Difference between revisions of "Development/Tutorials/Debugging/Using Error Messages"

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When you start a konsole and type the commands to start an application you
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= Qt 4 / kdelibs 4 =
will see all sorts of statements are printed in the konsole while the
+
application is running. All applications print these messages, to look
+
at them you have to know where to look. The application will have to be
+
compiled with the debugging enabled. So using a precompiled package from a distribution
+
probably will not give you this information. If you compiled the application
+
yourself, make sure the configure option "<tt>--disable-debug</tt>" was not used.
+
  
In KDE all debugging text-output can be switched on or off based on so
+
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.
called '''areas'''. One application can be one or more area. One part of the kde base libraries can be another area. Enabling/disabling these areas from being printed can be done using the '''kdebugdialog''' application. For simple debugging selecting all
+
sections is probably wise.
+
  
When you are debugging it is best to simply start a konsole and start the
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To use these functions in your code, you need to include the correct header file <syntaxhighlight lang="cpp-qt">
application from there. In a konsole you could simply type:
+
#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>
  
kicker
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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).
  
and in the konsole kicker could return a message like:
+
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.
  
ERROR: kicker is already running!
+
== Debug Areas ==
  
When a lot of output is written to the konsole it might go out of view before
+
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 could read it, therefor it is easy to create a text file which contains
+
kDebug(299) << "Registering a client interface to the KStatusNotifierWatcher";
all this information, to do so type the following:
+
</syntaxhighlight>
 +
The file <tt>kdebug.areas</tt> in the <tt>kdecore</tt> directory of kdelibs indicates that the number 299 is associated with "kdeui (KNotification)".
  
  application 2&gt;&amp;1 | tee debug.log
+
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 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.
  
where 'application' can be replaced with the application you are debugging.
+
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">
Afterwards you could open the file 'debug.log' to look at the messages again.
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add_definition(-DKDE_DEFAULT_DEBUG_AREA=<number>)
 +
</syntaxhighlight>
 +
to your <tt>CMakeLists.txt</tt> file.
  
If you are NOT starting the application from a konsole the messages will be
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== Improving Log Output ==
logged somewhere else, or they could have been discarded by the program that
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started your application.
+
  
If your application is started by clicking on an icon your best bet is to check
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There are a couple of useful environment variables to control the output of <tt>kDebug</tt> and friends.  <syntaxhighlight lang="bash">
the following log files. Beware; they contain logs for a lot of applications,  
+
export KDE_COLOR_DEBUG=1
not just the application you are debugging!
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</syntaxhighlight> will make them produce colored output, and <syntaxhighlight lang="bash">
 +
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.
  
'''Case 1: Graphical login (i.e. kdm, gdm, xdm, etc.'''
+
== Managing Lots of Output ==
  
The debug messages get redirected into the file {{path|~/.xsession-errors}} or
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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:
{{path|~/.X.err}} in your home directory (that is with a leading dot '.' also
+
# Use <tt>kdebugdialog</tt> to disable some logging areas to limit the amount of output generated
watch the Capital).
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# 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>.
  
'''Case 2: You are using startx:'''
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= Qt 5 / KDE Frameworks 5 =
  
Use the following command to restart your session:
+
<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 [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.
  startx 2&gt;&amp;1 | tee startx.log</pre>
+
  
so that all the debug messages of applications started at KDE's startup (and
+
In particular, for a library or plugin called "Foo", you should have a common header that contains the following declaration <syntaxhighlight lang="cpp-qt">
any application launched from the panel etc.) go to the file "startx.log"
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#include <QLoggingCategory>
 +
Q_DECLARE_LOGGING_CATEGORY(LOG_FOO)
 +
</syntaxhighlight> and exactly one source file containing <syntaxhighlight lang="cpp-qt">
 +
Q_LOGGING_CATEGORY(LOG_FOO, "foo")
 +
</syntaxhighlight>
  
== Links ==
+
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>.
  
