessagePattern 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 <tt>kDebug</tt>'s colored output:
 
{{bc-hl|lang=bash|code=
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 [https://github.com/Alhadis/OSC8-Adoption/ OSC-8]. For example, instead of printing the {{ic|%{file<nowiki>}</nowiki>}} (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 [https://develop.kde.org/docs/getting-started/building/ide/clion/ CLion], keep in mind that colorizing the hyperlinked text is currently broken. See  [https://youtrack.jetbrains.com/issue/IDEA-218793/Provide-visual-decoration-for-terminal-HTML-like-hyperlinks IDEA-218793].


== Managing Lots of Output ==
== 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:
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:
# [http://doc-snapshot.qt-project.org/qt5-stable/qloggingcategory.html#setFilterRules Disable some logging categories] to limit the amount of output generated
# [https://doc.qt.io/qt-5/qloggingcategory.html#setFilterRules Disable some logging categories] to limit the amount of output generated
# Increase the amount of scrollback in the terminal so that output is not lost; in Konsole, you can go to <tt>Settings > Edit Current Profile...</tt> and click on the <tt>Scrollback</tt> tab to change this. Konsole also has a useful search feature: just press <tt>Ctrl+Shift+F</tt> or click <tt>Find...</tt> on the <tt>Edit</tt> menu.
# 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.
# Save the output to a file; <tt>tee</tt> is useful for this.  For example, you can run <pre>application 2&gt;&amp;1 | tee debug.log</pre> to save the output to the file <tt>debug.log</tt> while still viewing it in the terminal.
# Save the output to a file. The UNIX command line <code>tee</code> is useful for this.  For example, you can run <code>application 2>&1 | tee ~/debug.log</code> or <code>application |& tee ~/debug.log</code> to save the output to the file <code>~/debug.log</code> while still viewing it in the terminal.
 
== Example ==
 
<pre>
export QT_FORCE_STDERR_LOGGING=1
export QT_MESSAGE_PATTERN="%{time yyyy.MM.dd, hh:mm:ss.zzz}, %{pid}, %{appname}, %{category}, %{type}, %{file}:%{line}, %{function} - %{message}"
export QT_LOGGING_RULES="*.debug=true;qt*.debug=false"
 
kcalc |& tee ~/a.txt
# kate ~/a.txt &
</pre>
 
You can append at the end of QT_MESSAGE_PATTERN the string ", %{backtrace}".
 
"qt*.debug=false" is there because there are few errors in the Qt Framework source code, because there are some very verbose log lines there (e.g. "qt.qpa.events", "qt.qpa.input.events", "qt.quick.hover.trace") and because we are more interested in the KDE source code than in the Qt source code.

Latest revision as of 17:51, 18 April 2024

A screen recording version is available https://www.youtube.com/watch?v=oTAfNb9Pcj4

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"
QT_LOGGING_RULES="*.debug=true;qt.*.debug=false"

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.

Example

export QT_FORCE_STDERR_LOGGING=1
export QT_MESSAGE_PATTERN="%{time yyyy.MM.dd, hh:mm:ss.zzz}, %{pid}, %{appname}, %{category}, %{type}, %{file}:%{line}, %{function} - %{message}"
export QT_LOGGING_RULES="*.debug=true;qt*.debug=false"

kcalc |& tee ~/a.txt
# kate ~/a.txt &

You can append at the end of QT_MESSAGE_PATTERN the string ", %{backtrace}".

"qt*.debug=false" is there because there are few errors in the Qt Framework source code, because there are some very verbose log lines there (e.g. "qt.qpa.events", "qt.qpa.input.events", "qt.quick.hover.trace") and because we are more interested in the KDE source code than in the Qt source code.