The debug messages are usually printed in C++ with the kDebug or kWarning statement. Example:
+
Logging lines then look like <syntaxhighlight lang="cpp-qt">
 +
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";
 +
</syntaxhighlight>
  
kDebug(1210) << "arbitrary message" << endl;
+
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).
kWarning(1210) << "this rather should not happen" << endl;
+
  
The number 1210 (so called ''debug area'') in this case represents kickerYou can omit the number.
+
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 <syntaxhighlight lang="cpp-qt">
 +
QLoggingCategory::setFilterRules(QStringLiteral("foo.debug = true"));
 +
</syntaxhighlight>
 +
somewhere in the application code, generally in the <tt>main()</tt> function.  Of course, you would typically disable this call in release versionsQt 5.3 will hopefully include a way to change this configuration externally, without recompiling.
  
See also: [http://api.kde.org/4.0-api/kdelibs-apidocs/kdecore/html/group__kdebug.html kDebug/kWarning API documentation].
+
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 {{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.
  
''Initial Author:'' [mailto:zander@kde.org Thomas Zander]
+
== Improving Logging Output ==
 +
 
 +
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: <syntaxhighlight lang="bash">
 +
c=`echo -e "\033"`
 +
export QT_MESSAGE_PATTERN="%{appname}(%{pid})/(%{category}) $c\[31m%{if-debug}$c\[34m%{endif}%{function}$c\[0m: %{message}"
 +
unset c
 +
</syntaxhighlight>
 +
 
 +
== 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:
 +
# [http://doc-snapshot.qt-project.org/qt5-stable/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.
 +
# Save the output to a file; <tt>tee</tt> is useful for this.  For example, you can run <syntaxhighlight lang="bash">application 2<&1 | tee debug.log</syntaxhighlight> to save the output to the file <tt>debug.log</tt> while still viewing it in the terminal.

Revision as of 18:33, 25 November 2013

Contents

Qt 4 / kdelibs 4

kdelibs provides a family of functions that output information to stderr, 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 ~/.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.

To use these functions in your code, you need to include the correct header file
#include <KDebug>
and then you can use the functions
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";

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

Note that the kDebug 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 cmake should be run with the -DCMAKE_BUILD_TYPE=debugfull argument. The other functions, however, will produce output no matter how the code was compiled.

Debug Areas

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 KStatusNotifierItemPrivate::registerToDaemon method in the kdeui library, for example, has the call
 kDebug(299) << "Registering a client interface to the KStatusNotifierWatcher";

The file kdebug.areas in the kdecore directory of kdelibs indicates that the number 299 is associated with "kdeui (KNotification)".

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

when executed from within the application kwalletmanager, with PID 642.

For applications, you can generally just omit the area number, and kDebug 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 kDebug and friends, you can just add
add_definition(-DKDE_DEFAULT_DEBUG_AREA=<number>)

to your CMakeLists.txt file.

Improving Log Output

There are a couple of useful environment variables to control the output of kDebug and friends.
export KDE_COLOR_DEBUG=1
will make them produce colored output, and
export KDE_DEBUG_TIMESTAMP=1
will include timestamps in the output.
export KDE_DEBUG_TIMESTAMP=2
can be used to include milliseconds in the timestamps.

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. Use kdebugdialog to disable some logging areas 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; tee is useful for this. For example, you can run
    application 2>&1 | tee debug.log
    to save the output to the file debug.log while still viewing it in the terminal. This can also be used to capture output from startx.

Qt 5 / KDE Frameworks 5

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.

In particular, for a library or plugin called "Foo", you should have a common header that contains the following declaration
#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 "plasma.engine.foo".

Logging lines then look like
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 kdelibs-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. 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 Konsole, you will see the logging output in that terminal window. Otherwise, it 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.

Improving Logging Output

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

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; tee is useful for this. For example, you can run
    application 2<&1 | tee debug.log
    to save the output to the file debug.log while still viewing it in the terminal.

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